Would it make sense to have Torbutton, when run in TBB, just report the Firefox version accurately? We're trying to help people track real Firefox versions after all.
Two reasons not to: 1. It would allow you to easily differentiate people who have not noticed the upgrade notice yet and are still vulnerable to the latest Firefox exploit. 2. It would allow you to differentiate TBB-stable from TBB-alpha.
#2 (closed) may not be a strong reason, though.. There will be likely be ways to fingerprint these two groups apart as we test out new features in alpha :/.
If we had a working autoupdater, I could be more easily convinced tracking the FF version closely was a good idea.