Welcome to the site Kenny
Thunderbird remembers your username/password for each account and outgoing server,and stores them in the password manager.
If ya try to access a server in which you dont have a username or password saved,it should prompt you to enter it and offer to remember what you have entered
My parents used to use Thunderbird so I know somewhat of it and I hope I have added to helping you! (Enough ppl help me here after all.. This is a good site)