The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
The Web Developer extension adds various web developer tools to the browser.
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.
TinEye is a reverse image search engine. The plugin adds a context menu item that allows you to search for an image to find out where it came from, how it is being used, if modified versions of the image exist, or to find higher resolution versions.
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)
If you have multiple logins for web based e-mail accounts (like Gmail and Yahoo! mail), then CookieSwap enables you to easily switch between those different accounts by swapping the 'cookies' that the sites use to know you.
Stop searching at PHP.net to simply remember some function signature. With this addon you can search functions quickly for the official function description directly into your browser without a new tab.
Plus, many tools to help PHP developers.
Testing tool for regular expressions with color highlighting (including submatches) and helpers for creating expressions
View Dependencies adds a tab to the Page Info window, in which it lists all the files which were loaded to show the current page.
One Click Validator. Provides you a button to validate a page using a validator like w3c or another user defined