A cross-browser bookmarklet to examine the box-model of any DOM element on a page. Runs on Internet Explorer, Safari (and Webkit based browsers), Mozilla (including Firefox and Camino).
A Firefox extension from Chris Pederick which adds in a toolbar of web development goodies, from disabling JavaScript, CSS and images through to manipulating cookie information, speed reports and validator tools and error consoles.
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
Dean Edwards takes Mozilla's newly introduced forEach method, and implements it for non-Mozilla browsers, for enumerating over objects and arrays.
Douglas Crockford's presentation on Advanced JavaScript. He covers topics such as inheritance, modules, debugging, efficiency and JSON.
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
Dean Edwards counters Dojo's Alex Russell's hideous hacks with two standards friendly approaches. Looking at the The DOM Content loaded problem and the sluggishness of walking a DOM Tree, Dean talks about speed improvements, including the use of XPaths.