Really I end up just using Firebug. I haven't needed extensions for much else. I don't usually conform to "pixel perfect" design translations or anything of that nature and I try to avoid working in a ton of JavaScript.
Firebug is good for finding out why element X isn't properly displaying the styles (usually because something more specific is set in another sheet) or to find out what javascript-added elements are being added to the page so I can manipulate those however I may need.
