definitely firebug and the web developer toolbar. my top two. oh and also the restore tabs one but i can't seem to find it again. i used that constantly!
That's a hard question. My favorite extension is the Web Developer toolbar. But, if I had to choose only one, the most essential is AdBlock Plus. That's the first extension I add to any new Firefox installation.
It is just a simple color picker...nothing hi-fi. I find it very helpful in modifying themes and templates. SEOpen is another which I use a lot.