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.
A Firefox extension to detect memory leaks caused by JavaScript. It pops up an alert dialogue when windows close if there is code still pointing to JavaScript objects.
Joe Hewitt, in this Dr Dobbs article talks about using the Firebug extensions to inspect and debug Ajax applications (or any browser-based JavaScript application). It also has CSS inspecting capabilities, editing JavaScript on the fly, inspecting HTTP request and responses, logging, breakpoint debugging and profiling. A useful howto.
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.
Apart from being an elegant debugger, FireBug offers the developer hooks that can be used from JavaScript, offering logging, assertions, performance measuring, and command line functions for inspection or traversing a document.
A collection of gotchas and guidance in Greasemonkey scripting. Positioned as an addition to Mark Pilgrim's dive into Greasemonkey
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.
All of the tools you need to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax are brought together into one seamless experience, including a debugger, error console, command line, and a variety of fun inspectors.