The Web Developer extension adds various web developer tools to the browser.
Displays a country flag depicting the location of the current website's server and provides a multitude of tools such as site safety checks, whois, translation, similar sites, validation, URL shortening, and more...
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...
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
A Statusbar with speed, percentage, time and loaded size (similar to Opera's one)
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Firefox 23+, you need 0.958 (under review)
Collections are groups of related add-ons that anyone can create and share.Explore Collections