Here is my situation. My ISP is Comcast and can always see my Mail if I log into Comcast. BUT, I find old and stogy and I have used Thunderbird for many years. Occasionally I do something and have to go to Comcast for information about the Inbound Ports and the Outbound Ports to undo my errors.

SO, you might find that your Log-In may be pushing all the way through to your ISP, not just TB!!

There's nothing like trying it!

I ... cannot remember the password.

Check with your email provider about password reset options.

I went to new site and changed Mozilla password

There is neither a "Mozilla password" nor one for Thunderbird. You do need a password to login to your email provider's server though. To change your account password you'd have to login to your account via browser. You can tell Thunderbird to remember your account password, so that you'd not have to type it again an again every time it is needed. Once you changed your account password Thunderbird will prompt you for the new password.

There was a problem with the support system at the time of your posting, so you might not have gotten a notice of the response(s) above. If you are still experiencing trouble when using the newest version, please post a new question using https://support.mozilla.org/en-US/questions/new/thunderbird