I have an existing Gmail account. I have repeatedly tried to add that account to Thunderbird. I enter all the required data and click on done. I am sent to Gmail to login to authorize Thunderbird access. When I click Allow Thunderbird indicates that either the username or password is invalid. Both are valid as I use them both to login to my Gmail account. I have Imap turned on for that Gmail account. I have checked all the Gmail suggested settings and all looks ok.