Firefox plugin to ease openid authentication
track changes made in Firebug
Firebug add-on for cookie information
Lookup words in dictionary.app from firefox.
Easy UI mockups in Firefox
Add S/MIME support to Gmail
Syncs OS X system proxy settings to Firefox automatically
Adds HTML validation to View Source
Take full page, and other, shots of your websites
Adds a cookie tab to your Page Info window
watch your http headers in real time, save them, reinject
firefox content to your mobile
Let's you view the source of a page directly in other applications