Remember sync isn't a backup system it uses the sync server to sync between devices. If you sync to Mozilla account without using backup data to restore the profile and bookmark or sessions any sync will clear what was there previously since it consider that a new sync. So any sync data on the sync server is gone as result of your sync data with the sync server. And if you don't remember the Mozilla account password that renders any data link to prior account gone as well as no users on here has access to your Mozilla account to help.