Here are my favourite plugins for mine and every other decent web developers' favourite browser - Firefox :) All are essential in my day to day web development and I'd cry if you took them away...
A Firefox addon that integrates with firebug (so you'll need to install that first). Great for analysing web site performance and suggesting performance improvements.
Adds a menu and toolbar to Firefox with lots of handy web developer tools, e.g. a link validator, a html validator, a 'small screen rendering' option for simulating mobile browsing and a bunch of other things too.
A web page analyser for reviewing your search engine optimisation criteria. Handy in its own right but very useful to the non SEO educated web developers out there.
A useful tool for easily and quickly viewing pages rendered as IE would. unfortunately this won't work for the very latest firefox release but hopefully they'll update the extension soon.