I an not aware of any issues with folders being set to read only. Except where folk copy profiles to read only media like CD's. So I think this might be more something on your system than a general Thunderbird issue.

What makes you think the folders are wet to read only?

Thanks, Matt! It turns out that TB-V115.9.0 was a victim instead of a perp. I had seen the file R/O set while TB was running and stuck, by selecting the email folders and viewing Properties. I expect that TB does this while running to keep other programs from screwing up the files somehow, or that it was an effect of a buffer overrun.

I had been able to get ONE email to slowly download at a time, and noticed that it was always the same one. At first, I thought that the emails were not being deleted on the server, but tracking what email it was and how large made me suspect duplication. And indeed it seems to have been!

So I spent yesterday breaking up an email logjam one at a time, apparently created by inadvertently emailing quite a few copies, like 20 or 30, of a 50 MB email containing several big jpeg photos from my phone. I think that TB experienced a buffer overrun from all that crap and locked up. Once I pulled out the last unwanted copy, it was like a dam bursting and presto, suddenly my inbox had 185 emails in it! So it wasn't TB's fault, it was mine. Seemingly a result of a twitchy forefinger on the phone's SEND button.