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).
Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.
Squared tabs, appmenu, add-on bar, small button view and many more 'old' features for Firefox 29+ (Windows / MacOSX / Linux).
All builds & changelogs on versions page!
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!
(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.
Evernote Clearly makes blog posts, articles and webpages clean and easy to read. Save them to Evernote to read anywhere.
Adds an about:addons-memory page that details add-on memory usage
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.