These are the add-ons that I have found and added to my firefox - some get used daily, others are a nice to have, but overall, a very robust and functional collection!
The first and only download manager/accelerator built inside Firefox!
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.
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!
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.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Integrates location bar and search bar into one. Shows search and url suggestions. Quickly switch search engine using search keywords. Provides different auto-complete popup styles for compact displays.
3D in your browser! FoxTab brings innovative 3D functionality to your Firefox.
New! Top Sites for FoxTab (aka Speed Dial).
Browse faster—Preview links and images without leaving your current page or tab. Just mouseover any link and a preview window appears to display the content.
Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, canonical links, WHOIS, robots and more.
A simple, clean and easy-to-use FREE Firefox extension for video download and Flash download. Supports Windows/Mac/Linux operating systems and the latest Mozilla Firefox. Now Supports Mozilla Firefox 4 and English, Spanish, Chinese language.
Add useful extra features to Gmail, like hierarchical labels, an unread message icon on your browser tab, file attachment icons, and more.
ShareThis for FireFox makes sharing simple. Share anything on the web with your friends on 45+ social sites including Facebook, Twitter, Email, Digg, Reddit and more. Track top trending shares by site and automatically save content to your ShareBox.