Let's say in the future Tor Browser is based on Firefox 90.0, Tor Browser Beta is based on Firefox 91.0b1, and Tor Browser Nightly is based on 92.0a1.
I propose Tor Browser uses the following versioning schemes:
Release: ${FF_MAJOR}.${FF_MINOR}.${PATCH} (e.g. 90.0.1)
Beta: ${FF_MAJOR}.${FF_MINOR}b${PATCH} (e.g. 91.0b101)
Nightly: ${FF_MAJOR}.${FF_MINOR}a1 (i.e. 92.0a1)
Where ${PATCH} is (${FF_PATCH}100) + ${TB_PATCH}), therefore we move the Firefox patch number into the third digit (hundreds) and we place the Tor Browser patch number in the first and second digits. (Initially, I began proposing (${FF_PATCH}10), but what-if-we-need-11-releases?). That would be a sad month, but probably safe-than-sorry.
I think we can re-use Firefox's alpha/nightly version number.
The main advantage of this scheme is that the underlying Firefox version is immediately clear when looking at a Tor Browser version number.