XULRunner

~ the goodbye ~

I met XULRunner without knowing it, which was the normal way to meet it. If you installed Songbird in 2007 because you wanted a music player that looked like Firefox, XULRunner was what you were actually installing. It was infrastructure with no face on it, and the entire design goal was that you would never notice.

The idea came out of the Mozilla Application Suite era. Gecko rendered the pages and XUL described the interface in XML. Put the two behind XPCOM and there was no technical reason the result had to be a browser, so Mozilla shipped the observation as a product: the platform with the browser stripped off the top and the empty space handed to somebody else.

The first stable developer preview arrived in February 2006, built on Mozilla 1.8. Configuration lived in SQLite databases, and an application was just a directory with a manifest telling the runtime where to start. For a while it looked like the thing a lot of desktop software was going to be built on.

~ the chronicle ~

~ the apps that shipped on it ~

The list of things that ran on XULRunner is stranger than you would guess. Songbird and its Nightingale fork were on it, as were the SeaMonkey suite and Instantbird, which you would expect from anything in Mozilla's own orbit.

Outside that orbit it picked up Google's AdWords Editor until 2010, IBM's Lotus Notes from version 8.5.1 onward, the Kiwix offline Wikipedia reader until 2017, and a 2007 adventure game called Simon the Sorcerer 4: Chaos Happens, which shipped an entire browser engine so it could draw its own menus.

That list is the argument for why XULRunner existed and also the argument for why nobody defended it. A music player, a mail client from IBM, an ad-buying tool, an offline encyclopedia and one adventure game. No two of those companies had any reason to notice each other, let alone to organize when Mozilla stopped shipping the layer underneath all of them.

~ the slow retirement ~

Mozilla stopped supporting XULRunner development in July 2015. Automated builds ended after the 41.0 cycle, the final release went out as 41.0.2 on 15 October 2015, and through early 2016 the Linux distributions quietly removed the package from their archives.

I was writing code professionally by 2015 and did not register any of this happening, which is the correct response to a runtime being retired. Nothing broke on the day. The applications still sitting on it kept running off the copy they had already bundled, and Firefox and Thunderbird kept accepting the -app argument, so a XULRunner application can still be launched by a browser that no longer admits to being one.

~ the electron question ~

XULRunner's premise was that a desktop application could be written in web technology and shipped on a browser engine. In 2013 Electron shipped that same premise on Chromium and it became the default way to build desktop software, to the point where people now complain about it as a category. Mozilla had been there seven years earlier with a worse developer story and lost the argument to its own retirement schedule.

In 2016 Mozilla started Positron, an attempt to build an Electron-compatible runtime on Gecko, essentially XULRunner with the Electron API bolted where XUL used to be. It was abandoned inside the year. The rest of the XUL story ended in 2017, when the extension system that had been built on it was replaced wholesale by WebExtensions.

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