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...
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...
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.
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)
A Statusbar with speed, percentage, time and loaded size (similar to Opera's one)
Collections are groups of related add-ons that anyone can create and share.Explore Collections