The easiest way is to copy the entire profile. Those steps are here:

BACKUP from old PC

1. ensure thunderbird is not running
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

RESTORE to new PC

1. ensure thunderbird is installed and NOT running
2. enter %appdata% in search window on task bar and press Enter key
3. 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
4. start thunderbird and all should be there

BUT From your post, it appears you are using an older version. Is the version on new PC the same? If not, the safest conversion would be to install the same version on new PC. That would ensure total compatibility.

ALTERNATE APPROACH

Okay, if you don't want to do that, and just import the message folders, you can do this: - exit thunderbird - in windows file explorer, locate the Local Folders folder of old profile and copy the message folders (but not the msf files) to the Local Folders folder on new PC - if the account is POP, the account needs to have already been installed on new PC. Copying the message folders will overlay any messages you have already downloaded. If that isn't an issue, locate the Mail\<accountname> folder on old PC and copy everything there to same location on new PC.

Before trying to transfer or modifying folders, can I just uninstall the newest version and return to older one? By the way, I installed the older version about ten years ago and updated that one since. Also, though it shouldn't matter, my PC is old, running 32-bit Windows 7.

Yes, you could uninstall current version and go with the version you're using on old pc. If you're using 32-bit PC, you cannot use the latest release.

Thanks. I just want to clarify my question; if I uninstall the current version on this PC, will the older version be recovered completely or will I have to reinstall the older Thunderbird? By the way, the "new" version (Thunderbird 102.15.1) works just fine on my 32-bit PC. I haven't downloaded or installed the update to 115.10.1.

Ah! I got all of it back. Just Tools>Import>Select "Import from another Thunderbird installation" then choose for all four types and make sure the path is for the older profile. Thanks again!