Sync isn't designed to be used as a backup device, but merely to sync data between multiple devices, so the outcome can be uncertain if you try to restore personal data via Sync. It is not guaranteed that you can restore data from the Sync server in case of problems with your OS or because you otherwise lost data. Best is always to backup your personal data locally to be sure to be able to recover from issues. *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles If you did reset the password of the Firefox Sync Account because you forgot this password then this reset will have removed all data stores on the Sync server. This is because this password is used to encrypt the data locally before uploading to the Sync server, so only that original password can decrypt content that is stored on the Sync server.