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Working on a presentation for work has given me an opportunity to read up about the history of the Web Browser. Turns out to be not such a simple thing to unravel, but I've given it a go.

For many years I thought it would be a mistake to mix my hobbies with my professional life. Blurring the boundary would prevent me defining a clear boundary between my work time and my relaxation time. I thought it could also lead to things I enjoy becoming contaminated, irreversibly harming the joy I get from them. It's not that I didn't want to enjoy my work: quite the opposite in fact. I felt in order to enjoy both I needed to maintain a separation.

As my life has progressed I've changed my opinion on this. It's great to separate work and play, but there's also immense joy to be had from doing something you love as a professional endeavour. Mixing the two together has the potential to amplify the joy from both.

Working for Jolla was what really brought this home to me. Smartphone development, user privacy and control, and Sailfish OS in particular were always part of the life I separated from my career. When I started working at Jolla I thought I was taking a risk. Would I lose my passion? Would I regret knowing what goes on inside the "sausage factory"?

My concerns were unfounded and, with this experience in hand, I now make it my aim to bring my personal passions into my professional life as well.

Now I'm at the Turing I'm no longer developing for Sailfish OS during work hours. As readers of my gecko dev diaries will know, upgrading the Sailfish browser has been one of my main activities outside of work. Finding opportunities to bring this Sailfish development into my professional world has been one of my objectives and recently just such an opportunity arose.

It's only a small overlap: in November I'll be giving a presentation about browsers at the Turing. The title of the talk will be "The Anatomy of a Browser: Embedded Mobile Lizards". Lizards being a reference to Gecko.

To help with this I've been digging a little into the history of browsers. They have a rich, fascinating and often fractious history that I find fascinating and one I want to talk a little more about today.

But to understand the history, we first need to understand a little about the internals of a browser.

Browser Internals

What are the various pieces that make up a browser? Broadly speaking we can see it as being made up of five parts:

1. Protocol client (HTTP/S, WS/S, file, FTP,...).
2. JavaScript engine.
3. DOM - Document Object Model.
4. Layout/rendering engine (HTML, CSS, SVG).
5. Media encoder/decoder (JPEG, PNG, audio, video,...).
6. User interface.

That's already quite a lot to think about, but each of these can be broken down into many more pieces. Let's look at them in a bit more detail.
 
The protocol client handles network interactions. It opens a network connection, sends a message to the server, then waits for and interprets the response. If the protocol uses a secure transport layer it handles certificate validation, checking certificate revocation, data encryption and integrity. The latest releases of Firefox and Chrome support HTTP, HTTPS, WebSockets, Secure WebSockets, Secure Real-time Transport Protocol, file access and probably others I'm not aware of. Firefox and Chrome used to support FTP but have since dropped it. Firefox dropped support in version 90 (July 2021) while Chrome dropped it in version 95 (September 2021). Unlike the rendering and JavaScript engines, protocol clients tend not to be given their own bespoke names separate from the browser. Maybe this is because they're often built from other libraries offering support for specific protocols. Nevertheless the protocol client is both a crucial and complex part of the browser.

I've listed the DOM as a separate piece of the browser, but it's usually tightly coupled with the layout and rendering engines. The DOM defines the internal data structures used to represent the page being rendered. For HTML, XML or SVG documents these are hierarchically built from nodes that have a parent and multiple (possibly zero) children. Typically the document structure will map naturally onto the DOM, with XML elements and attributes mapping onto nodes. Child nodes in the document will map to child nodes in the DOM. In practice nodes are likely to be represented as class objects in the code containing references to child nodes. The DOM is usually part of the rendering engine separate from the JavaScript engine, but if it weren't for JavaScript the DOM might be considered as just an implementation details. The existence of JavaScript elevates the DOM to something Web developers have to have a good understanding of, as we'll see.

The JavaScript engine allows execution of JavaScript code. JavaScript has an odd history. Originally invented at Netscape by Brendan Eich, you might think that the JavaScript language has something to do with Java. In fact they're very different. Java is a strictly-typed object-oriented garbage-collected language that compiles down to a bytecode representation that can be executed by a Java Virtual Machine. Although at one point Java "applets" that ran in the browser were a thing, you rarely see these nowadays (they're not supported without installing a plugin). Java is still used in server applications and to be honest, given Sun's expertise and revenue rested primarily with servers I always found it rather surprising that it was ever anything other than server-focused. JavaScript on the other hand is very much a client-side, dynamically-typed event-based scripting language with prototype-based object orientation. In recent years it's also become popular as a server-side language for reasons that I won't go in to here. Both Java and JavaScript are "curly-brace" languages with similarities to C++; and while I realise I've managed to make them sound quite similar, they're actually totally different. The only reason they share a name is that in a bit to ride the wave of Java's popularity, Netscape signed a licensing agreement with Sun to use the name. Marketing genius or ontological vandalism? You decide.

Another key difference between JavaScript and Java is that the DOM is a first-class entity in JavaScript. Although they live in different parts of the browser, the development of the DOM is tightly intertwined with that of the language. When first released by Netscape JavaScript could interact with only certain elements of the page, most notably form elements. The name now given to the set of elements exposed at that time is DOM Level 0. Access to the full document didn't come until DOM Level 1. While JavaScript is a perfectly good language even without the DOM, in a browser context the two are tightly coupled.

Although JavaScript refers to and can modify the DOM, the DOM implementation is part of the layout and rendering engine. When we refer to browser engines (WebKit, Gecko, Blink,...) we're usually referring to this layout/render engine portion of the browser. The layout engine takes the document, structured using the DOM, and lays it out as elements on the page in the way they'll be viewed by the user. This allows the browser to build up the equivalent of a scene graph which is then rendered by the rendering engine to some sort of canvas (the screen or an offscreen buffer). This rendering usually uses an appropriate render backend, for example on the Sailfish Browser it calls a serious of GLES commands. The layout engine follows a strict set of rules for positioning elements on the page. The HTML/CSS box model is used for rendering most items, but there are exceptions. For example SVG has its own rendering model which Gecko also supports as part of the same DOM hierarchy.

HTML and SVG documents embed or reference large numbers of other file types, which the browser has to support as well. These multimedia files include images, audio, animations and video. Historically browser support for different multimedia elements has been a mess, often delegated to some other operating system component (e.g. Windows Media Player, ffmpeg, gstreamer). Each file type will have its own decoder and there may be Digital Rights Management involved as well (e.g. Widevine). In practice browsers tend to separate raster and vector images from video and audio. The former have been tightly integrated into HTML for decades whereas the latter two only became standardised in HTML 5 with the introduction of the audio and video tags. These allow audio and video to be embedded with customisable controls.

