Should I figure out how to update the win 10 machine to 128.5.2 before transferring the profile?

Yep. I've no idea where the button would be in that old a version but I'd save the profiles and also a separate address file somewhere in case it goes horribly wrong.

Hi, I had the same problem. After reading lots of suggestions everyone was saying find the profile folder, copy it and replace on your new machine. But! copying the profile folder does not work, you need to copy the whole "Thunderbird" folder from your old machine and replace the Thunderbird folder on your new machine. On both Win10 and 11 it's in the same place. Also any mail you've downloaded onto your new system will download again so nothing is lost.

This is the easiest way to find it

   1. ensure thunderbird is not running on old machine 
   2. enter %appdata% in search window on task bar and press Enter key 
   3. this should open windows file explorer in Appdata\roaming folder 
   4. highlight the Thunderbird folder and copy to external media 
   5. ensure thunderbird is installed on second computer and NOT running 
   6. enter %appdata% in search window on task bar and press Enter key 
   7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it 
   8. start thunderbird and all should be there 

This process copies everything that is in the profile. Be sure that thunderbird is NOT running on either PC.