Firebug is a free, open source in-browser web development tool for the Firefox web browser. It’s many features include: on-the-fly HTML and CSS editing for tweaking or debugging, a Console for logging, analyzing and debugging JavaScript, and an intuitive Document Object Model (DOM) inspection tool to help you quickly see how the elements of a web page relates to one another.

Firebug’s popularity is so immense it’s one of the few Firefox extensions that have its own extensions (like YSlow and FirePHP)!