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.
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