First, I am not a technical guru, but the screenshot of your junk folder has an empty location. that indicates that it doesn't actually exist. From my experience, some email providers do not use the word 'junk' for the junk folder. I encourage checking the account online to see what is the default and then set thunderbird to use that folder. See attached.

Thanks, David, I'll look into that. The subject account is a Microsoft one, a work-related Outlook site that was functioning correctly in this regard up until a few days ago. One of the other accounts I have is also a Microsoft one, but more of a 'consumer' (Hotmail) one, which may be different in that regard; it continues to work properly. Anyhow, I'll open the Webmail access to the subject account and see how things are organized there. Thanks much for the tip, and I'll report back here when I find out anything definitive.

Well, I don't think this has anything to do with the main Microsoft account. I'm using POP, not IMAP, so there's no 2-way communication when the program automatically downloads new mail; that is, I don't see how a change in the Microsoft (their server) account can change anything in Thunderbird.

What I have done, however, is to open the Thunderbird profiles to take a look at the two Microsoft accounts, thinking that there really should be no difference in how they are set up. But there are differences. The attached screenshots show the two accounts, and how there is no Junk folder under the second one. Moreover, there is no Archive folder either, but then I've never archived anything in that account, so maybe Thunderbird didn't create one... yet. Not sure how that works. Anyhow, there must have been a Junk folder there at some point, because a few days ago it was working. I wonder if I can create (just manually add) a Junk folder in that tree? I am loathe to do that for fear of really screwing something up.

UPDATE: Physician, heal thyself. Well, for some strange reason, the Junk folder magically reappeared, although the icon for the one account was the wrong color. I clicked on Default for the color, under Properties, which turned the little color sample box gold, but made the icon in the left pane red, just like the other two accounts. There was a bogus junk message in the re-created Junk folder; bogus, in that it opened a totally blank message pane, and I could not delete the message, that made Thunderbird crash. But a quick Repair, under Properties, took care of that issue so now it's all working just as it should again. Must have been some Artificial Intelligence 'bot' that took temporary residence in Thunderbird. I dunno.

I'm pleased that all is working. :)