My guess is that 5.0[sane] would be around the time they changed the extension system (ca. 10.0[insane]) to take into account the rapid release cycle breaking extensions that relied on a max version number. Internally, I can't off the top of my head think of what changed internally since 10.0[insane], but I'd guess the introduction of Australis (29.0[insane]) would make it 6.0[sane] (unless there was another really major change which would push the sane version number to 7.0 or even 8.0). So FF 30.0[insane] would be 6.1[sane].
Another way of doing it is that the major version number only changes when an ESR is branched off, and the minor version number corresponds to an insane-major-number. So 4->4, 10->5, 17->6, 24->7, so 30.0[insane] would be 7.6[sane].
Perhaps if we can work out what version number Firefox should have, we can encourage disgruntled Firefox users to refer to it by it's sane version, and hope this will catch on.
Tamerlan Tsarnaev comes out as Tristan Thompseen
Tamerlan Tsarnayev comes out as Tristan Thozomas
and that is that
To think of it as spherical
Is frankly quite hysterical.