An online copy of O'Reilly's 2002 book Creating Applications with Mozilla. It focused on pre-1.0 versions of Mozilla, so details about the application structure has changed, but the XUL elements are relatively stable.
JavaScript documentation surrounding version 1.5, 1.6 (supported by Firefox 1.5 and Mozilla 1.8), and version 1.7 (supported by Firefox 2.0). Backed up by mailing lists, newsgroups and an IRC channel
Building extensions for Firefox 1.5. The Firefox and Mozilla platforms keep changing the extensions enough to out-date tutorials. This is the official documentation, covering the directory structure, the manifest, XUL Overlays, chrome URIs and packaging
A collection of gotchas and guidance in Greasemonkey scripting. Positioned as an addition to Mark Pilgrim's dive into Greasemonkey