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 User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.
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...
The Web Developer extension adds various web developer tools to the browser.
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager.
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)".
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox...
View HTTP headers of a page and while browsing.
The most beautiful yet the simplest add-on that makes a strong colorful appeal. Colors every tab in a different color and makes them easy to distinguish while beautifying the overall appearance of the interface. For Seamonkey & Flock. An essential. F...
Add, modify and filter the HTTP request headers sent to web servers. This addon is particularly useful for Mobile web development, HTTP testing and privacy.
UnMHT allows you to view MHT (MHTML) web archive format files, and save complete web pages, including text and graphics, into a single MHT file in Firefox/SeaMonkey.