If you have 4 Yahoo, or any other type, accounts, there should be 4 smtp servers in Tools/Account Settings, Outgoing Server (SMTP) at the bottom of the left pane. Give each one a unique Description, and set the User Name as the email address for each account. Then, select an account in the left pane of Account Settings and click Outgoing Server (SMTP) in the right pane so the smtp server's User Name matches the User Name of the selected account. Once you have all the accounts lined up with their proper smtp, you might need to remove the passwords or oauth tokens from Saved Passwords in Preferences, then restart TB and enter the correct password in the OAuth window when asked to allow TB access (assuming you're using the Yahoo servers for the Frontier accounts). Similar topic: https://support.mozilla.org/en-US/questions/1324013