Thunderbird is provided by the FireFox Browser people, Mozilla. It's not a Windows Microsoft-default program.

"All software is updated."

Which might be why you are having issues. Updates will set new functions, features and settings.

You might start with this article:

https://proconnect.intuit.com/community/general-login/help/e-mailing-a-lacerte-client-file-to-anothe...