I don't remember where I read this, but wasn't the version number bumped because it's a separate branch from trunk rather than development on the 4.x branch?
FWIW, there seems to be general agreement that FF5 is better than FF4. So as long as they stick to higher version number == better softare we ought to be ok.
Actually, I think they will be releasing a major version every 6 WEEKs. The current Firefox 6 is on Aurora channel for 2 more weeks and then on beta for 6 weeks. Then the release is set. The 2 week delay only exists between FF 5 (which was done faster) and FF 6 and will not exist in next versions.
It pretty much guarantees more exposure on tech sites, I dare say 'version 5 is out!' yields more coverage than 'version 4.1 is out!' no matter what the actual improvements are.
Also faster turnaround on major version numbers likely gives the impression that more is happening with the software than actually is, worked for Chrome ('hory shet, it's already at version 13!! this software must be so great!!').
Actually the effect is already weaning off, all the Chrome and FF dick-waving is causing everyone to start ignoring major version announcements, even for software that does not follow that model.