Use "Siphon" to sync addons across different computers.
- Adblock Plus
- AutoCopy
- Auto Context - user popup menu on text selection
- AutoHideStatusbar
- Bar Tab
- Better Gmail
- Better Youtube
- Clear Cache button / Clear Private Data
- Context Search - adds option to select search engine on right-click context menu
- Define Word / Define
- DownloadHelper
- Download Statusbar
- Fission
- Flashblock
- FlashGot
- Gmark
- Google Gears
- Google Toolbar
- Greasemonkey
- Keyconfig
- Menu Editor
- Mouse Gestures Redox / All-in-one Gestures
- Multiproxy Switch
- Omnibar - adds capability similar to Google Chrome's location bar
- Persona for firefox - Lightweight theme manager from Mozilla (in-built since Firefox 3.6)
- Split Browser
- Tab Mix Plus
- URL Suffix - Continued
- Voyage - History management
- Weave / Firefox Sync