Without question it would be Firebug (getfirebug.com) - I use it every day for debugging HTML, CSS and Javascript and for checking less transparent issues such as HTTP requests.

I used to be a 100% Firefox user, and Firebug's the best around. However, now that I'm using Chrom(e/ium) more often, I've grown accustomed to the built-in developer tools. I still think Firebug's got a bit of an edge.