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).
Never be annoyed by a Flash animation again! Blocks Flash so it won't get in your way, but if you want to see it, just click on...
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!
Just another speed dial extension. Fast and easy to use.
NoSquint allows you to adjust the text-only and full-page (both text and images) zoom levels as well as color settings both globally (for all sites) and per site.
(former Google Dictionary and Google Translate)
In-line dictionary for any word you point to or double-click; whole page translation of any web page in another language.
The greatest user script engine on the Internet (a fork of Greasemonkey).
Make any pages' search functionality available in the Search Bar (or "search box")...
This 15KB restartless add-on will add a "File" -> "Restart" menu item and a ctrl/cmd + alt + r hotkey to restart FIrefox or Seamonkey.
Customize application menus:
Rearrange or remove menuitems from the main context menu (right-click menu) and main menubar (File Edit View etc.)
Expands the context menu's 'Search for' item into a list of installed search engines, allowing you to choose the engine you want to use for each search.