Follow the steps to configure your POP3 accounts in Mozilla Thunder Bird

1.  Open Thunder Bird.
2.  Choose >> Tools >> Account Settings.
3.  Click “Add Accounts”.
4.  In pop up Window choose >> “Email” >> click Next.
5.   In the next page Type your name
6.  Enter your full email address  eg: xxx@domainname.com and click Next.
7.  Choose “POP” and type your pop3 settings mail.domainname.com and clik Next
8.  In the next page you can type your desired name for outgoing mails.
9.  Next >> Next >> Finish.

Then
1.  Click Outgoing Server (SMTP)
2.  Click Add
3.  In description : specify your domainname
4.  In server name : mail.domainname.com
5. In username : xxxx@domainname.com
6.  Use Secure Connection : TLS, if Available.

Now your email is configured in Mozilla Thunder Bird.