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).
Collector's Note STU - PEN - DO!!!
The first and only download manager/accelerator built inside Firefox!
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)".
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!
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
A customizable mouse gestures extension which enables to execute various commands and user scripts with six types of gestures.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
This extension allows you to export your saved passwords and disabled login hosts using XML or CSV files that can be imported later.
View HTTP headers of a page and while browsing.
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up -- It will actually rebuild your extensions individually into installable .xpi files.