http://www.longfocus.com/firefox/gmanager/ - Lets me manage my gmail accounts, and keep tabs on how much email and spam each account has.
http://fasterfox.mozdev.org/ - Another must have, allows you to tweak FF to enable the browser to run faster, things like allowing more connections per webserver and such.
http://adblock.mozdev.org/ - Must have, can block ads and other items you don't want to see, allows for regexp matches.
http://www.customizegoogle.com/ - This one you need to have it will do things like force gmail over SSL, customize the display of ads, and change the way pages are displayed and ranked in google.
http://188.8.131.52/theonekea/tabprefs/ - You love tabs? We all love tabs. This one lets you manage the way your tabs behave. I love the fact that I can switch tabs by holding my mouse over them. It probably saves me 500 clicks per day :)
http://www.mrtech.com/extensions/local_install/ - With the number of extensions you see I run, and always playing with another one or two, you know we need a solid extension manager. Well this tool takes the cake, its the best extension manager out there.
http://ietab.mozdev.org/ - Lets you run certain tabs with the IE engine, and use regular expressions to auto load certain sites in ie. For example I load *.msn.com Domains with IE from within FF.
http://users.rcn.com/shoofy/forecastfox_enhanced/ - Gotta keep in touch with the weather and sat maps right in your FF status bar.
http://smoothwheel.mozdev.org/ - Makes the scroll wheel on a mouse act more "smooth", and allows you to tune the way the wheel works in FF.
http://jennifermadden.com/ - This one pretties up the view source, color coding the html and such. Makes it easier to read and analyze the source.
http://mozmonkey.com/ - (Switch Proxy) This one allows you to manage and switch proxy servers quickly. I am often doing things that I don't want to be tracked to me, so I use this and my list of proxy servers to better hide myself as needed.
http://searchsidebar.mozdev.org/ - This one does search result aggregation, its pretty good, but I prefer using Copernic Agent for the most part.
https://addons.mozilla.org/extensions/moreinfo.php?id=1912 - This extensions allows you to search for a person on many sites at once. Its pretty handy, but I don't use it a whole lot.
I'm currently playing with this extension :
http://showcase.uworks.net/ - It shows the open tabs in a mini view, I'm not sure how useful it is yet.