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).
Protect your privacy. See who's tracking your web browsing and block them with Ghostery.
United States English Spellchecker for Firefox, Thunderbird and SeaMonkey by Kevin Atkinson and Geoff Kuenning.
To enable, right-click in a multi-line text box, select "Languages", then "English (United States)".
Monitors web pages for updates. Useful for websites that don't provide Atom or RSS feeds.
Jump-to-word buttons and highlighting for the search box content
Connect securely to https websites by checking certificates with network notaries. See http://www.perspectives-project.org
Note from Mozilla: This add-on has been discontinued. Try Add-on Update Checker instead.
Notifies you when updates are available for your extensions and themes.