Here's a useful list of Firefox extensions. I have been using these extensions to extend Firefox's functionality and they work great :)
Adblock 0.5.2.039
Add Bookmark Here 0.5.3
BlogThis 0.2.1
BugMeNot 0.6.1
Download Statusbar 0.9.2
eWebMail Color and Graphics 1.6
Ext2Abc 0.2.1
fireFTP 0.86.2
FoxyTunes 1.0
ListZilla 0.5.1
Nuke Anything 0.2
QuickNote 0.6
ReloadEvery 0.5
SpellBound 0.7.3
SpellBound Win32 Components 0.7.0
Tweak Network 1.0
US Department of Homeland Insecurity Idiocy Level 0.1.1.1
Wikipedia 0.6.0.7