The default is set up outgoing servers (SMTP) in the account settings tab.

Do you have separate smtp servers for each account? If not, and they all send on the same smtp server, recipients might see your messages as having been sent from one account, no matter which account appears in the From field. This is especially true with gmail accounts. If you have a distinct smtp for each account, and set each account to send through the corresponding smtp, you can choose the sending account by clicking the From drop-down, and recipients will see and reply to that account.

https://support.mozilla.org/en-US/questions/1350593