Tree-style tabs + Colored Tabs extension is a joy to use. You'd think that Tree-styled tabs would not play nice with other tab extensions but the author makes large efforts to interoperate.
Favourite: PDF plugin for Mac OS X. This uses the mac's native pdf rendered and makes viewing pdf's a joy rather than a pain. http://code.google.com/p/firefox-mac-pdf/
Shameless plug: If you are a google docs user you might want to look gdocsbar [http://www.gdocsbar.com]. an update with some more features like download, is on the works and will be released shortly.
mozrepl This has so much potential. You can telnet into your firefox instance and run javascript commands, on documents and on the chrome. When I get some time I have ideas about integrating this with emacs (there already is mozreply.el , but it does little more than telnetting in).
Let's you take a picture of a part of your page by rearranging a square. Helps so you don't have to go to paint or photoshop to crop your recent screenshot.
I used Mouse Gestures Redox for a long while then I switched to firegestures. I can't remember why exactly, I think something to do with speed & a larger library of gestures.
Awesome plugin for quick/constant perf measurement. It shows how long it took the browser to get the first byte, the total time for page rendering, the total size of page and the total number of requests. For example, for this page, the stats are :
I use Safari more than Firefox these days. But when I use Firefox I quite like Vimperator, especially since I can write simple but functional extensions for it easily.
