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.
Christian Heilmann's excellent guide to writing unobtrusive JavaScript. Takes the reader from the inline JavaScript world right through to a clean dynamically attached events with all the JavaScript code in external files. Covers DOM Scripting. This is the key reference to unobtrusive scripting.