IE Tab - can open the current page or a selected link embedding Internet Explorer in tabs of Mozilla/Firefox. Very useful for those IE only pages.
FoxyTunes - Listen to music while surfing the Web, and control your favorite media player without ever leaving the browser.
Gmail Manager - Allows you to manage multiple Gmail accounts and receive new mail notifications. Displays your account details including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.
Foxmarks Bookmark Synchronizer - Install Foxmarks on each computer, and it will work silently in the background to keep your bookmarks synchronized
Add Bookmark Here - adds an eponymously titled menu item to every submenu in the Bookmarks menu (like in Opera), enabling easy bookmarking in any folder.
ColorZilla - adds a color picker (like the dropper tool in Photoshop) to the status bar and brings Page Zoom magnification to Firefox
FEBE - FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions.