Google Toolbar for Firefox
Google toolbar featuring enhanced search box, safe browsing, spellchecker, feed subscription, auto web-forms fill, sites page-rank statistics and other,

Performancing
Performancing for Firefox is a full featured blog editor that sits right within Firefox

Fasterfox
Dynamic speed increases can be obtained with Fasterfox’s unique prefetching mechanism, which recycles idle bandwidth by silently loading and caching all of the links on the page you are browsing.

Tab Mix Plus
Tab Mix Plus enhances Firefox’s tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more.

FireFTP
FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.

SEOpen
SEOpen is an extension for Firefox to help with search engine optimization. It provides Google, Yahoo, MSN, Alexa SEO tools at the click of a mouse.