Firebug integration with Firefox is awesome. Despite Mozilla Firefox comes with its own Developers Tool, Firebug is a MUST HAVE for every web developer out there.
With CSS: You can modify CSS definitions on the fly, How will be seen this H1 in red color? Done, just change it in FireBug, review and write the definitive code in your CSS file, the same if you want to add some margin, padding o change font-size.
With HTML: You can remove elements, add elements and modify elements in DOM tree or HTML Code on the fly. Very handle.
You are curious about a web site, its code or its CSS? Just FireBugize it.
I can't make no one web project without using FireBug.