I'm not a fan of Mozilla support, so far. Not much to go on.

When I try to import mail from Mac OS Mail, I do not get the opportunity to specify which folders, it just runs off and imports something that has no data.

How do I get TB to import the correct files?

Export the folders from Mail, extract the mbox files, copy into TB:

https://bugzilla.mozilla.org/show_bug.cgi?id=688166#c23

Unfortunately, the reason I'm going BACK to TB is that A) Mail crashes quite regularly, and B) the precipitating event is a crash that somehow has Mail in a spinning-ball loop, and nothing I do can break the loop (all the way to restarting the computer in Safe Mode).

I believe there is a way to reinstall a specific app in the OS, so I will see if I can reinstall just the Mail app. Then, hopefully, the export method will work.

Perhaps you can find the .mbox files without running Mail:

https://www.lifewire.com/find-and-open-stored-attachments-1172808