Does the user name/email address in the 'From' section of your email (1) match the User name in your SMTP Settings (2) and the email address in your Server Settings (3)?

1 - Hamburger (top right)/Account Settings - click on the name of the account through which you are sending the email. On the right is the return email address called 'Email Address' unless you have put another email address in 'Reply-to Address'.

2 - Hamburger (top right)/Account Settings/Outgoing Server (SMTP). Highlight the server through which you are sending the email and the User Name is displayed at the bottom. It can be changed using the 'Edit' button.

3 - Hamburger (top right)/Account Settings - click on the 'Server Settings' under the account through which you are sending the email. The email address is in the 'User Name' field.

The above did not work. However, I logged into the Startmail.com web mail and added my newest laptop as a trusted device. It then gave me a password for the new laptop. Upon entering that new password into Thunderbird when it ask for a password to retrieve email, I entered the password for the new PC and it worked. The PC password is not the same password used to access the website email. Nowhere on the internet searches did I find this. I just found it by accident. You may share this tidbit if you wish.

Not sure what you mean by having found it by accident, but FWIW the procedure to configure a mail client to use with StartMail seems to be documented in StartMail Support website:

https://support.startmail.com/hc/en-us/categories/360001112237-Email-Clients