You might be interested in our HTML & CSS Reference plug-in for Firefox:
It provides detailed browser compatibility information, and example code, on every HTML & CSS element from within Firefox. It's completely free as well.
I might indeed. You might, too.
Installing this led me to go and fix Firebug, which wasn't showing me styles. The fix was to uninstall and reinstall Firefox – yes, the whole browser, not the debugger that was exhibiting the problem. Go figure.
I found Firescope somewhat useful, but often too elementary. When I look up an HTML tag, what I really want to know is all the attributes, styles, and events that apply to that tag, as well as information about what browsers do and don't support each one. Firescope only gives me the basic information, but the price is right.