If this is your first time starting Thunderbird, it should automatically come up with a Set Up an Existing Email Account window as shown in the image. If you already have accounts set up and are looking to add another, click on Local Folders in the lefthand menu bar and then Email under "Set up an Account"
Enter your Name, Email and Password and click on Continue
Verify that the server settings say the following (replacing domain.tld with your domain):
Incoming: IMAP mail.domain.tld SSL
Outgoing: SMTP mail.domain.tld SSL
Username: youremailaddress@domain.tld
If everything looks good, hit Done, and your setup should be done! If not, click on Manual Config
Make sure the server host name for both Incoming and Outgoing is mail.domain.tld
If you are configuring IMAP, select SSL (or SSL/TLS), using port 993, and Normal Password authentication.
If you are configuring POP3, select SSL (or SSL/TLS), using port 995, and Normal Password authentication. (Note that we do not suggest POP3 setups except in special cases, ask us for more info if needed)
For SMTP, select SSL/TLS, using port 465 and Normal Password authentication.
Make sure that for the Username under both Incoming and Outgoing your full email address is filled in
Click on Done
Your email is now setup! If you'd like, send a email to yourself to test functionality