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/