How do you efficently copy recipients from old email to create a new mail/conversation in Thunderbird?

There is one other method that avoids the conversation-tree problem but has its own defect:

Copy recipients from email source

The source is accessible via Ctrl+u (or View > Message Source). You can copy several email addresses and paste them in the e.g. To-field of a new email. On pressing enter the string is split automatically and spread out in one field per address.

For this string it will work fine:

Alice Test <[email protected]>, Bob Test <[email protected]>

The defect: if there is a newline between Bob Test and <[email protected]>, Bob Test will be extracted to an extra field and sending the email will fail.