'send 'mass' (messages addressed to >1 recipient) mailings to individuals seperately' button

[BlipBertMon]

The topic pretty much concisely describes the objective functionality.

This specifically mitigates using Cc: and Bcc: fields for 2nd and subsequent recipients who are otherwise at parity with one another, and prevents leaking e-mail addresses among the >1 recipients, and reduces the iterative finding and 'edit message as new' slog to just a different, single delivery-mode click.

[BlipBertMon]

... and doesn't effectively lower the priority of any message with the 'undisclosed-recipients' address.

[Jon]

Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.

[MattAuSupport]

The mail merge addon offers what you seek as well as numerous other personalisation options.

https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/?src=ss