When moving the profile, everything is moved. So, it remains to the original users. Maybe if you explain your intent, we may be able to give a more focused response.

For a reason I don’t understand my newer PC has a different user name to the source PC although I am the only user. I wish to transfer Thunderbird to the newer PC (currently running Win 10) whereas the source PC (also running Win 10) is not upgradeable and will be retired! I guess my query is should I set up a new user on the newer PC to match that of the source? Thanks Terry

The user name at c:\users\<userid> is irrelevant to thunderbird. That variable is established when you turn PC on the first time and respond to a few Windows questions. What is important is pasting the copied thunderbird folder at the ...appdata\roamig folder to overlay the existing one.