ForecostFox 0.5.9
Dictionary Search 0.7
SwitchProxyTool 0.7 (I only use it for improved security)
QuickTabPrefToggle 0.0.4
ColorZilla 0.8.2
Adblock.
Heehee. I kinda went crazy with the extensions.
Adblock and Gmail Notifier. I try to keep FF as clean as I can.
Google released their own Gmail notifier a while back. I'm not sure if they have it on Linux distros, but I use it for Windows and its great. You might want to take a look at it.