Hey man -
I just read something yesterday that said there _is_ a version of Firebug ready for FF3. http://ajaxian.com/archives/firebug-11-and-getfirebugcom.
Might be worth checking it out. Interesting that you wrote about this. I meant to upgrade to FF3 last night. I've been trying to use Safari because if its "zippiness", but can't get past losing the extensions (Safari's aren't worth a damn and they're more hacks than extensions, IMO). Particularly Firebug and Greasemonkey.