The Web Developer extension adds various web developer tools to the browser.
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
View HTTP headers of a page and while browsing.
Firefox OS Simulator is a test environment for Firefox OS. Use it to test your apps in a Firefox OS-like environment that looks and feels like a mobile phone. After installing it, go to Web Developer > Firefox OS Simulator to access its features.
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.
Wappalyzer is a browser extension that identifies software on websites.
Firebug extension to view which CSS rules are actually used.
Need to test your web pages out in Internet Explorer, Chrome, Safari, Opera (or anything else for that matter)?
Open With adds options to various parts of your browser for opening the current page in another browser.