Reference

Mozilla Developer Center: XUL Reference
Describes the values and attributes that the Mozilla XPToolkit widgets support, as well as the genealogy and structure of valid XUL documents.
http://developer.mozilla.org/en/docs/XUL_Reference

Mozilla XUL and Script Reference
A easy-to-read reference to XUL programming.
http://www.xulplanet.com/

XPToolkit
Background, design documents, manuals and references for the Mozilla cross-platform user interface toolkit.
http://www.mozilla.org/xpfe/