The best security you can get in a web browser!
Allow active content to run only from sites you trust, and protect yourself against XSS and Clickjacking attacks.
Use the Evernote Web Clipper to save things you see on the web into your Evernote account.
URL Fixer fixes common typos in URLs entered in the address bar. It will fix errors like "google.con" or "youtube,com". You have the option of being asked to confirm corrections before they are applied.
A productive way to organize, read and share the content of your favorite feeds, blogs and news sites. Feedly offers a seamless transition from Google Reader.
The greatest user script engine on the Internet (a fork of Greasemonkey).
Help make Firefox better. The Add-on Compatibility Reporter enables you to report if the Firefox add-ons that you're using are working properly.
Allows URI texts written in webpages to be loaded by double clicks.
This 15KB restartless add-on will add a "File" -> "Restart" menu item and a ctrl/cmd + alt + r hotkey to restart Firefox or Seamonkey.
Useful tools for the nightly tester.