Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
The first and only download manager/accelerator built inside Firefox!
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.
Protect your privacy. See who's tracking your web browsing and block them with Ghostery.
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
The Web Developer extension adds various web developer tools to the browser.
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
View HTTP headers of a page and while browsing.
The greatest user script engine on the Internet (a fork of Greasemonkey).