Software's goal is to advance, upgrade, add features, improve upon the existing, and basically just... get better.
Firefox 3, doesn't so much work for me.
Firebug breaks a lot more than it used it, and by breaks, I mean DOM Inspector doesn't come shipped with Firefox de jure, but it doesn't really get installed with Firefox 3 de facto. And to me, one whom considers themselves "advanced", but feel trustworthy enough of a software to just let the default work fine enough. (I'm thinking of the "trustworthiness" of Firefox when they declared Apple untrustworthy for including Safari with its apple update.
Basically, if Firefox 3 would default install DOM Inspector, I would be happy. Web Developer toolbar should give me better advice since I don't have DOM Inspector installed, and Firebug should show the blue line around an object when I inspect it.
Thats it, that'll win my vote.