Finally we have the user interface, which is the bit that we most associate with the browser. This is a little ironic given I'd argue the depth, complexity and maintenance burden is weighted towards the other layers. But most people aren't really concerned with the rendering or JavaScript engine, they care about whether a particular user interface feature is supported or not.

And to be fair, the user interface doesn't just display an address bar. It also has to provide tabs, JavaScript pop-ups, permissions dialogues, Settings controls, password management functionality, bookmarks, history management and a whole lot more.

In the embedded browser space the user interface is intentionally minimal. The idea is that the browser gets embedded into some other application which provides the user interface elements needed over and above those provided by the rendered Web page itself. On Sailfish OS this minimal interface is provided by the WebView. The additional capabilities are managed through the WebView's Application Programming Interface. On Sailfish OS there's also a Qt-based user interface to the browser, which brings its own complexity. For simplicity I've grouped together the user interface of the browser and the application programming interface of the embeddable WebView in the "Interface" section in the diagram.

During my time upgrading the Sailfish Browser from ESR 78 to ESR 91 I routinely referred to it as a Gecko upgrade. The name Gecko covers the DOM, layout engine and rendering engine but typically doesn't include the JavaScript engine or user interface. The user interface is typically referred to by the name of the browser itself. For example Firefox uses the Gecko rendering engine, the SpiderMonkey JavaScript engine and the Firefox user interface. For Safari it's WebKit, Nitro and Safari. For Chrome it's Blink, V8 and Chrome. And so on.

Now that we've broken down the different parts of the browser we're equipped to delve into the history of Web browsers in more detail.

Libwww

We're going to start our history in 1990 when Tim-Berners Lee and Jean-François Groff, both working at CERN, created the HTTP protocol and HTML language that still define the Web today. It fascinates me that Tim-Berners Lee is so well-known as the inventor of the Web, but pioneers like Jean-François Groff and Nicola Pellow, who were there at the beginning, are scarcely recorded. But the Computer History Museum has documented a fascinating interview with Jean-François in which he gives am explanation of the very first Web engine.
 
Right at the start the history is a bit messy. The WorldWideWeb browser was the graphical HTML browser (and editor) written by Tim-Berners Lee in Objective-C to run on NeXTSTEP. The first version was completed at the end of 1990 with the browser being later renamed to Nexus. The code was re-written in C by Tim and Jean-François and turned into the Libwww library to become the very first browser engine. This was then used by Nicola Pellow at CERN to write the Line Mode Browser which was text-based, usable over telnet and released in 1991.
 
This was not just the birth of the Web, but also the genesis of structures that now define what it means to be a Web browser. These same structures can be seen in how browsers are built today.

Libwww and the Line Mode Browser that were created from it continued to be developed right up until 2017. Although the library is written in C it applies an object-oriented approach. Structures have constructors and destructors with the me context variable often used in places where you might find this or self in an object-oriented language. Reading this code in the early noughties had a profound influence on me, shaping my own style of C coding to this day.

/*	Create a Context Object
**	-----------------------
*/
PRIVATE Context * Context_new (LineMode *lm, HTRequest *request, LMState state)
{
    Context * me;
    if ((me = (Context  *) HT_CALLOC(1, sizeof (Context))) == NULL)
        HT_OUTOFMEM("Context_new");
    me->state = state;
    me->request = request;
    me->lm = lm;
    HTRequest_setContext(request, (void *) me); 
    HTList_addObject(lm->active, (void *) me);
    return me;
}

Besides the Line Mode Browser, Libwww was used in countless other projects. My own port to RISC OS from 2004 is still available. I used it to extend a forensic analysis tool for use on the Web (that the University I worked for later patented).

More notably it was also used by the Amaya lightweight Web editor developed at INRIA and the Mosaic Browser developed at the NCSA. The Mosaic browser was popular in its day and the NCSA spun out a commercial entity in the form of Spyglass Mosaic which built on the NCSA Mosaic code. The company was set up to licence the browser to other companies.

Trident

This Microsoft duly did. The browser engine of Internet Explorer — called Trident — was built on the Mosaic technology. The first version of Internet Explorer shipped without JavaScript support (the language hadn't been invented yet), but when it arrived in IE 3 in 1996 it was powered by Microsoft's Chakra JavaScript (nee JScript) engine.

The licensing agreement struck with Spyglass required Microsoft to pay a small monthly fee with additionally a portion of all non-Windoww revenue from the browser going to Spyglass.

As anyone who experienced the browser wars at that time will know, Microsoft proceeded to give Internet Explorer away for free with Windows. This ultimately earned them a lawsuit from Spyglass (settled out of court for $8 million) and an antitrust lawsuit from the US Government (eventually resulting in Microsoft having to change its approach to interoperability).

Internet Explorer remained as a core component of Windows until Windows 10, after which the company finally switched to offering Edge as the default browser. While Edge is built on Google's Blink engine, even that wasn't enough to dislodge Trident entirely. It remains to this day as the rendering engine powering Edge's compatibility mode. While it's not clear whether any of the original code can still be found in Edge (seems unlikely), a thirty-five year legacy is pretty good going.

Gecko

Internet Explorer's arch rival during the browser wars was Netscape Navigator, offered to consumers by countless dial-up Internet providers bundling it on free CDs alongside their own dial-up software and configurations. Netscape was the first browser to incorporate JavaScript support, which it did using the SpiderMonkey JavaScript interpreter in 1995.

Running up to 2000 Netscape completely re-wrote their browser engine. The result was what we now know as Gecko and which powers both Firefox and the Sailfish Browser. The purpose of the re-write was ostensibly to improve standards compliance and maintainability. But the highly abstracted code — arguably what has allowed the renderer to remain relevant to this day — resulted in poor performance. Netscape Navigator was a large programme, incorporating not just a browser but also a full email client and Website editor. In an attempt to improve performance in 2002 the components were split up to form Firefox as a stand-alone Web browser and Thunderbird as a stand-alone email client. My recollection is that this was controversial at the time and didn't improve performance a great deal. But the separation stuck. Splitting email from Web and dropping editing entirely seems to have resonated with users.

Gecko, in the form of Firefox, has experienced ups and downs. Browser statistics are notoriously subjective, but Statscounter registers Firefox market share as having dropped to just over 3% as of January 2024, having peaked in January 2010 at just over 30%.

There's plenty more to say about Gecko's history, not least in relation to its use as an embeddable component, but let's put that aside for today and I'll return to it in a future post.

