Thunderbird neither accepts or denies email passwords. Email providers servers do. Is your account POP or IMAP? If POP do you have less secure apps turned on at gmail? If IMAP are you using oauth2 authentication or normal password?

To access and edit what you told Thunderbird to remember go to... Options-Security-Passwords-Saved Passwords Click Show Passwords Right click the password and select Edit.

Do not bother doing this until you answer the questions about your setup.

I too have this problem. Google suggested that I turn don't allow less secure app. I did and no email from Google. Then I turn it off, as before, and still no emails. I restarted too.

Now what?