Rated 4 out of 5 stars
Your firebug extension is great! I've just sliced complex website consisting of 30 subpages and I'm going to use it in my next project.
Adding new rule in firebug in HTML tab in style tab would be nice. There should be an option "add new rule below this in code" and I wouldn't have to go to "CSS" tab to add new rules any more.
Firebug's genarated code is ugly(especially firefox specific properties "-moz..."), but after filtering it through W3C CSS validator it's nice. You should add automated W3C filtering while uploading.
Cheers, keep developing this extension;)