it is more likely caused by the use of snap installers in Ubuntu and therefore mint.

why are you not using a current release. 128 is also out of support.

Hello Matt, I don't know why I am not using a "current release". The Thunderbird on my Mint machine came with the Thunderbird that I have. I want desperately to solve my current problem first. I can look at changing to a newer release later if there is a case for it. I do not understand the reference to "snap installers".

I'd be inclined to first update win7 to 128, then attempt the move to linux with version 128.

Install and start 128 on the linux system, let it create a directory, then copy your windows profile directory into that. It should create xxxx.default-release directory. (Profiles named xxxxx.default in not used in normal practice, so I'd avoid that.)

Then describe your problem, if any.

Thank you Wayne, Unfortunately, Thunderbird regards Win7 as such an outdated operating system, that it aborts any attempt to upgrade on that platform. Apart from that detail, I have tried what you suggest. I plan to list the contents of the xxxxx.profile-default folder on the two machines to see if they at least have content items with the same names. Do you know where I could get Thunderbird 115.18.0 to install on the Mint machine?