Gecko remains relevant today as the most popular alternative to the WebKit/Blink family of browsers. While technically open source, both WebKit and Blink are directed by large corporations with few concessions to open source development methodologies. Mozilla on the other hand is a not-for-profit foundation that embraces the spirit of open source as well as the letter. For many, Gecko is an important bulwark against a corporate-controlled browser monoculture.

An interesting twist in Gecko's development comes from its adoption of the Rust language. Developed by Mozilla employee Graydon Hoare and officially adopted by Mozilla in 2009, Mozilla has been gradually moving Gecko's internal components from C++ to Rust.

This led to the development of the Servo engine, written wholly in Rust as a Mozilla research project. While never intended to replace Gecko, elements of the Servo engine were integrated back into the Gecko's WebRender rendering engine.

Servo is currently available as an engine with an intentionally bare-bones user interface. Mozilla divested itself of Servo in 2020, but development continues with the aim of specifying a WebView API during 2024 for use as an embeddable engine.

Presto

We're going to jump ahead a little in the diagram and turn our attention to the Opera browser. There are many unique and fascinating facets to Opera that it won't be possible to explore fully here, but it's still worth skimming the surface. Opera is unusual in that it was, for a long time, one of the few independent commercial browsers. When first released in 1995 it was shareware (requiring payment after a trial period). There was no JavaScript support (the language hadn't been invented yet) and at the outset the rendering engine wasn't named separately to the browser. This changed in 2000 with the introduction of the Elektra rendering engine and the Linear A JavaScript engine. In 2003 Opera switched to using what they claimed to be a new rendering engine, the internally developed Presto, alongside a new Linear B JavaScript engine. While the Presto name stuck, Opera's JavaScript engines have enjoyed periodic renaming: the Futhark JavaScript engine in 2008, followed by the Carakan JavaScript engine in 2010. Since the browser and all of these engines are closed source it's impossible to know to what extent they were really new technology as compared to an evolution of existing code.

Through much of its life Opera forged its own path. It was the first mainstream browser to introduce tabs. It integrated a (very good) email client long after Mozilla had disentangled Firefox and Thunderbird from Netscape Navigator. It even integrated its own Web server at one point. Opera also made a point of sticking to W3C standards while other browsers were still trying to lock users in to a proprietary Web.

Perhaps it's for this reason that there was much disappointment when Opera switched to using Blink and V8 in 2013, soon after Google and announced it would fork WebKit. To find out how it got to this point we'll need to go back a bit again and look at the evolution of WebKit.

WebKit and Blink

At this point in time WebKit is the most popular engine for accessing the Web (in either its WebKit or Blink variants). Moreover it's also the go-to browser engine for use in embedded scenarios as we'll see shortly.

Initially part of the KDE project, WebKit provided the engine for Konquerer, the default browser for the KDE desktop environment. At that point the engine was referred to by the name KHTML, alongside the KJS JavaScript engine. It was picked up by Apple in 2001, apparently because of its small code footprint. Apple renamed KHTML and KJS to WebCore and JavaScriptCore respectively with the WebKit project encompassing both.

Contributions to WebKit came from both Apple and the KDE project, as well as from the Qt Project which offered the QtWebKit embeddable widget. Sailfish OS supported use of QtWebKit up until its deprecation in Sailfish OS 4.4 and removal in 4.5, the functionality being replaced by the Gecko WebView API.

In 2008 Google introduced its own Chrome browser also built on WebKit but using the new and Google-developed V8 JavaScript engine. Google's advertising for it emphasise speed (start times and JavaScript execution in particular). Chrome also had an — at the time — unusual sandboxing model with each tab executed as a separate process. This meant that crashes triggered by WebKit or V8 would only bring down a single tab, leaving other tabs and the browser intact.

Although built using many open source components, Chrome itself is made available under a proprietary licence. The Chromium project, also developed by Google, is a fully open source implementation of Chrome, but with the proprietary components removed.

From the outset Google had to make changes to WebKit to support its use in Chrome. Still it took another five years before Google officially forked WebKit in 2013, creating the Blink browser engine. Consequently Chrome now uses both its own renderer and JavaScript combination: Blink and V8.

One of the attractive features of the Blink engine, also particularly relevant to Sailfish OS, is its embedding API which allows it to be used separately from Chrome (or Chromium) and embedded in independent applications. A common example of this usage can be found in the Electron framework, which uses Blink for rendering.

This embeddable design, which neatly separates the chrome from the engine, also makes Blink attractive for use by other browser developers. As noted earlier, Opera switched from Presto and Caraken to Blink and V8 for rendering and JavaScript respectively. Microsoft similarly chose Blink and V8 as the basis for its Edge browser in 2019.

Qt introduced the Qt WebEngine component, wrapping Blink and V8 to offer an embeddable browser, around the release of Qt 5.2 in 2013. This was intended to replace QtWebKit, which was ultimately removed in Qt 5.6. The closest KDE has to a default browser is Falkon, which uses the Qt WebEngine. This therefore completed a strange cycle, with KHTML having been started as part of KDE, forked by Apple, forked again by Google and then integrated back in to KDE via Qt.

LibWeb

An unexpected entrant into the browser space was recently announced in the form of Ladybird. To understand why Ladybird exists, it helps to understand a little about Serenity OS, the operating system project it grew out of and which it has now eclipsed. According to the FAQ of Serenity OS the developers try to "maximize hackability, accountability, and fun(!) by implementing everything ourselves.". And that includes the Web browser: the project developed its own renderer and JavaScript engine in the form of the imaginatively-named LibWWW and LibJS.

Recently the main Serenity OS developer, Andreas King, refocused his attention from the operating system to the Ladybird browser. Ladybird is built using the LibWeb and LibJS browser components of Serenity OS, but which he now develops independently. This arguably represents the first new engine to be introduced with the aim of being a fully-fledged browser for over twenty years, making for a particularly interesting development.

NetSurf

Last but not least we have NetSurf, which like Ladybird, is a bit of an outlier. Like Ladybird it was originally developed for exclusive use on a non-mainstream operating system.

The first version of NetSurf was released in 2002. At that time it was developed exclusively for use on RISC OS, the operating system that powered the Acorn Archimedes (the first publicly available computer to use an ARM processor).

RISC OS is very different from most other operating systems available today. It makes no attempt to be Unix-like and has its own distinctive and cooperatively multitasking desktop environment. This heritage means that the browser is incredibly lightweight, with good CSS support but without viable JavaScript.

During the early days of development JavaScript support was considered out-of-scope for the browser. The reason for this is interesting: it wasn't for lack of a usable JavaScript interpreter, but because the browser lacked a standards-compliant DOM. It turns out JavaScript isn't especially useful without a standards-compliant way to access the elements of a Web page.

