The first time you send mail in a newly-installed account, a small dialogue window should pop up asking you to enter your email password for that account. (There's an option for TBird to remember the password too.) Can you please check whether the dialogue window has popped up but is hidden behind the main TBird window or some other window on your screen? It might be there but you can't see it. If that's not the case then there are two other things to check: 1) In Account Settings, select Outgoing Server (SMTP) and make sure that all the server settings are correct; 2) In Preferences > Security > Passwords > Saved passwords, see whether the correct password is saved for the outgoing server for each of your accounts.