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.