Install each version in a separate subfolder of Program Files, such as Thunderbird68, Thunderbird78, Thunderbirdbeta. At the end of the setup, don't run TB, but launch Profile Manager with Winkey+R, type

<path to TB> -p e.g. C:\Program Files (x86)\Thunderbird78R\thunderbird.exe -p

Create a new profile, name it to distinguish the version, e.g. profile78. Uncheck the box 'Use the selected profile without asking at startup'.

another offer I got in another place is to run run the portable version of Thunderbird 68 as its profile folder is self-contained.

https://portableapps.com/apps/internet/thunderbird-portable-legacy-68