
Adblock Plus
Ever been annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page? Install Adblock Plus now and get rid of them. - Yeah, this one isn't really related to web development, but it's probably the coolest Firefox extension around, so it deserves mention...

Web Developer
This extension provides a number of tools, from quick links to validation services to ability to edit CSS on the fly, from clearing the cache or form variables to outlining block level elements on a page... Dozens of nearly indispensible tools for any web developer.