Angus Croll covers the new 'use strict' operator introduces in EcmaScript 5th edition and how to get started using it. He offers a set of unit test to see how well supported strict mode is in your browser.
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).
Exploring the features of Firebug, with code examples covering using the console, timing code, stack traces, debugging and breakpoints, profiling, tracing HTTP requests. An excellent overview of Firebug's functionality
Svend Tofte puts together a detailed guide to setting up and using the Venkman debugger. From basic debugging to watchlists and call stacks, breakpoints, and more advanced techniques like meta comments and profiling. It also contains a list of other useful Venkman resources.
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
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.
Mark Pilgrim dissects the security issues of the 0.3 Greasemonkey, and describes how the new architecture of Greasemonkey works, pointing out pitfalls and how to address them.