Firebug is so large that it has add-ons of its own. That's right--add-ons for an add-on. Optional Firebug panels include CodeBurner for extending Firebug with HTML and CSS reference materials, and FireFinder for matching CSS selectors or XPath expressions to various page elements. These are just two of the myriad Firebug add-ons available on the Mozilla add-ons site.
I use Chrome for my day to day browsing, but Firebug keeps me coming back to Firefox whenever I need to develop or debug a webpage.
Note: The link takes you to getfirebug.com, where you can install the add-on directly into your Firefox browser.