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...
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
Wappalyzer is a browser extension that identifies software on websites.
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.
Displays Server Type, Headers, IP Address, Location Flag, and links to Whois Reports. Shows links to check server status and cache when page fails to load.