Step Wise Manual Fastmail mail settings for thunderbird
If you don't have a Fastmail mail address. you can create an email id from here then you can Fastmail email login.
Open Thunderbird Software App. Click on Email
When you click on the email, pop-up Set UP an Existing Account Screen.
Enter Your name, Fastmail Email address and Password then press Continue.
Click on the Manual config menu at bottom.
Enter Below Fastmail incoming server settings.
In incoming click on the dropdown menu and select IMAP.
Add server host :mail.messagingengine.com
Port: 993
select SSL/TLS
Enter Below Fastmail outgoing server settings.
In outgoing select IMAP.
Add server host :mail.messagingengine.com
Port: 587
select SSL/TLS
Tap on Done. Now you are ready to use your Fastmail email account on your Thunderbird.