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
Douglas Crockford's presentation on Advanced JavaScript. He covers topics such as inheritance, modules, debugging, efficiency and JSON.
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.