Despite the lack of JavaScript support NetSurf still managed to find a niche as a fast and lightweight browser, growing beyond RISC OS. As of today there are downloadable packages available for RISC OS, GTK (Linux), Haiku, AmigoOS, Atari and experimentally for Windows.

The Truth About Browsers

Browser history is a tangled Web. While writing this it quickly became clear that, when it comes to browsers, any generalised claim is likely to turn out false. The date a browser came into existence? Do you mean the date the project was first thought of? The first commit? The first release? An alpha release? A beta release? Release 1.0? Is a particular engine entirely new, the redevelopment of an old engine, or just a rename? To what extent does the code of one engine flow into another when they both share libraries? Every browser out there is like the Ship of Theseus at this point. When we talk about a browser engine are we talking about the renderer, the layout engine, the JavaScript engine, the chrome? Sometimes these things can be separated, other times they're intrinsically tied together. Is it good to have a single reference engine that all browsers use for a consistent experience across the Web, or should we be championing diversity as way to prevent any single entity taking control? Do we even know how to calculate browser market share?

Even the question of what a browser is, presented in anything other than the most abstract terms, is likely to suffer exceptions.

What is clear is that browsers have become deeply integrated into our lives. Whether using a computer or smartphone, access to a browser has become a necessity. Over time they've continued to become more capable and more technically complex. Combined with their convoluted history, that makes them fascinating objects of study.

Now that I'm no longer writing a daily Gecko Dev Diary I've had the time to reflect on my experiences upgrading the Sailfish browser to Gecko ESR 91.9,1 over the last year. Check out my Gecko Retrospective to read about what I think went well, what went badly and where things might go in the future.

Way back last year in August 2023, before actually starting the process of upgrading the Gecko engine in Sailfish OS from ESR 78 to ESR 91, I wrote a preamble in which I set out my objectives and sketched a brief plan for how to achieve them. Although the work isn't entirely complete, after 339 days I consider the main bulk of my work on the project to be complete. We're now in the mopping up stage. That means it's a good time to look back at the process, find out what went well, what went badly, what I've learned from the experience and how I feel about things. If the preamble was the opening bracket, this retrospective can be considered its closing partner. Together they're the bookends encapsulating all the diary entries in between.

The Journey

Back when I started I hadn't quite appreciated how long this whole process was going to take. Although somewhere between half a year and a year seemed reasonable, the final 339 day tally is a little closer to the latter than I'd hoped. Moreover a year in theory feels much shorter than a year in practice. Adjusting for the fact I'm employed full-time to not do Gecko work, in practice I must have worked only around three hours a day during the week and twelve hours at the weekend. Two thirds of that time was spent coding and the other third writing up the diary entries. Given that the 339 days was made up of 244 weekdays and 48 weekends, I can be a bit more precise about how much time I actually spent on it.

$ time gecko-dev

real    48w 3d 0h 0m 0.000s
code    5w 1d 12h 0m 0.000s
diary   2w 4d 4h 0m 0.000s

Let's convert that into work time. This is interesting because practically speaking this is the "Full Time Equivalent" (FTE) or the amount of person-hours needed to complete the project from a commercial perspective. Typically the work would of course be distributed between multiple people to speed up project implementation, so the real time would be shorter.

$ time --work gecko-dev

real    67w 4d 0h 0m 0.000s
code    23w 1d 2h 0m 0.000s
diary   11w 3d 1h 0m 0.000s

Let's consider now how those days were partitioned into tasks. The following diagram shows the linear sequence of how I spent each day of work. This oversimplifies things a little given I didn't always complete tasks sequentially, but is pretty close to reality.
 
On day 149 I gave a presentation of this work at FOSDEM'24, including an earlier version of this diagram. I thought I was about half way through the work at that stage, but this turned out to be an underestimate, as is clear from this diagram. I was in fact only 11/25 of the way through.

The longest task of 87 days involved getting the WebView render pipeline working. In comparison getting the first successful build to complete took only 45 days. Both of these were quite dark and gloomy times. Without a working build it's impossible to debug or test the code, whilst without a working renderer nothing else can be effectively tested. Both periods felt like dark tunnels that took an age to emerge from.

Following these in terms of length of task were PDF Printing at 28 days, the WebGL renderer at 25 days and the Sec-Fetch-* headers at 15 days. These were the only tasks that took more than 14 days, which is a bit of a cut-off point for me. After two weeks of writing daily about the tasks it becomes really hard to write more without sounding (and seeming) a bit lost and exhausted.

It was particularly frustrating how long it took to get the WebView rendering working, given the browser already worked nicely. It could have been worse though: I had a clear plan which involved gradually stripping out and adjusting pieces of code to align with the code in the ESR 78 implementation. This gradual convergence towards ESR 78 meant I knew the task was inevitably going to be time-limited, also allowing me to identify progress on a daily basis. As it turned out I had to do it twice: first removing code, then adding code back in again. But it did eventually get there.

Realising on Day 254, after all this, that I'd broken the WebGL rendering process was also a bit of a low point. By then I really wanted to move on from the render pipeline.

But eventually I did emerge from all of these tunnels and the joy of getting something to work is a crucial counterbalance to the frustration when it isn't. In retrospect the low points were all worth it for the sake of the enjoyment I also got out of it.

Apart from how things turned out in practice it's also interesting to compare how closely it matched my initial expectations. Returning to the preamble once again, it's clear I was expecting a long haul, but I also had experience to draw from my previous involvement in browser upgrades at Jolla:
 
I think it's fair to say that I did follow this approach, starting with getting things to compile and then focusing on the details afterwards. This led on to the following decision about the structure of the work:
  Looking back I did broadly follow this structure. I got the build to complete, then I applied the patches needed to get rendering working and only after that did I apply the other patches. I did diverge from this advice in one important respect. Rather than applying all of the remaining patches I actually only applied a minimal set required to get the render working.

In hindsight I think that was the right thing to do. But it also felt like a natural consequence of the situation I found myself in. Given the upstream code changes the patches I did apply needed quite a lot of work to get them to stick. That gave me the impression that many of the existing patches might turn out to be redundant, superseded by changes in the upstream code.

By applying only the patches that were necessary it give me the opportunity to potentially avoid patches which were no longer relevant in a more intentional way. Hopefully the patches I've ended up with are closer to the minimum required and have a slightly cleaner structure than would otherwise have been the case.

But practically speaking I think my original plan was a good one and, in retrospect, I followed it pretty closely.

