XULRunner is a project that has been around since the inception of Firefox, and predates Adobe AIR by quiet a margin. For those who have not really looked into it too much, XULRunner has a lot of similarities to Adobe AIR – it is a cross platform runtime that allows you to run code written in XUL – an XML derivative very similar to HTML.
A little more information: XUL elements add certain features that are required to add things like menus, menu items, toolbars, toolbar buttons and the like into your application, and can be mixed and matched with HTML. XULRunner also exposes extra functions to JavaScript, allowing you to access files on the host computer, read and write from SQLite databases, and do a lot of things that AIR does in this regard.
I haven’t played with XUL a whole lot since AIR came out, as documentation has always fallen behind AIR’s. And performing some advanced functions, like populating a tree component with SQLite results, or manipulating an image, are a little more involved than with AIR.
However, XULRunner has been really branching out to a lot of platform – Windows, Mac OS and Linux support was all there before AIR even came on the scene. There is also SkyOS support, and today the first “pre-alpha” of XULRunner is available for Windows Mobile (codenamed “Fennec”). There are also lightweight ports available for mobile Linux distros, and for Symbian S60.
This really opens a lot of doors for developers – your desktop applications and mobile applications can all share the same codebase. XUL itself is actually quiet nice to write in, and I was always impressed by how it always took advantage of native OS components libraries.
Anyway, I think the XULRunner team deserves very high praise for what they have accomplished – they have always been ahead of the game when with their platform independent runtime, and they are continuing to push support to an impressive array of platforms.