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
CustomizeGoogle - adds tons of features to Google and makes searching a lot easier and also customizes GMail and removes ads
Flashblock - puts a play button where Flash is usually embedded
IE Tab - already mentioned
NBA Scoreboard - a must have if you're an NBA fan, access every score and stat with one click from the sidebar
NoScript - best way to block Javascripts and it blocks all ads from a certain site that we all use
Sage - the best RSS reader for Firefox and it's all integrated into the sidebar
How people can still use IE, I have no idea.