> The items mentioned above should be done periodically, not just once

While helpful advice - if you need to do this manual maintenance regularly, the application is broken.

Possibly, hard to know over this medium. I’ve never encountered the problem, but added the tasks to my cleanup command.

Not to denigrate the hard work of developers (particularly open source developers) who work on very complex application software like Thunderbird, but software designed for regular users needs to cope without cleanup scripts. (Yes, yes, I know that reinstalling Windows from scratch is a time honoured tradition...)

Sure. My cleanup script however does multiple things, tbird maintenance is only a tiny part.

Maybe the old folders used mbox, which stuffs entire folders of emails in a single file, while the current stardard (for a while now) is Maildir, a file per email.