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 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.
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!
Session Manager saves and restores the state of all or some windows - either when you want it or automatically at startup and after crashes. It can also automatically save the state of open windows individually.
Open Bookmarks/History/Homepage/URL/Search in new tabs, and more utilities for tabbed browsing, including Protect/Lock/Faviconize/Unload Tab, Reload Every, Select and manipulate multiple tabs simultaneously, Tab Stacking/Coloring, etc.
Automatically find user scripts on Userscripts.org (requires Greasemonkey or Scriptish)
View and manage downloads from a tidy statusbar - without the download window getting in the way of your web browsing.
Thanks Tekzilla for your awesome review! Click here to see their video: http://youtu.be/BUDqWrJ-Vao
TabGroups Manager helps to classify and organize tabs by dividing them into Groups. Because tabs become easier to manage, you can never have too many! TabGroups Manager is ideal for tab power users.
A full-featured scientific calculator which can be placed in any Firefox tool-bar or the Australis PanelUI.
Automatically highlight and count all occurrences of the selected text with just a click (substitute for Ctrl-F with highlight option). Another click in the document removes highlighting.
Supports to customize the Default Character Encoding for each sites.
Note from Mozilla: This add-on has been discontinued. Try DragIt instead.
Allows you to open links in new tabs, and do more with drag and drop gestures easier.