Destination Gecko

Let's now consider where the journey took us. The point of all this work was to take the browser engine from ESR 78 to ESR 91. What does this give us?

Abstractly speaking, one of the most compelling reasons to want to upgrade is because websites routinely attempt to fingerprint browsers and serve different content depending on the result. This practice is as old as the hills, yet remains as common today as it is problematic. I understand that different browsers have different capabilities and that website creators will be blamed (unfairly) if a page renders poorly as a result of a user failing to keep their browser up-to-date. But you'd have thought at the very least browsers could test for features rather than versions.

When browsing using ESR 78 it's not uncommon for a site to chastise its own customers. Updating the engine on Sailfish OS is one way to reduce the chance of seeing these invectives, even if just changing the user agent string is often just as effective as a browser upgrade without any of the effort.

One of the worst offenders is Cloudflare, which routinely blocks the Sailfish browser from accessing sites on its content delivery network. Upgrading to ESR 91 seems to circumvent this in at least some cases.

But browser upgrades also bring genuine improvements as well. New features, improved stability, increased security and bug fixes. There have been a total of 45 point releases between the previous Sailfish OS engine of 78.15.0 and the upgraded version at 91.9.1. Each of these point releases has brought improvements, although not all will be relevant to Sailfish OS. Major releases (e.g. from 78 to 79) will typically include new features, stability improvements and security fixes, whereas point releases (e.g. 91.1.0 to 91.2.0) will often only include security and regression fixes.

Working through the Firefox changelogs the following are some of the obvious improvements that have a direct impact on the Sailfish browser:

1. Certificate performance improvements (80.0.1).
2. WebGL rendering improvements (80.0.1).
3. Support for viewing more filetypes (81.0).
4. Improved element rendering (81.0.1, 86.0.1).
5. Improved PDF export (81.0.1, 85.0.1, 90.0.2).
6. Increased startup and rendering speeds (82.0).
7. Fixes for WebSocket message duplication (82.0.2).
8. SpiderMonkey JavaScript performance improvements (83.0).
9. An HTTPS-Only mode option (83.0).
10. Improved shared memory performance (84.0).
11. Increased cookie and supercookie isolation (85.0, 86.0, 89.0, 90.0, 91.0).
12. Deprecation of WebRTC DTLS 1.0 (86.0).
13. Private browsing compatibility improvements (87.0).
14. Increased referrer privacy (87.0, 88.0).
15. Working hyperlinks in PDF export (90.0).
16. Removal of FTP support (90.0).
17. Improved user-action response times (91.0).
18. Fixes for microsoft.com certificate errors (91.4.1).
19. Many crash bug fixes (81.0.1, 82.0.1, 85.0.1).

If you look through the changelogs you'll also notice several references to enabling WebRender for various platforms at various times. These are the main changes that haven't been introduced to the Sailfish OS version since it would require a much bigger change to the rendering pipeline and the consequences of making this change are unclear at this point.

In addition to the above changes, there were 15 critical, 115 high severity, 68 medium severity and 30 low severity security fixes combined into these updates. The importance of these can be best understand with reference to Mozilla's security classification:

• Critical: Vulnerability can be used to run attacker code and install software, requiring no user interaction beyond normal browsing.
• High: Vulnerability can be used to gather sensitive data from sites in other windows or inject data or code into those sites, requiring no more than normal browsing actions.
• Moderate: Vulnerabilities that would otherwise be High or Critical except they only work in uncommon non-default configurations or require the user to perform complicated and/or unlikely steps.
• Low: Minor security vulnerabilities such as Denial of Service attacks, minor data leaks, or spoofs. (Undetectable spoofs of SSL indicia would have "High" impact because those are generally used to steal sensitive data intended for other sites.)

These are all great changes to have. Probably the most important for daily use on a phone are the efficiency and performance improvements. Based on feedback on the Sailfish OS forum, users also seem to be happy with the results in this respect, with many users claiming the browser feels faster and more responsive.

Whether this is actually the case is hard to say. My tests using various performance measurement tools don't suggest significant performance improvements. But I must admit to having the same feeling of improved responsiveness. I suspect that may be due to the upstream changes in version 91.0 that claim to have improved responsiveness for user-interactions by 10-20%. That would make a noticeable improvement for users in a way that may not show up in benchmarks. It's my suspicion that the page loading feedback that's used to drive the progress bar on Sailfish OS has also been improved, although I've not found any explicit changes that would do this.

What do all of these changes mean for the state of the code? The upgrade from ESR 78 to ESR 91 also, surprisingly for me, brought with it a larger codebase. Mozilla has been intentionally transitioning code from C++ to Rust, with the number of lines of Rust code increasing by 14%. But the number of lines of C++ code also increased by 3% and for the total combined C++, JavaScript and Rust code this increased by 7%. Plotting the lines of code categorised by language, these increases are clearly visible.
 
Although proportionally there's been a bigger increase in Rust code than C++, in absolute terms the increase in both is almost identical (380607 lines of Rust code added compared to 384080 lines of C++ code).

In the above diagram Docs refers to content that relates to documentation. Build refers to scripts used to manage the build pipeline. IDL refers to interface definition files.

It's worth pausing to consider the code needed to build the Gecko engine. Gecko has experienced several changes through its life accumulating a mixture technologies as it goes. As a result the build system is a strange combination of Build (the mozilla build system), Python, Make, ninja, GN and Cargo. At certain points the build system compiles Rust into native binaries that then become part of the build pipeline itself. This causes havoc for the scratchbox2 cross-platform build engine Sailfish OS uses. No small part of the work in getting gecko working for Sailfish OS involves taming these build systems.

Although the numbers for IDL shown in the graph are low compared to the other languages, I nevertheless wanted to include it because it's such a critical part of the way Gecko works. The combination of C++/Rust and JavaScript means that there needs to be a really solid way to expose native methods to JavaScript and JavaScript methods to native code. The type systems aren't equivalent and so this requires a careful arrangement. Gecko supports this using its Interface Definition Language. IDL files read a bit like C++ header files but are more generic. Any interface defined using IDL can be exposed both natively and to the JavaScript layer. It's critical glue that holds everything together.

The numbers shown in the graph are measured in millions of lines of code. They're big numbers, but it's worth bearing in mind that Gecko is a relative minnow when it comes to code size in the world of browsers. For comparison I ran the same code analysis on the Chromium source. I was pretty surprised by how large Chromium is compared to Gecko.
 
Chromium contains over four times the code: 154 981 674 lines of code for Chromium compared to a paltry 37 361 820 lines of code for Gecko ESR 91. It's also interesting to compare the range of technologies involved in the two projects. Chromium introduces TypeScript, Go, Java, Objective-C, Lua, AppleScript, TCL and WASM, although some of these will be target-specific.

