Radical proposal of the day: Can we use the Firefox major version number in the Tor Browser version? This would mean that TBB versions would be 24.0, 24.1, 24.1.1, etc.
Why consider making this change? Because doing so would allow us to greatly simplify the patches needed for bug #4234 (Use Firefox updater).
For the Firefox updater to work correctly, it needs to be able to check the version of an advertised update against the version of the software that is currently installed. At the same time, for add-on compatibility the browser version itself must be close to that of the Firefox release it is based on.
We already have patches for #4234 that modify our Firefox build process to pass the TBB version through and use it inside all of the updater-related code (and to use the 24.x Firefox version when we need to do so). However, the patches are messy and I don't think Mozilla could ever accept them; Mozilla has no need for a bundle version that is separate from the Firefox version.
If we used a version number for TBB that was close to the Firefox version number, we would not need to patch anything; we would just use our version number as THE version number. Doing this would also allow us to eliminate the torbrowser.version hidden preference. And it might help end-users who want to figure out what version of Firefox the Tor Browser is based on.