XUL Reference
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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.
Mozilla XUL and Script Reference - A easy-to-read reference to XUL programming.
XPToolkit - Background, design documents, manuals and references for the Mozilla cross-platform user interface toolkit. Help build the largest human-edited directory on the web.