Installed right now in Iceweasel on Statler A2:
Downthemall - easily the best download manager I've ever used. I use chrome on my windows side, but I keep firefox with DTA installed as it is sometimes needed and chrome's built in download manager is lolzy bad.
LastPass - How I keep track of my gazillion character random passwords...
TabMixPlus - until duplicate tab is a default setting in firefox, this stays.
Torbutton - For any anonymous browsing needs...
Link Target Display / Compact Menu 2 - First one allows me to hide my status bar. Second, makes the File menu bar into a small icon... I need all the screen real estate I can get on my netbook.
Have Had Installed in the Past:
AdBlockPlus - When I reinstalled #! I was using this but then I felt guilty about basically destroying the revenue source that supports my favorite sites. Yes, I know you can whitelist sites, but after the nth time of doing that after switching distros I just got lazy and stopped...
Omnibar - I used this until I realized it's way easier just to go into about:config and change keyword.url.
LocationBar2 - Like this extension but doesn't really change my life and I've been trying to keep firefox's extensions to a minimum since it already runs like a dog on my netbook.
^^^Ditto for Fission, Downloadstatusbar.
All in One Sidebar - Experimented with this on the netbook and ultimately decided that it was a system hog and really not much more convenient than firefox's defaults. But seriously, firefox needs to get the ability to open bookmark manager, history, and downloads in a tab, which is how chrome handles these things.
Speedial/Fast Dial - These are cool but the thumbnails just don't handle gmail, google reader, well and end up looking really ugly. Firefox could learn so much from Chrome's new tab page...