It works great on a Mac (not “MAC”) and migrating works too. What exactly have you done and what exactly is not working?

Actually it is "mac" as-in "macOS". If all you can provide is meaningless spelling options, I do not want YOUR help!

The computer is Mac and the operating system is macOS.

I could have helped you much more if you hadn't over-reacted to a piddly correction.

"piddling"? You revealed who you are in YOUR over-reactive initial response. I do not need help from your sort.

So what does this actually mean?

Having tried more than a dozen sets of downloaded instructions, including those from Mozilla Thunderbird and Apple,

Keep in mind we know what you tell us, so I have no real idea at all what you have already done beyond that rather bland statement and that you assessment is none of what you did actually worked for you. Why it did not work I have even less idea of as I do not know what you did. To put your statement it into perspective. I want to change the oil in my tractor. How do I do that? I have looked at multiple Youtube videos none of them worked! I therefore conclude Mobil does not make a suitable oil.

I would suggest you can not offer me the sort of detailed instructions I would need to succeed and likewise I can not offer you more than generalities because you really have not said anything on which to base direct instructions. At least Rick knows about the menus in an Apple, I last used an apple computer when they were black and white displays and had about a 7inch screen some 30 years ago. It kept reformatting my disks because put the wrong one in the slot. I learned then that I was not an Apple fan.

In general terms I would be telling you to;

1. Open the troubleshooting information on your windows 11 system version of Thunderbird.
2. Select the open button beside the Profile entry in the Troubleshooting information.
3. Once the windows file manager opens close Thunderbird (it can not be running in task manager)
4. In the windows file manger select Press Ctrl+A (this is the windows shortcut to select all)
5. Drag the selected items into a suitable drive for transfer to the apple device. (note that windows returns from a copy a long time before it actually completes, so you will need to wait. Perhaps make a cup of coffee before pulling the plug on the drive being written, or shut down windows with a restart option and wait until the restart completes. That will ensure a sane copy not some sort of incomplete mess)
6. Now on your Apple device, open Thunderbird and repeat the steps in Troubleshooting information until you open the profile in finder
7. Again close Thunderbird
8. Copy all of the copied file and folders onto your apple product replacing the ones in the profile folder, so expect your apple to get concerned about replacing existing files. That is the intent.
9. Start Thunderbird.

When seeking support in a peer forum as you are here, you are drawing on the knowledge of other users of the product and their knowledge of the operating system they run it on. These statistics are the most recent available show that less than 5% of Thunderbird users do so on an Apple product (Darwin) so you are fishing in a relatively shallower pool looking for Apple support than you might expect. Perhaps one volunteer in 20 will be using your operating system so you will need to be very specific in your information to get the outcome you want.

What a waste of time!

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