StumbleUpon
StumbleUpon is the easiest way to discover new and interesting photos, videos, web pages and more, recommended by a community of experts. Tell us what you are interested in and we recommend page after page of amazing stuff you’d never expect to see.
YSlow
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.
Pocket (formerly Read It Later)
When you find something on the web that you want to view later, put it in Pocket. It automatically syncs to your phone, tablet or computer so you can view it at any time, even without an internet connection.
Personas Plus
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
Xmarks Sync
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!
ImTranslator - Online Translator, Dictionary, TTS
ImTranslator provides access to the online translation powered by Google Translate, Microsoft, Babylon for 66 languages. It includes dictionary, text-to-speech, back translation. Complete Video Tutorial: http://youtu.be/UwK6529XZIQ
Stylish
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.
NoScript Security Suite
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.
Greasemonkey
Customize the way a web page displays or behaves, by using small bits of JavaScript.