Can I assume you're aware that there are global junk controls in Settings, plus controls for each account that can override the Global ones?

There's also the Message Filters feature that can mark messages as spam. Have you checked them out? A poorly-formed filter rule could mark everything as spam.

You can't assume that the "Thunderbird thinks this message is Junk mail" banner means that Thunderbird is making that decision. You get that same banner even for messages that were moved to the junk folder by the mail system.

As I said in my original message, junk filters are turned off globally and for each account, so yes I am aware of those. Additionally, there are NO filters set up of any kind in my accounts. As I mentioned, this started suddenly several days ago after years of not having this problem and not changing anything in my setup. Also, I am aware that gmail considering mail as junk would trigger a notice from Thunderbird, but as I also indicated gmail is NOT filtering and is not flagging these messages as junk. Further, no messages are being sent to the Junk folder -- they are all appearing in my Inbox and are getting flagged as junk.

I've tried everything I could think of, considering that I've used this client for many years and have become quite familiar with its' functionality, and I'm at a loss to understand why it is doing this. I am only left with the possibility that it is something within the Thunderbird application that is causing this issue and I can't figure out how to stop it.

Maybe the adaptive junk filter database has become corrupt? I used to use other Bayesian email filters, and that happened once or twice, resulting in erratic detection. I believe choosing the "Reset Training Data" button will fix that, though you will have to train it from scratch again.

And maybe turning on the "Enable adaptive junk filter logging" option will reveal something. I've never tried that.

Well, I did try the "Reset Training Data" feature already, but it made no difference. Right now, since it's not supposed to be filtering anything the junk filter logs have nothing in them, however that option was not selected (since I wasn't filtering). I've turned the logging on, but I'm not expecting anything to show up since there is no filtering/detection of any kind. I'll have to check it when I get more emails flagged as junk to see if it logs anything.

A brute-force possibility would be to remove the account from Thunderbird and re-add it. It could be painful or not depending on how many messages are on the server.

All my messages are kept on the mail servers. I only use Thunderbird as a client (nothing is stored locally), so it would take quite some time to reload all the messages for all my accounts (5) in this application, and set up each account all over again. Also, this is a relatively new computer, so Thunderbird hasn't been active on this pc for very long -- I had to install and set it up from scratch, so I'd really hate to have to do that all over again so soon. I prefer Thunderbird, but unfortunately if I had to start over with it, especially with no guarantee it would solve the problem, I might just as well find another client to use.

Well, none of the suggestions so far has eliminated the problem. In fact, despite telling Thunderbird that email from senders is 'Not Junk', it continues to mark the same senders as junk, and I have to select the button to tell it that it isn't junk every time.

This is becoming impossible to manage.

If you've disabled all TB junk controls for all accounts, the junk/spam filtering is being done by the mail provider:

https://support.mozilla.org/en-US/questions/1404881

Unfortunately, we have already considered that possibility (and double-checked it) and there are no spam or junk filters being applied in my gmail accounts. Also, even if they were, the affected mail messages would be moved into the spam or junk mail folders on the mail server and I'd see them there -- all of the mail I am discussing here is delivered into my Inbox. Even mail that I indicate is 'Not Junk' by clicking the appropriate button on the preview pane, gets filtered again as possible junk the next time there is a message from that sender.

Also, as a side note, if the messages were being flagged by gmail, the flame icon would be showing as grayed out, these messages are all showing the orange flame, indicating that Thunderbird is flagging them...