How to get a Group of e-mail addresses into the TO field of a message

What do you mean by "groups of email addresses"? Are you referring to Thunderbird's Mailing List feature? This article described how to use Mailing Lists. It's written with an older version of Thunderbird in mind, but it's still relevant: https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird A variation of the above: select the containing Address Book in the Address Book tab and you will see the Mailing list listed among the other addresses. Select it, and in the right-hand pane, click the "Write" button.