Please do not post your email address in a publc forum because it is of no use to anyone except those who may want to abuse it. All comments in this question forum are auto sent to the email address you used for the Firefox Account. It is also visible to anyone in the world as it is a public forum. If you want to get back into your original gmail account then that is between you and gmail. First you have to regain access to your account and be able to see all emails in the webmail account before you can then reset access in the Thunderbird account. Please see the information at this link as it may be of help in getting that all sorted out. * https://support.google.com/accounts/answer/6294825?hl=en For your original gmail email address which you set up as a mail account in Thunderbird. It is likely that the account cannot access the server because if your google account got hacked then the hacker would have changed the password and that would prompt a new OAuth2 being set up for the mail account. But if you do not know the 'new' password then it will not connect. But do you still see all the previously downloaded emails in that gmail mail account? Did you have it set up as IMAP or POP?