You need to copy the Thunderbird folder from AppData\Roaming in your old user account to the same place in your new user account.

Or did Microsoft helpfully obliterate the old user account's data?

For some reason, I can't paste that folder onto the new new account. Maybe because there's already the Thunderbird folder, perhaps created when the account was created.

I presume you have something like this: This one is original User Account name containg the old thunderbird data.

Logon to old User Account. C:\Users\<OLD Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\

Make sure Thunderbird is closed/exited.

Access old User Account and copy the Thunderbird folder to an external drive.

then logon to new Windows User Account. I'm assuming Thunderbird is set up to run on all the Windows 'User Accounts'. Exit thunderbird if it is open.

nothing can overwrite if you have Thunderbird open.

Access the new : C:\Users\<NEW Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\

Does this Roaming\Thunderbird folder ready exist ? I'm presuming yes.

Check the Roaming\Thunderbird folder Properties, on 'Security' tab, do you have 'full control' in permissions for both the old and the new User Accounts on the computer?

Copy paste the Roaming\Thunderbird folder from the external drive into the new User Account\AppData\Roaming folder overwriting the default "Thunderbird" folder. Thus replacing new 'Thunderbird' folder with old one.