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 Web Developer extension adds various web developer tools to the browser.
Xmarks is the #1 bookmarking and open tab add-on. Keep your bookmarks, and open tabs backed up and synchronized across computers and browsers. Mobile phones with Premium access. Utilize LastPass to sync passwords!
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox...
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
Monitors web pages for updates. Useful for websites that don't provide Atom or RSS feeds.
Bring back your personal and powerful Firefox Button!
This extension provides advanced configuration for your browser. It allows you to easily change many additional options, which are not displayed in the Firefox Options dialog.