It ’s only a minor thing on first glance, but the code that makes the GMail interface work in our browsers was updated in some significant ways a while back. Unless you were a user of GMail for your domain, or Google Apps or whatever it’s called now.
But now, the new shiney interface has been migrated to users of GMail for their domain too.
I suspect a Firefox tabbed browser will somehow, under the surface, benefit from running two copies of the same JavaScript rather than two that are dissimilar — this is only an educated guess and I’m assuming (overly simplistically probably) that memory fragmentation (etc) will be hurt less by this change.
Anyway, it’s a good thing for users of both interfaces (ahem).