Note: All IMAP usage from October 2022 will require Device Password.
Open Thunderbird and find the menu that says “Create a new account”
In the next window, “Would you like a new email address?”: click on “Skip this and use my existing email”
In the “Mail Account Setup” :
Email address: firstname.lastname@example.org
Password: Your password
Click on “Continue”
Next window: click on “Manual config”
Incoming : Choose IMAP, Server hostname: mail.psmail.net, Port: 993, SSL: SSL/TLS, Authentication: Normal Password
Outgoing: Choose SMTP, Server hostname: mail.psmail.net, Port: 465, SSL: SSL, Authentication: Normal Password.
Password: your password.
Use Test or Re-Test to make sure all these works correct.