A bit off the wall, but where did you download your Thunderbird from? I have some bad feelings about the version offered in the Microsoft store. Mostly because it works in some sort of sandbox that is not explained anywhere clearly.

I downloaded it from Thunderbird.net site.

Weird. (my one word synopsis)

Did any of those things work before the profile migration? Where was your profile on the old machine Vs new? I would expect some variance in the user name in the appdata path, but is it on the same drive letter?

Everything worked (and still works) perfectly on the old computer (Windows 10).

Everything is identical on file location - c: drive - under identical computer "user" name.

SOLVED! -- For anyone else running into these types of issues on Windows 11, there is a simple solution. It took me a month to figure it out, but here it is: Install it as "administrator." Right-click on the .exe installer, choose "Run as Admin," and let it run through the installation. Once I did this everything runs perfectly on it now 😁

One question to add - I have Thunderbird 128.5.2 (32 bit) running in Windows 10. Can I install the 64-bit version in Windows 11 and then transfer the profile folder?

Hi @ElviraKate - I think this would work, yes. Just make sure you're running the same version on both - v128.5.2.

The trtansition is the hard bit,

On the old computer use Thunderbird to select the profile folder via the Troubleshooting information link on the help menu.

This will open windows file manger or explorer. Now shut down Thunderbird On the toolbar in the file explorer select the three dots Choose select all. Then click the copy icon on the same bar.

Now past all of that onto a suitably large USB stick. a 64Gb one would almost certainly not be to small.

In the file explorer on your new computer go to your USB drive and select all and copy from that media. Start Thunderbird and again go to the profile folder via the troubleshooting information. Close Thunderbird Now use the paste command in the toolbar of file explored and replace the profile contents Thunderbird has created if you do not get prompted to replace some file (lots of files usually) you are pasting in the wrong place.

Note installing as an admin creates other issues down the track. Unless you share the computer and want every user to have access to the program but not update it I would not go hat route.

Note The procedure above bypasses this https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation which is known to cause issues when moving from a 32 bit to a 64 bits version.