Destination Sailfish

So far we've considered the differences between ESR 78 and ESR 91 in some detail, but none of this has touched on the actual changes needed to get the code to run on Sailfish OS.

As any Sailfish OS developer will be aware, Sailfish OS uses RPMs for packaging software, a technology that originated on Red Hat Linux as the Red Hat Package Management system. Work started on RPM in 1995, a good ten years before the initial release of git and two years before Netscape started work on Gecko. Back then it was commonplace for software to be provided in the form of a tarball and in some ways the RPM build process reflects this. Distribution-specific changes are provided as patches applied directly to the upstream source. These patches are all listed in the spec file which is passed to the rpm tool to perform the build. On Sailfish OS this is all hidden behind sfdk which is itself a wrapper for the scratchbox2 sb2 tool. It's a complex layered system with multiple abstractions.

The point is that even now on Sailfish OS packages that use upstream code can pull directly from the upstream repositories, rather than having to use Sailfish-specific implementations. Any Sailfish OS specific changes can then be applied onto this code in the form of patches. It's not a process I enjoy working with because patching is a lot messier and less flexible than working with commits in a repository. Even though it's possible to convert a patch list into a series of commits and back again this adds an extra step and contrains what actions can be performed at different times.

The benefit is that we always retain a very clean and clear distinction between the upstream code and the Sailfish OS specific changes, with the latter being encapsulated in the patches to be applied. We can use this separation to discover how the changes needed to get Gecko ESR 78 to work with Sailfish OS differ compared to those needed for ESR 91.
 
This figure shows only a very high-level view, but nevertheless tells a story. Note that unlike the previous figures the y-axis of this chart uses a logarithmic scale to account for the big differences in scale between different languages. This can make the values harder to read so, for clarity, here they are in tabular form.
 

Language	ESR 78 added	ESR 78 removed	ESR 91 added	ESR 91 removed
C++	22 726	606	22 476	631
Docs	510	2	508	12
Build	28 558	20 350	19 320	11 090
JavaScript	158	6	170	43
Rust	544	175	498	180
IDL	29	3	39	6

These numbers represent the actual code I've been working on for the last year. In general the number of lines added or removed has reduced as we've moved from ESR 78 to ESR 91. This is a good thing. The fewer changes made to the upstream code the better. In general the difference isn't huge, but it does exist. The total number of patches reduced from 98 to 84. The number of lines added to ESR 91 was 82% of the number of lines added to ESR 78. The number of lines removed from ESR 91 was only 57% of the number removed from ESR 91.

Interestingly, while there were fewer change made, the differences practically balance themselves out. Overall the patches to ESR 78 increased the code size by 31 383 lines compared to 31 049 lines for ESR 91. That's astonishingly similar.

These numbers don't quite capture all of the changes because they relate only to the gecko code. There were also changes needed in the other four components that make up the Sailfish browser stack, as well as to the EmbedLIte code (which is handled separately from gecko but ends up in the same xulrunner package). Let's briefly take a look at these other components.
 
The gecko renderer is by far the largest of the components. The qtmozembed component provides a QT wrapper around the renderer. The embedlite-components package adds the privileged JavaScript shims needed for Sailfish OS, largely replacing equivalent privileged JavaScript that would typically run in Firefox. The sailfish-components-webview component provides Qt components needed in order to support both the browser and WebView (for example the pop-up dialogues), but also provides the code needed to offer the rendering engine as a WebView component to other Qt apps. Finally the sailfish-browser component is the actual browser app you run when you open the browser on your phone.

Apart from the gecko renderer all of these are Sailfish-specific packages, so they don't have any "upstream" code. The Jolla repositories are the upstream repositories for these. Consequently there's no need to apply patches and we can work on the code directly. That means that when analysing changes for these we're just using the commits that take the code from ESR 78 versions to ESR 91 versions. Between them they accumulated 169 commits with the following additions and removals (these numbers also including the changes to the gecko source):
 

Language	Lines added	Lines removed
C++	23 456	1 281
Docs	508	12
Build	19 724	11 381
JavaScript	452	114
Rust	498	180
IDL	52	17
QML	14	14
Total	44 704	12 999

This table essentially captures the sum total of the changes needed to move from one version to the next. As you can see, the majority of the additions have been to C++ code. The build scripts saw rather a lot of churn. I'm very surprised to see more Rust additions than JavaScript additions. The QML code changed very little, which is perhaps to be expected given the external appearance, renderer aside, is almost identical. That was intentional: there's always scope to improve the Sailfish browser user interface, but my objective with this work was to get the renderer upgraded as quickly as possible. Changing the interface would have been a diversion.

Mental Health

I put a lot of myself into the Gecko upgrade. Working on it practically every day for a year, even if not full-time, required a level of commitment that I wouldn't typically give outside of my work hours. This is a personal perspective: the world is blessed with many people who commit far more for far less reward and who don't then feel the need to tell the world about it in a blog post.

Nevertheless, this was a big deal for me. I'm not a natural blogger so the prospect of writing about my coding on a daily basis was daunting at the outset. But it turned out to be surprisingly easy. Writing about specific tasks is very different from having to come up with inventive and interesting topics to write about on a daily basis.

Having to write daily diary entries undoubtedly helped keep me on track and working on the project every day. The need to have at least a few paragraphs to write about drove me to do the coding work.

There were a few occasions when I struggled with this. Typically on a Friday night after having spent two and a half hours on public transport returning from work. Having to then write up a diary entry in a tired state of semi-consciousness was not always ideal. But these cases were relatively rare.

There were also occasions — mostly in the middle of the work to get the various rendering pipelines working — when the work really got me down. Writing the diary entries made me very conscious of the progress I was — or in many cases wasn't — making. In the middle of the trough when it's really not clear whether it will be possible to come up with a solution, some of those occasions felt quite dark. If I hadn't been writing the diary I can imagine myself choosing to take a break and then having that break go on for several days.

But, and this is a big but, I was supported the entire way through by the amazing Sailfish community who responded to my posts on Mastodon and the forum, always encouraging and supportive. I'm not a social person and this was a bit of a shock for me. People out there in the Sailfish community and beyond really are the most encouraging and thoughtful people you could hope to interact with.

The amazing images and poetry from the likes of Thigg (thigg) and Leif-Jöran Olsson (ljo) are beautiful cases in point.
 
But there are so many people who helped and contributed in so many ways, I couldn't possibly mention everyone here. I apologise for not mentioning you all individually, but I'm really grateful.

