Do you know the password of the Mozilla Account used for Sync as that is mandatory to be able to connect to the Sync server ? If you will have to reset the password for the Mozilla account because you do not remember this password then that 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. You can install the About Sync extension to check what data is stored on the Sync server via the <b>about:sync</b> page provided by this extension. *About Sync https://addons.mozilla.org/firefox/addon/about-sync/ Close and restart Firefox to access the <b>about:sync</b> page added by this extension (see the about:about page for possible about pages).