Sync isn't a backup system it sync between devices. If you login with new laptop without backup data present aka profile/bookmarks/session data it will clear the sync to match the new laptop login which most likely didn't have any data to sync to so the Sync clears out the old sync data rendering any data gone.

When Firefox sync's it sync's to a Sync server which is why if you ever change your password it removes what has been synced since the password is the encryption for the connection. It does not sync device to device only as above states.