Besides the community I also have to mention Joanna, my wife, who's sacrificed more than anyone else for the sake of me spending three hours each day and most of my weekends on gecko development. She carried me through this.

With all of this support, I found the experience surprisingly effortless. Perhaps the biggest challenge, as it turns out, was being able to find a suitable point to wind things down. Dropping off from posting diary entries every day and having a very clear purpose for my free time has been hard to manage in a measured way. It was too much of a cliff edge and, if I do this again, I think I'd want to look into ways to mitigate this. But I don't yet have a good solution: writing these diary entries doesn't lend itself to a tapered reduction of work.

Future Work

Future work for this project comes in two forms. There's the future work needed to achieve the (hopefully) near-term goal of getting the browser released to users as part of Sailfish OS. Then there's the longer term goal of what to do beyond that.

As I write this the current situation is that three out of five pull requests have been merged into Jolla's repositories. The remaining three have been through a couple of review rounds already. So the immediate task is to get them through review and merged in. This alone won't result in their release as part of Sailfish OS as they're currently being merged into bespoke ESR 91 branches. Jolla will need to merge these into the main branch before they can become part of any official Sailfish OS release.

It's nevertheless exciting to see that as part of the recent upgrade from Sailfish OS 4.6.0.13 to 4.6.0.15, several changes to libhybris were included that will support the move to ESR 91. As readers of my diary entries will know, there were several issues that caused the browser to crash or hang which were ultimately traced back to libhybris and which, looking at the changelog, will now be fixed. If ESR 91 does go out in some future Sailfish OS release, this will make the transition much smoother.

At present I've been building exclusively for aarch64. The build will need to be tested and potentially amended for armv7hl and i486 targets. On top of this, it appears that getting the browser to work on native platforms such as the emulator and the PinePhone, where there is no libhybris layer, will also require some additional work.

In the longer term, there are two, maybe three, objectives. The obvious next step after the release of ESR 91 would be to move to the next ESR release, which is 102.15.1. Checking the various release notes we can see that ESR 91.9.1 was released on 20 May 2022, whereas ESR 102.15.1 was released on 12 September 2023. That's a gap of around 16 months. So far the upgrade from ESR 78 has taken 13 months, so it looks like we may have an opportunity to catch up with Firefox ESR latest. In practice though it's usually around 12 months between ESR releases so some acceleration will be needed if we're to properly keep up. It's worth noting that the extended service releases have a much longer support cycle than other releases, which can lead to some overlap. For example both ESR 115.15.0 and ESR 128.2.0 were released on 3 September 2024.

Besides the obvious upgrade to the renderer engine it would also be great to add features to the browser. On the Sailfish OS Forum Niels (fingus) suggested supporting MPRIS for the video and audio controls of the browser. That's the sort of thing I'd love to add, but which would require some research and effort to investigate and implement. I'd also love to introduce support for reader mode, scrollbars and maybe even extensions. There's no shortage of interesting ideas for things to work on.

The third objective would be to properly support the WebRender compositor on Sailfish OS. It's not clear how much work this would involve, but it's potentially substantial. Integrating this with the Sailfish OS render pipeline could be quite a challenge.

Finally there's plenty of scope to make important improvements to the browser build process. Updating Rust, fixing the multi-process hang — which remains a significant barrier to reducing build times — and introducing a build cache would all help to make development easier.

Lessons Learned

The main outcome of this work for me has been the reaffirmation that the browser is a critical component of Sailfish OS. The better the browser the more usable Sailfish OS becomes as a daily driver. Make no mistake, the reason I wanted to do this work was for entirely selfish reasons: Sailfish OS is my mobile phone operating system of choice. I enjoy using it and I want it to remain relevant so that it continues to be supported. Upgrading the browser is my way of helping ensure this happens; it's my itch and I've been scratching it.

But I've learnt a whole lot more than this and not just from the process of development, but also from the experience of writing a daily diary about it. I'd like to think that the work has helped demonstrate the importance and benefit of open source, for users of course, but also for Jolla. Jolla invested heavily in ensuring the browser is open source. Not just in giving the code the right licence and making the source available, but also in documenting it, following an open development model and supporting the community in making it accessible. In no way was this a "free" browser upgrade for Jolla, but I hope it goes some small way to justifying this open source strategy. I'd also like to think the diary entries have demonstrated some of the benefits of being open about progress as well.

I've also learnt more than I'd like to admit about Brownian debugging. This is the process of performing a random walk, changing bits of the code en route, until it works. It may not be the most efficient debugging approach and it may be that an element of strategic direction improves matters, but as long as the problem space can be constrained I've found Brownian debugging can be unexpectedly effective. Given enough time and patience.

There's a follow-up to this, which is that it also demonstrates how much can be achieved without the benefit of understanding or insight, but relying on perseverance alone. I'm definitely more familiar with the gecko code than when I started, but the gaps in my knowledge remain prodigious. Armed only with my abilities in Brownian debugging and enough time to deploy them, I managed to make some progress.

I admit this wasn't my first involvement in upgrading the browser. While working at Jolla I contributed to the upgrade from ESR 60 to ESR 68, and then again from ESR 68 to ESR 78. But that was as part of a team with an incredible depth of knowledge of the browser and impressive software development skills. When I started this process I wasn't at all certain whether I'd be able to make any meaningful contribution to the next upgrade. I'm now much more confident that not only has this been possible, but that I'd be able to do it again.

It's been great to feel some purpose within the Sailfish OS community. I really enjoyed working for Jolla, not least because it felt worthwhile contributing to an operating system I love using, but also contributing to the community I felt a part of. Doing this work has served as a great way to continue feeling like I have something to contribute.

Writing the development diaries was, I hope, helpful in demonstrating that work was continuing on the browser: it hadn't been forgotten or left to decay. It gave me a lot more visibility than I would have got otherwise. Crucially though it made me realise that there are many, many, Sailfish OS developers putting in similar or greater levels of commitment, for ports and apps and bug checking, who may not have the same visibility because they're not writing a diary, but who nevertheless put in more work and deserve the same appreciation that I've felt privileged to have received from the community.

It's been a week since I last posted a diary entry. It turns out that not writing a diary entry everyday is almost as hard as writing one. If I'm not wandering aimlessly around the house trying to find a purpose, I find I'm sitting on the sofa quietly mumbling gdb backtrace nonsense to myself.

Thankfully there are still some tasks to do, addressing review comments and reviewing others' commits. I'll talk about these happenings today. But I also want to share some retrospective analysis of the project: what went well, what went badly and how the whole process felt as an experience. I'll be expanding on all that in a future post.

