Best Firefox Add Ons for Web Developers
ColorZilla is a nice add-on for Firefox which helps web developers in all the issues related to colors. With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program.
IE Tab V2
This Add-On helps you check your webpage view in IE direct from the Firefox browser. This Add-On Embed IE in a Firefox tab.
User Agent Switcher
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser. With this you can view how your website displays on various browsers and mobile