Killer Firefox Extension => FireBug

Out of all the Firefox extensions I’ve used my recent favorite is FireBug. If you do any sort of web development then you NEED to get this extension. It consists mainly of a console, debugger, and DOM inspector. These are much needed tools for web application development. My favorite feature of Firebug is the console where you can do live JavaScript authoring and testing. It reminds me a lot of Venkman. If you haven’t tried FireBug I highly recommend giving it a test drive.