Securing your data with Sync involves creating a unique password, which plays a crucial role in encrypting your data for complete privacy. This encryption is end-to-end: your data is encrypted before it ever leaves your browser and can only be decrypted by another instance of Firefox. Once your data reaches a Mozilla-operated server for storage, it's already in an encrypted state, ensuring that not even Mozilla can access or decrypt this information.

support.mozilla.org/en-US/kb/sync