If you have a gmail IMAP account and you are using 'Authentication Method: 'Normal Password' This means you have not been using the method of Authentication as preferred by gmail which is OAuth2. It means in gmail webmail account you would have been using the 'less secure apps' option. Periodically, gmail may have blocked you and forced you to sign in to webmail and reset the 'less secure apps' option. Gmail do state that in their help pages claiming it is done to check you really are you. However, gmail is now moving away from allowing that 'less secure apps' option. As you can see from the link they sent to you, gmail now requires you to switch on '2-Step Verification' and then generate 'an app specific password'. In Thunderbird, when prompted to input password, you input the 'app generated password'.

BUT - There is another option - in Thunderbird gmail mail account - change the 'Authentication Method': to 'OAuth2' for both incoming and outgoing SMTP. See example images below. OAuth2 is actually the preferred method that gmail use for IMAP accounts. You need to make sure your Thunderbird Preferences are set to allow cookies. Menu app icon > Preferences > Privacy & Security Web Content : select 'Accept cookies from sites'

Restart Thunderbird Google will prompt for normal password and this process is to allow Thunderbird access to server. Then a token - long set of numbers/letters is stored in the passwords manager. From then on the token is used by thunderbird to access server.

If you choose to use the 'Authentcation Method : OAuth2' then you do not need to switch to 2 step nor need to generate any app password.

So you have two options and it is up to you on what you prefer to use.

Thank you, yes this was the problem.