What is it with huge blocks of text today. I struggle to read it but I think I got it.

First questions are do you have an antivirus program and which is it?

That is the usual cause when you get told messages are downloading and then they just stop. Or you get the same few and it never ends, you just get the same start of the new list over and over. It is also a leading cause of issues about write privileges etc. Without being technical Thunderbird does not share well so if you have an antivirus scanning files on access (most windows users do) or a backup program trying to maintain a copy of the files in the web (be it some sort of streaming backup or a modified storage location as the default ones are not usually included in web backups)

Disabling the scanning of mail is sometime all that is required. But the more comprehensive the third party antivirus the more likely it is to mess things in the nest. Note that you can uninstall third part antivirus products and windows defender will just step in to cover you security wise. So do not be afraid of removing the antivirus to see if that is the issue.

Thanks, Matt, I really appreciate you taking the time to reply. (And apologies for the big block of text.) I think I've solved the issue, which *was* (as you suggested and which I'd read elsewhere) related to an AV situation, but not exactly as you thought. Short and very non-tech version: some evil thing (malware or trojan) seems to have "snuck through" my AV program (Windows Defender) and lodged itself in my some Thunderbird Inbox sub-sub-sub directory. Although I followed all of Defender's prompts and performed various suggested procedures, it just wasn't doing the job of either quarantining or removing the bad stuff, nor was MalwareBytes, which I also have installed. So my conclusion was that these two programs just weren't robust enough, and I ended up signing up for a Norton AV program (the "Standard" one), after which I ran various scans and whatnot -- and immediately after doing so, two things happened: (1) Thunderbird went back to acting normally, and (2) Defender stopped sending me all these alarming pop-ups about "There's a threat!" etc. (These pop-ups, which had never been a thing for me before, had begun happening frequently, at about the same time that Thunderbird started malfunctioning, so it seemed obvious there was a connection. I didn't mention it in my original post because I didn't want the big block of text to be any bigger or blockier.) Anyway, more than 24 hours later, it seems that all is well again. Thanks again for your input.