See the end of my blog post under profile per install here https://thunderbirdtweaks.blogspot.com/

Nice try, but the new fresh installed thunderbird on Win 10 is older than my Win7. Just read the specs in my question.  :( :(

hanheeze1 said

Nice try, but the new fresh installed thunderbird on Win 10 is older than my Win7. Just read the specs in my question.  :( :(

Copy the profile from a greater version to a lesser version. That is exactly when the downgrade protection kicks in. It is the profile that it is checking for a last used with and your new install is a lesser version. It is also exactly what is described at the end of the article. I have changed the heading so when I send folks there with your issues they actually read the section. I will not be pasting the stuff here.

Please post back only after the downgrade over ride command line does not work.

I have been following the detailed instructions to migrate my 10 years of thunderbird emails in some 50 file and folders (plus a few thousand addresses) from my windows 7 to a windows 10 computer. This involves moving data f rom one profile folder to the new machine. I have been trying for days to follow the intricate instructions but it just does not work and none of the files and folders will transfer although some 14gbs is transferred on my external hard disc. Can somebody give me some more helpful instruction please? I am at my wit's end. I cannot be the only user struggling with the death of windows 7. Vic PS Surely with both computers on the same network it should be possible to do a more simple network transfer..... just a thought.

You really need your own question @VicYouel I think this one is done and the original poster does not need the additional email that each response creates.

But to summarize. Copy to your external device. Open your new copy of Thunderbird and from the trouble shooting information in the help menu open the profile folder. Close down Thunderbird

In your copy from windows 7, locate the folder level with the file prefs.js

Copy everything in that folder and it's sub folder and paste it into the profile folder you opened from the troubleshooting information.

You should be prompted to replace a number of files as most of the top level files will exist in your new profile. If you are not prompted to replace, go back to the paste step and make sure the location you are pasting to is correct. Note that you can not copy and paste the top level "profile folder (something.default is probably the name) unless you want to jump through further hoops about profile folders being from older versions.

What I have outlines is the simplest method as it bypasses downgrade protections and the profile per install features which are causing the most angst with profile moves.