Sometimes, the real error is that the server name, security setting, spelling of email address cause the issue, resulting in the 'incorrect password' message.

When I try to set up my existing email address (enter full name, email address and password), Thunderbird finds a configuration for POP3 (incoming: POP3, STARTTLS, pop3.starstream.net; outgoing: SMTP, no encryption, smtp.starstream.net and Username: jmcarron). I then click on Done and get the error message Unable to log in at server. I'm using the exact same password that I use successfully logging into Wave webmail. I'm at a loss for what to try next.

Do you have antivirus other than windows Defender? That is a regular cause of authentication problems. THe issue can arise from mail scanning, but in the setup wizard it is more often phishing or encrypted communication scanning that is the error.

I run Linux, not Windows, so I don't use Windows Defender.

Linux huh, that would have been good in the initial post, along with the distribution and the source of the install files.

Are you using one of these fancy new sandboxed applications that simply does not have permissions to actually function. Canonical adopted them in their last release and hence all the derivative distributions. But I am still guessing as you did not mention what repository you used or what distribution. But SNAPs have become something of an issue for some Linux users. https://snapcraft.io/docs/security-policies

The OS is Ubuntu, release is 24.04. Is Thunderbird one of the fancy new sandboxed applications you are referring to?