This change is only a change to the name of this account, nothing else changed and this is still about the ''accounts.firefox''.''com'' website, so likely something else went wrong. *https://support.mozilla.org/en-US/kb/firefox-accounts-renamed-mozilla-accounts Did you possibly reset the password of the Sync account because you forgot the password? Resetting the password for the Mozilla account because you do not remember the password will remove all data stored on the Sync server because this password is used to encrypt your data locally for security reasons before it is uploaded to the Sync server and only this password or a possible recovery key can decrypt this data.