What is XUL?
"The XML User Interface Language (XUL) is a markup language for creating rich dynamic user interfaces. It is a part of the Mozilla browser and related applications and is available as part of Gecko. It is designed to be portable and is available on all versions of Windows, Macintosh as well as Linux and other Unix flavours. With XUL and other Gecko components, you can create sophisticated applications without special tools.
XUL was designed for creating the user interface of the Mozilla application including the web browser, mail client and page editor. XUL may be used to create these types of applications. However, it may also be used any place you would currently use a web application, for instance, when you need to be able to retrieve resources from the network and require a richer user interface. Like HTML, in XUL you can create an interface using a markup language, use CSS style sheets to define appearance and use JavaScript for behavior. You also have access to programming interfaces for reading and writing to remote content over the network and for calling web services. Unlike HTML however, XUL provides a rich set of user interface widgets for creating menus, toolbars, tabbed panels, and hierarchical trees to give a few examples."
See also: MozillaAsPlatform
XUL tutorials and useful developer links
XUL Trees - XUL provides a way to create tabular or hierarchical lists using a tree. We're planning on replacing the current JavaScript sidebar with a XUL sidebar. If you're running Firefox or Mozilla you can go to View > Sidebar > bookmarks to get an idea of what we're talking about. This will replace all the functionality of the current sidebar and create a more 'traditional' tree view of the Outline pane. We can also implement drag-and-drop for moving about nodes rather than the current move-up, move-down, promote, demote functions.
RDF Datasources - a look at additional datasources and how to use your own RDF files as datasources.
Examples of every XUL element - I'm finding this resource to be pretty indespensible; you can even download the entire thing so it can be a local reference. brent.
XUL and Python
My Chrome Oven: Generating XUL with Python
Mozilla 2.0 platform must-haves - interesting to note how much discussion there is on this blog post about adding Python support into Mozilla platform. Perhaps we're really ahead of the game on this one and that XUL / Python is the preferred combination over the current XUL / JavaScript.