Don't you need to set-up the accounts in the first place? In Thunderbird it's just an ad-hoc text entry field, useful for when you are using a catch-all account and you need to respond from a random service specific email address.

For example say my account is [email protected] but fetchmail pulls in my gmail emails. If I want to send from [email protected] or [email protected] I don't need to configure the three accounts separately I just edit the from field as I'm composing the emails.

You literally can't do what I said in my comment and you explained this in your comment "pick any of my emails or domains from the dropdown" (which means you had added those beforehand) and you still said you can do that. I hope it was just an oversight and not the belief :)

Ah! I misinterpreted. Yes all of the accounts have to be “added” prior to this functionality in Apple mail. I stand corrected