Some time ago, I wrote a post in this blog about the essential Firefox extensions for developers. After some time, that post is out of date, so I changed some of those extensions, and started using some new. Here is the updated list:
Firebug: Javascript debugging, graphical check of CSS properties (moving the cursor to an area of the web, you can see the html, css properties...), loading time statistics per file, and a infinite set of option.
Web developer: Disabling CSS, Javascript, Resizing browser window to standard formats...
HTML Validator: Offline version fo the w3c validator.
SearchStatus: PageRank, and Alexa rank information.