Some best-of-breed FireFox extentions for web developers: 

The Mozilla Firefox browser is probably the greatest webbrowser in the world on it’s own. But ever since it’s extension feature matured around the release of Firefox 1.0 it has been an absolute killer application! For web developers it has become the ultimate tool to inspect, debug and test and for users it has become the greatest platform for making a browser that suit their specific needs.

Read the list: Extending Firefox for Web Developers