XUL Tutorial
A guided tutorial that will help you get started with XUL, originally from XULPlanet.
XUL (XML User Interface Language) is Mozilla's XML-based language for building user interfaces of applications like Firefox. The term XUL is sometimes used to refer to the whole Mozilla platform (e.g. XUL applications are applications using XUL and other components of the platform).
XUL Controls lists some of the common controls provided by XUL.
This collection of XUL demos used to be available as a web page, but can no longer be viewed in Firefox since support for Remote XUL was disabled. There is a XULRunner application containing the XUL Periodic Table which can be opened with Gecko based browsers. You can get it here. See: XULRunner_tips#Using_Firefox_3_to_run_XULRunner_applications for instructions on running XULRunner apps in Firefox.