re :, does each recipient get an email with its email address in the To: field? Do the other email addresses also get the addresses of the other recipients?

If you enter several email addresses in a TO field then all recipients of that email will get a copy of that email and it will display all email addresses you put in the TO field. So everyone will see who else got that email.

Just to be clear . . .

In an email addressed to a, b, and c in the "to" field, 3 emails will be created (one each for a, b, and c) and each email will have 3 "to" fields each containing the addresses of a, b, an c?

Just to be clear . . . In an email addressed to a, b, and c in the "to" field, 3 emails will be created (one each for a, b, and c) and each email will have 3 "to" fields each containing the addresses of a, b, an c?

Yes.

The alternative is to use this addon to send individual emails with a single To: https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/