Passwords can only be set or modified via the webmail account accessed using a browser. As you cannot remember the password to logon to that account I suggest you use the 'Forgotten Password' option and try to regain access to your webmail account. Once you have managed to get access to the webmail account then you can use that password in Thunderbird.

You could see if that pasword is still stored in Thunderbird. But the instructions vary depending upon what version you are using, so we would need to know that information.

In current version : Menu app icon > Preferences > Privacy & Security Under 'Passwords' section Click on 'Saved Passwords' click on 'Show Passwords'

In version 68 Options > Security > Passwords Click on 'Saved Passwords' click on 'Show Passwords' see image below