Based on this article I just saw on the right hand column (didn't find this via search) https://support.mozilla.org/en-US/kb/unable-launch-older-version-profile and according to this https://bugzilla.mozilla.org/show_bug.cgi?id=1616495 older profiles should go through TB 68.5 first.

Could there be a workaround of exporting to Apple Mail and then re-exporting to TB via the built-in wizard?

I think lets back up to the start. what is the actual problem because I think this is off track already really.

Hi Matt. Thank you for your reply. Basically I've found that old TB like v52 seemed more stable so wanted to use it on Big Sur. I tried but it didn't work. So instead I tried loading 52's profile using v78. It worked except passwords were missing. Finally I tried v68 and all seems okay although do miss v52.

Hello there We have read your message. We try to help you.

Yes ,indeed the versions you can play with a Macintosh are the versions that will play on these s Mac systems.

Also Thunderbird version 78.0 is not compatible with apple devices but you can try and run them.

That is why some versions of Thunderbird won t do as they should have to do. With Mac books.etcetra. But they play good for most of the time. Older versions of Thunderbird has stopped with updates for security and more.

Do you want migrate your profile from Thunderbird Mac or inbox to Mac mail from there on to paste it in Thunderbird windows? It s possible.

Or just from a older profile to 78.0 version of Thunderbird?

https://translate.google.com/translate?hl=en&sl=auto&tl=en&u=https%3A%2F%2Fsupport.mozilla.org%2Fnl%2Fkb%2Fthunderbird-os-x

https://support.mozilla.org/nl/questions/1344541#answer-1429468

Greetings Firefox volunteer.

While your "preferring" old software. Just have a look at how many serious security bugs have been fixed in subsequent versions. https://www.mozilla.org/en-US/security/known-vulnerabilities/thunderbird/

Like those that stuck to XP while Rome burned. There is reason no one involved in the project will recommend old software except as a temporary step in fixing some issue.

Hello there again.

Yes, they are planning to keep on using it.

Greetings Firefox volunteer.