O'Reilly Network articles about this topic:
Remote Application Development with Mozilla
This article explores the uses for remote XUL (loaded from a Web server), contrasts its capabilities with those of local XUL (installed on a user's computer), explains how to deploy remote XUL, and gives examples of existing applications.
XUL Tools and What They Mean
Don't get your widgets and your skins confused. Ian Oeschger expalains the various XUL components and what they do.
Other documents about this topic:
Below are other references available on the web for this topic. Since other sites may change their links, please if you find any that may need to be updated.
Fooling with XUL
XML.com managing editor Edd Dumbill takes a hack at using XUL to alter the sidebar in the Mozilla browser. [Source: XML.com]
The XUL Programmer's Reference Manual
An online reference manual for working in XUL, the XML-based User Interface Language. [Source: mozilla.org]
Neil Deakin's very valuable and well-written introduction to the eXtensible User-interface Language.