I have quite a number, but they work smoothly and so does FF itself:
Clear fields [clears address bar, google bar]
Colorful Tabs [different color for each tab]
Dictionary Tooltip [this is EXCELLENT]
FireFTP [very handy]
FirePhish
Flashblock
Forecastfox
Highlighter
no-referrer
NoScript
PDF Download
QuickNote
RestartFirefox
SearchBar Autosizer
Tab Clicking Options
Tabbrowser preferences
Oops!!
also
AdBlock Plus and AdBlock Filter-G
All-in-One-Gestures
All-in-One-Sidebar
Auto Copy