One of the problems with Mozilla’s plan to hide Firefox version numbers is that the replacement of “You’re running the latest version” only succeeds if people have confidence that the check is working. Speaking for myself, the last time I checked About:Firefox, I was convinced that it was broken until I verified that the update I was expecting was Mac-only, which was why it wasn’t showing up on Windows.
The biggest, of course, is breaking deeply ingrained user expectations (where to find the version number) for no real discernible benefit.