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.