Mozilla’s Sync Service

This document describes how Sync is deployed at Mozilla.


This document is not complete. Consult the Services team for authoritative information.


Mozilla’s Sync service is comprised of the following services:

Mozilla operates many instances of the Storage Service. We call these nodes. Each node is independent from the others and has no knowledge that other nodes exist.

When a new account is provisioned, the Registration Service assigns a user to a node. The node is chosen based on which nodes have capacity, etc. After node assignment, clients connect directly to that specific node. All Sync operations are performed against that client’s assigned Sync node.

The user registration service is hosted on If you download Firefox from Mozilla and set up Sync, this is where it will connect by default.

Easy Setup Service

Mozilla hosts an instance of the Easy Setup service at When you pair two devices by entering codes, they communicate through this service.

Crypto Record Semantics

Storage Format 6 does not explicitly define semantics for how crypto records are managed, leaving it up to the clients to agree on behavior. This section documents the behavior in Mozilla clients.

Clients and Key Management

Sync clients can differ in their abilities to manage keys and their associated crypto records on the server. There exist 2 tiers of clients:

  • Tier 1 Client - Supports key generation and management.
  • Tier 2 Client - Supports key consumption only.

Tier 1 clients are full Sync clients. They can provision accounts from empty servers, reset server data, change keys around, etc. Tier 2 clients are simpler clients that only support reading of keys (no writing).

The main reason why different tiers of clients exist is that cryptography, security, and the management of keys is hard and that these problems should be left to professionals. It is extremely easy for a client to introduce subtle differences that could compromise the integrity of data security. By providing a facility for clients that don’t modify keys, we are reducing the surface area on which a new client may error as well as decreasing the number of clients which need to be validated for proper behavior.

Mozilla provides the following Tier 1 Clients:

  • Firefox (on desktop)
  • Firefox (on mobile - aka Fennec)

Tier 2 Client Behavior

Tier 2 clients never perform updates to the crypto collection. Instead, they read records and get the data they need. If the data they need is unavailable (i.e. the record it wants isn’t found), it gives up and tries again later.

Tier 2 clients do support creating new collections on the server. When a Tier 2 client wishes to create a new collection, it will need to use a Collection Key Bundle for that collection. Normally, a new Collection Key Bundle would be created and uploaded. However, since Tier 2 clients must not modify the crypto collection, they resort to other means.