Firefox SYNC uses latest standards in cryptography to secure and encrypt your data. After you start the process of Sync operation you will create an account and a recovery key. That recovery key is used to encrypt the data which is later uploaded to a cloud server. Mozilla Corporation does not have access to your vault because the key which decrypts the data is known only by you and it's stored on your computer.
In other words, you are the only one who has access to the data you sync over Mozilla servers.