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 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