The very long, very cryptic string underneath Saved Passwords is not your Google password. It is an OAuth2 authentication token which is stored there after you did successfully authenticate with your Google password the last time. OAuth2 is the Google preferred authentication method for some time now.

If you can't remember your Google account password you'd need to check with Google about password recovery options for your account.

Note, Yahoo also switched to OAuth2 authentication not too long ago. If you haven't set up your Yahoo account in Thunderbird for OAuth2 yet, you'll probably have to do that sooner or later to be able to receive mail.

To add christ1's information, google had an outage recently which caused some users' oauth tokens (the part stored in Thunderbird) to expire.

Ok, thank you both for the information. Just so I'm clear, Thunderbird will no longer be storing the actual passwords for gmail accounts because it is not possible for it to save those passwords?

Thanks again.

The short answer is yes.

The longer answer, to properly understand, is you need to know that no version of Thunderbird has ever saved a password for accounts that are set to the oauth authentication. It will only ever save a token, a recording of the fact that you have already authenticated to the resource you want to access, and which is used to allow you future access. It is this token that you had lost.