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.
Mark Pilgrim's essential online book about developing Greasemonkey scripts. Excellent starting point for Greasemonkey developers. Covers everything from setting up a Greasemonkey scripts to common DOM coding idioms, as well as a brief rundown of XPath, and case studies of Greasemonkey scripts.