Did you forget the password of the Sync account ? Did you previously had problems with syncing data ? ---- Data currently stored on a specific device isn't affected if you would reset the password of the Sync account. A reset only affects data stored on the Sync server and you would merely have to reconnect each device using the new password. You would need to use a recovery key to regain access to data stored on the Sync server. To be more safe you can disconnect other devices before resetting the password and backup profile data on desktop devices. *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles