I have the universe & multiverse repos enabled, so I'm not sure where, exactly, this came from, but the rc? package was upgraded a day or two ago to "firefox 3.0+nobinonly-0ubuntu0.8.04.1" which I assume is a long-winded version of 3.0-final. But I can't say for sure. Works very well though.
EDIT: I also use synaptic rather than mintupdate. Mintupdate generally works well but misses things sometimes, I find.