The two most useful extension I use are:
Tabbrowser Extensions - Has every thing you could every want related to tabs: session saving, editable tab context menus, appearance, too many tabs behavior, auto-reload, undo tab close, rearrange tabs, and many more features.
SpellBound - Will spellcheck any textbox.
Some others I use are:
FoxyTunes
Adblock
Show Image
Bookmark Duplicate Detector
Bookmark Backup
Bookmark LinkChecker
Calculator
DownloadWith
DerBrowserTimer - Adds clock and alarm functions
EZNav
Full Screen Context
Nuke Image
Nuke Anything
Paste Happy
Stop/Reload - Combined Stop and Reload button
Triple Select - Third click selects paragraph
Word Count
Zoomy - Increase/Decrease Text Size
All of these and more can be found at
The Extensions Mirror.