There are two add-ons for FF that every web developer needs and they are Web Developer Toolbar and Firebug, the Firebug extension YSlow comes in very handy too.
Web Developer Toolbar has great cache and cookie control down to individual cookies. Firebug lets you mess with the DOM and CSS directly for a page and YSlow is good for page weight and response times.
Be careful having Firebug enabled for JavaScript heavy sites as they really slow down. GMail will warn you about this and tell you to turn it off. I have had a problem trying to disbable sites with the menus when the tool is embedded in the browser at the bottom but opening it in its own windows the sites menu works fine.
I personnaly also like IE Tab which means I can quickly view a page in IE without leaving FF and I can also get it to load anything that only works in IE with it.