These are the addons that I consider essential to my every day browsing experience. Every addon has been tested extensively by me and is a permanent fixture in every Firefox installation that I use.
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.
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.
Automate Firefox. Record and replay repetitious work. If you love the Firefox web browser, but are tired of repetitive tasks like visiting the same sites every days, filling out forms, and remembering passwords, then iMacros for Firefox is the soluti...
The most lightweight and easiest translator. Translate between more than 50 languages with one click or a hot key. Translate from one word to a whole page.
Self contained add-on for downloading youtube videos to mp3 for easy listening on podcasting gadgets like a Zune or iPod
Evernote Clearly makes blog posts, articles and webpages clean and easy to read. Save them to Evernote to read anywhere.
FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up -- It will actually rebuild your extensions individually into installable .xpi files.
Visual bookmarks for Firefox. (Works with all latest versions of Firefox)
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.