the safest way is to create a new account for tuta and then copy any desired messages from yahoo and then delete yahoo account

You can't add Tuta accounts to TB in the usual way, but there is an add-on that allows access:

https://www.reddit.com/r/tutanota/comments/v0edct/can_tutanota_be_used_on_thunderbird/

https://addons.thunderbird.net/en-us/thunderbird/addon/tuta-mail-quick-access/

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