A few days ago, Mozilla released Firefox 3 Release Candidate 1 and I've been furiously testing it ever since. Let me take a moment to note that Mozilla is simply calling this "Firefox 3 Release Candidate". I've decided to number this release candidate since my experience in this area suggests there will be more than one. Who knows, maybe we'll get lucky!
I am happy to report that after testing this release, I can say without a doubt that the Gmail colored label problem has been fixed. For me, this problem has been a show stopper. With it cleared up, I can now begin using Firefox 3 on my production workstation, rather than just my test bench.
This version is not perfect, of course. There are still a few problems, but nothing I would consider major. Review the Release Notes linked above for a list of known problems.
By the way, it recently came to my attention that disabling "snippets" in Gmail caused the Firefox 3 colored label bug to disappear. Although this workaround was suitable for many, I found it annoying.