First up I want to highlight the really nice fix from from Vlad G (vlagged) that's fixed certificate overrides. When the browser accesses a site using HTTPS but receives a bad certificate, it'll present the user with an error page. This is to avoid the user mistakenly believing that the data being sent from the site, or that they're sending back to it, is being done so securely (i.e. using a channel that maintains integrity and confidentiality).

There are still some sites that don't support HTTPS which provide no more security than a site with a broken or bad certificate. But the latter is worse in that could give the user a false sense of security. If there's a failure it could be an attack. Exposing this to the user in an overt way is critical for them to make an informed decision about how to proceed.

That means that Firefox typically offers the option to view the site anyway, as long as the warning page has been prominently presented. It's hard to imagine anyone could miss the bad certificate page given the way it's presented on Sailfish OS.

Changes to the underlying API however mean that the ESR 91 upgrade broke the option to bypass the certificate check. With this broken code, selecting the option to "view the page anyway" caused the browser to return the same certificate warning page rather than actually going to the site. That's because the button is supposed to record a "certificate override" stating that the browser should ignore the bad certificate. But the method to perform this has changed parameters, so it was throwing up an error rather than recording the site details.

Here we can see the blame for the interface file where the new parameter was added by R. Martinho Fernandes:

$ git blame -c --date=short \
    security/manager/ssl/nsICertOverrideService.idl -L 84,113
[...]
345598abc1d6f (Masatoshi Kimura 2019-06-11)  void rememberValidityOverride(
                                                     in AUTF8String aHostName,
e368dc9c853cf (Ehsan Akhgari  2012-08-22)            in int32_t aPort,
7ea1bcd018a32 (R. Martinho Fernandes 2021-06-01)     in jsval aOriginAttributes,
61d16d4cd9588 (kaie@kuix.de   2007-10-03)            in nsIX509Cert aCert,
e368dc9c853cf (Ehsan Akhgari  2012-08-22)            in uint32_t aOverrideBits,
f2e2a270fb9a4 (kaie@kuix.de   2007-11-19)            in boolean aTemporary);

If we check out the commit 7ea1bcd018a32 that introduced the change, we can see it takes us to differential revision D91962:

$ git log -1 7ea1bcd018a32
commit 7ea1bcd018a32a8e98216cf9f675c49d582950ca
Author: R. Martinho Fernandes <bugs@rmf.io>
Date:   Tue Jun 1 06:55:07 2021 +0000

    Bug 1597600 - make certificate overrides depend on origin attributes 
    r=keeler,geckoview-reviewers,smaug,agi
    
    Differential Revision: https://phabricator.services.mozilla.com/D91962

This was noticed by Vlad who dug down into the code and found the source of the error, fixing it by adding the missing parameter. It's great detective work and a nice fix.

As I write this I've approved the change myself and hopefully it'll be picked up and reviewed by Jolla so that it can be merged swiftly in to the ESR 91 branch.

Besides this I also received really nice review feedback from sailors Raine (rainemak) and Mal (mlehtima) in relation to my gecko-dev pull request. This pull request is the one that actually performs the upgrade from ESR 78 to ESR 91. The upgrade takes the form of redirecting a submodule to a different upstream commit so is a super-simple change. The bigger part is the set of patches that then has to be applied on top of the upstream code to make it all work. This is all in the pull request as well.

Raine and Mal clearly spent some time going through the commits and I'm really grateful for their suggestions. The result was better preference handling and several commits being merged or removed entirely. On Mal's request I also rebased the code to bring in recent changes needed to support Jolla's updated SDK. The totality of these changes all make for a much cleaner commit history and impoved code.

This pull request, along with the pull requests for the sailfish-browser and sailfish-components-webview repositories, are now awaiting a second review from the Jolla team. As far as I'm aware I've completed the changes requested, but there may still be a few more review rounds to go before they get merged in.

The other change this week has been to the package tarballs available from my website. These are very much still testing packages and you should read all of the info in the installation instructions before you use them. Nevertheless these new packages should provide some nice improvements over the previous ones. The main changes are:

1. The user agent patch has now been applied, so problems with pages serving desktop rather than mobile variants should now be largely resolved.
2. Vlad's HTTPS fix is included so you can now view pages with broken TLS certificates if you so choose.

If you're still on ESR 78 you can just follow the installation instructions from before. If you already have the previous ESR 91 packages installed you can follow the same instructions but should skip steps 4 (copying the browser profile) and 7 (creating a symlink to the ESR 78 library directory).

You should, however, still do step 5 to clean out the old settings files. This won't clear cookies, tabs, history or anything like that, but it will reset any preferences you've changed using about:config. In summary, if you already have ESR 91 installed you should complete the following steps:

1. Download the tarball to your phone.

   $ curl -O https://www.flypig.co.uk/dnload/dnload/sailfishos/gecko/gecko-dev-esr91-release.tar.bz2
   

2. Unpack the tarball:

   $ tar -xvf gecko-dev-esr91-release.tar.bz2
   

3. Ensure the browser and any apps using the WebView are closed.
4. [Skip this step]
5. Remove settings that need to be updated for ESR 91:

   $ rm ~/.local/share/org.sailfishos/browser/.mozilla/ua-update.json
   $ rm -rf ~/.local/share/org.sailfishos/browser/.mozilla/startupCache
   $ rm ~/.local/share/org.sailfishos/browser/.mozilla/prefs.js
   $ rm ~/.local/share/org.sailfishos/browser/__PREFS_WRITTEN__
   

6. Install the ESR 91 packages:

   $ devel-su rpm -U --force \
       xulrunner-qt5-91.*.rpm \
       xulrunner-qt5-misc-91.*.rpm \
       qtmozembed-qt5-1.*.rpm \
       sailfish-components-webview-qt5-1.*.rpm \
       sailfish-components-webview-qt5-pickers-1.*.rpm \
       sailfish-components-webview-qt5-popups-1.*.rpm \
       embedlite-components-qt5-1.*.rpm \
       sailfish-browser-2.*.rpm \
       sailfish-browser-settings-2.*.rpm \
       mapplauncherd-booster-browser-0.*.rpm
   

7. [Skip this step]
8. Finally kill the browser booster:

   $ killall booster-browser
   

9. You're good to go!

As always, if you run in to trouble, drop me an email, create an issue on GitHub, or post a message in the Sailfish OS forum.

That summarises the changes this week. I'm still writing up my retrospective of the project so far and will post it once it's ready.

If you'd like to read any of my other gecko diary entries, they're all available on my Gecko-dev Diary page.