'Username or password incorrect' is a standard error message that you get from Thunderbird when it fails to connect with an email server. It's not always helpful, in my opinion, because sometimes the real reason is different: you can have the correct username and password and still the error message appears. Are you trying to set up your Gmail account in Thunderbird for the first time? Gmail has some peculiarities, unlike most other email providers, and the process isn't always smooth. I recommend you start here: https://support.mozilla.org/en-US/kb/thunderbird-and-gmail If you have tried the automatic account configuration and are still having problems then you will need to check the settings, which are given here: https://support.google.com/mail/troubleshooter/1668960?rd=1 Notice that you have to enable Gmail to work with a client like Thunderbird: yes you can log into the account via webmail, but the server won't respond to an external client until you set it to do so.