*Foxmarks
If you use more than one computer then Foxmarks might become handy as it keeps your bookmarks and passwords (optionally) synchronized.
*Adblock Plus
This is a must-have add-on for Firefox.
here's my list:
FireFTP - FTP Client
Downthemall - Download Manager
IE Tab - Views websites in IE in one click
Colorzilla - Show's the color of a website portion where you point your cursor
MeasureIt! - Measures the selected website's area
Firebug - Advanced error console
ScreenGrab - Makes a screenshot of the website you are viewing