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
A BetterExplained article that discusses how to reduce the initial load time of JavaScript files. Covers measuring load times, and a number of approaches to delaying the loading of a JavaScript file. Also covers minifying JavaScript and maintaining a debug version of code. Also covers an AJAX onLoad pattern to dynamically include JavaScript after page load, and HTTP caching of external JavaScript files
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.