If I was able to revert TB to the previous version, I would - and am still tempted to research the mechanics of doing this.

Don't bother. I for one have simply given up trying to help those that do. It is just a huge mess in enough instances to put me off even trying.

I would actually think the issue might well be antivirus related. I was sort of thinking that and then I got to the POP messages being duplicated and that is classic antivirus interference.

When Thunderbird retrieves mail in the circumstance you use, it writes the server reference for each mail to the popstate.dat file in Thunderbird. But this is done at the end of the retrieve cycle, not after each mail so if for some reason the get messages hangs up and does not terminate normally the file is not updated. This used to be a special trick of Nortons, but I have heard of it of most antivirus products over the years.

Sometime clearing out the junk/spam folder on the server will fix that issue as the antivirus just drops the ball when it encounters malformed email and that is the most likely place to get that. Otherwise you will be looking for a mail that arrived just after the last dated email. Far simpler is to simply disable mail scanning until you have downloaded the troublemaker. Malformed mail is not really a risk in Thunderbird, only to the stability of the scanning tool.

You might find your inability to write is also antivirus related. Generally we recommend disabling any on access scanning in the antivirus for the Thunderbird profile folder. This has two immediate benefits.

Compacting folders becomes much faster and more reliable. This has a secondary benefit of removing old deleted mail that a subsequent definition update in the antivirus that sees it quarantine your entire inbox, or delete it, because it still contains an old deleted email that has a threat in it that the AV could not detect when the email arrived.

There is a very significant reduction in files being unable to be updated due to contention issues on the disk. Contention has many possible causes, but the antivirus is the most common. Some other possibilities are the profile folder being in a location that is cloud synced (often as a result of the profile being deliberately included in the "My Documents" or Documents" folders) which our Googles and Microsoft all want to place in the cloud so we pay them to store it. Other issues are streaming backups or even on a network storage location because network storage access can be slow.

You might also want to check in your profile for files with numbers in them like the prefs.js file. You should have one, if your antivirus is really acting badly you might have 99 or more. When Thunderbird can not open a file, it tends to create a new numbered version, save the data to it and never refer to it again. This looks to the user like setting changes do not get saved because when they were saved they were also lost.

All very interesting stuff, thankyou.

I solved my specific issue by sheer fluke a few days after posting, but wanted to ensure it was stable before posting back. So tempting fate... I made a small change to the POP3 config that I can only assume was tantamount to creating a new account. In my case it was toggling between two alternate POP3 server configs, but I presume any alteration would have the same effect.

Your various comments about the file storage & housekeeping with Thunderbird are very interesting, and I thankyou for mentioning them. Definitely a few bits to keep in mind!