OK, you asked for it...
AdBlock Plus - blocks ads...I use this one to make DST faster for when I have to go there.
ColorZilla - color picker in your browser
Download Statusbar - all my downloads show up at the bottom of my screen instead of in a new window - love this one!
Fast Dial - when you open a new tab, this comes up and you program it to have all your favorite sites in it, so they're all just one click away! Awesome, awesome, awesome. I use this 100% of the time...they did an update the other day that caused it to not work and I was lost, lol. It's been fixed, though.
FireFTP - free FTP program inside my browser - again, love it, use it all the time.
Flashblock - blocks flash from playing unless you tell it to play - sooo nice to not see those annoying flash ads anymore!
FoxyTunes - allows me to control my iTunes (also works with TONS of other media players) from within Firefox, and also shows me title/artist of the song playing
IE Tab - opens a tab that "reads" like IE
ReminderFox - a calendar/to-do list feature that allows you to put in things you need to do by certain dates and mark them as finished - use this one ALL the time as well
Tab Preview - gives you a preview of the page you're on when you hover over the tabs you have open...so if I am here at SSD, and have gmail and my ct board open, I can hover over either tab to see what's going on there without ever leaving SSD's tab.
Undo Closed Tabs Button - allows you to reopen a tab you accidentally closed without having to go up into your history, etc. I'm really bad for closing things without thinking and then having to find it again, lol - this saves me a lot of trouble.