I've been a Firefox user for a long time so I almost feel obliged to respond to this.
The ones I use are:
Adblock Plus -
must-have, the regular Adblock has tons of bugs, this one is great
Ablock Filterset.G Updater -
synchronizes Adblock with Filterset.G
All-in-One Sidebar -
makes Firefox 10x better, check out the picture below
adds tons of features to Google and makes searching a lot easier and also customizes GMail and removes ads
puts a play button where Flash is usually embedded
IE Tab -
NBA Scoreboard -
a must have if you're an NBA fan, access every score and stat with one click from the sidebar
the best RSS reader for Firefox and it's all integrated into the sidebar
How people can still use IE, I have no idea.