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.
Increase your browsing privacy, protection, and speed.
Automate Firefox. Record and replay repetitive tasks. If you're tired of manually visiting the same sites, filling out forms, downloading files and extracting data, then iMacros is for you! Save time, effort and money with iMacros browser automation!
The Web Developer extension adds various web developer tools to the browser.
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
The greatest user script engine on the Internet (a fork of Greasemonkey).
Protects you against tracking through 'free', centralized, content delivery. It prevents a lot of requests from reaching networks like Google Hosted Libraries, and serves local files to keep sites from breaking. Complements regular content blockers.
Firebug command line autocomplete.