Web Developer Extension
The Web Developer Extension created for Firefox by Chris Pederick adds a toolbar to Firefox with an amazing array of useful developer tools. Developers have the ability through the browser toolbar to outline page elements, manage cookies, disable the cache, get CSS and form information, as well as a multitude of other tools. Also worth highlighting is the ability to edit CSS on the fly, providing an indispensable tool for CSS debugging. The Web Developer Extension is extremely helpful and is a crucial reference tool for any developer. It's a free download, runs on Windows, Mac OSX and Linux and is available here: www.chrispederick.com.