If you have made a copy of the entire "Thunderbird" folder before upgrading to Win12 you now may - Thunderbird shut down - replace the actual "Thunderbird" folder in ...\AppData\Roaming\... with the saved copy and restart TB.

Thanks for your reply. Before I do a replace, I’d like to give a description of the folder Thunderbird that I created just prior to the move to W11. There is something that puzzles me and causes me to hesitate. I will begin with a somewhat detailed description of the Thunderbird folder (that you can probably skip over) and then a cause and question about my hesitation. The Thunderbird folder was one of the subfolders of C:\Users\rsuth\AppData\Roaming\. It is 8.94 Gb, containing 72 subfolders and 11067 files. Two of the files are installs.ini and profiles.ini. Four of the subfolders are Crash Reports (1010 bytes, 101 files, subfolder events), Pending Pings (empty), Profiles (5.16 Gb, 467 files, 33 subfolders; one of the subfolders is odp….default, containing the files times and json, the other subfolder is gqf….default-release, containing many files and subfolders), and another subfolder named Thunderbird. This second named Thunderbird (3.77 Gb, 34 subfolders, 497 files) has three subfolders: Crash Reports, Pending Pings, and Profiles as well as the files installs.ini and profiles.ini. My hesitation is based upon a folder named Thunderbird containing a subfolder with the same name. So my questions is this: does one Thunderbird folder containing a subfolder with the same name effect the instructions you gave to shut down Thunderbird, then replace the current Thunderbird folder with the first one I described above, or does something else/different need to be done before any replacing?

A folder Thunderbird containing a sub-folder named Thunderbird is not normal. It means that someone has made a faulty copy & paste action with another (?) Thunderbird folder in the file explorer. Does the sub-folder Thunderbird contain a Mail and an ImapMail folder? Anyhow you can proceed to the replacement as I indicated in my first reply. If it succeeds we'll explore the sub-folder Thunderbird a bit later.

Definitely possible that I screwed up something regarding saving folders back at the start of my problems.

I used file explorer to search the Thunderbird sub-folder for a Mail folder and an ImapMail folder. No matches found for ImapMail. The search for the word mail found two folders named Mail. The date modified on each was only 1 minute different than the other. (Actually, the search found two more folders with mail in their titles: mail.comcast.net. Their dates modified were only 1 minute apart.) I looked at the properties of the two Mail folders. The properties were not the same. 3.70 Gb for 1, 5 Gb for the other; 59 files and 63 files; 6 folders both.

So then I tried your suggestion: “Thunderbird shut down - replace the actual "Thunderbird" folder in ...\AppData\Roaming\... with the saved copy and restart TB.” I did that and then loaded Thunderbird - and it appeared with most – if not all – of the settings that had been there before the W11 upgrade! After a moment’s pause, Thunderbird, as usual, asked me to enter my password so that I could download my email from Comcast. I did so. Thunderbird’s response was that the password was incorrect, try again. So I did – and got the same result. I then shut down Thunderbird , my other open programs and the computer itself. After a few minutes, I rebooted and loaded Thunderbird. It works!! It looks like what I remember from before the W11 problem. The settings, folders, etc. are what I remember – or very close.

So I begin by saying thank you for the help that you’ve given. I feel comfortable again, but with a tiny bit of hesitation. I want to wait until tomorrow morning, after a complete night’s shutdown, before I try Thunderbird again. Right now I feel that all will be fine, but I will wait until then before I click the “Solved the problem” button.