Warning: web geekery ahead.
I don’t know of any other CSS editor that gives you a real-time, editable box model diagram of any selected element. Hell, I don’t know of one that does box model diagrams at all. And that Style tab next to Layout? That’ll show you all the CSS styles applied, including those inherited from parent elements, and it’ll cross out any rules that have been trumped by something else. You can turn off each line, to remove one rule at a time until you find the problem, or you can just edit the properties and values.
That screenshot shows the first thing I tried to debug. That’s a layout error that’s had me puzzled since I built the site a year ago. (See how the yellow stuff overlaps the left column?) Two seconds with Firebug and I can see the problem; two minutes later it’s solved.
Web developer tool of the year? No contest.