foxsync has no server. It sends nothing to its authors and has no analytics.

After you pair a device, foxsync sends messages, such as approval requests and their answers, straight to that device over a WebRTC peer-to-peer link. Each message is encrypted and authenticated with keys from the pairing, on top of WebRTC's own encryption.

If you type a relay URL, foxsync sends the sealed pairing answer and reconnect offers through that relay. The relay is a server that you choose or run. It can see box ids, the IP addresses of both ends, the times and the size of each text. It cannot read the texts. foxsync does not provide a relay.

Pair records and keys stay in this browser. The phone page URL and the relay URL that you type stay in the add-on's local storage.