The ProtonMail Bridge is a tool that utilizes Thunderbird and other similar clients’ capabilities via ProtonMail without compromising privacy; however, there might be problems that arise in the process. For instance, if you are trying to connect ProtonMail Bridge to Thunderbird or vice versa and it is not working, try checking if there are any updates to either of the clients since it is possible that any of them is outdated which can lead to compatibility issues.
Proceed to the ProtonMail Bridge and look at the connection settings. Make sure that any IMAP and SMTP provided in Thunderbird is correctly input. The correct ports usually are localhost 127.0.0.1 for the IMAP incoming and SMTP outgoing server. Further, check whether the password saved in Thunderbird corresponds to that provided in ProtonMail Bridge’s custom app password.
If the connection issues persist, both ProtonMail Bridge and the Thunderbird email client should be restarted in order to reconnect them to each other. For more consistent types of these measures, you can simply wait until they have delivered such a mailer and eliminate any firewall or VPN if possible which may inhibit the connection. They may also advise you on how to set up the application and troubleshoot conflicts with Thunderbird.

More Info: https://www.resetprotonpassword.com/

More Info: https://www.resetprotonpassword.com/how-to-reset-my-password-for-protonmail/