The deal with mine, is the encryption is left to the client. The server just stores a binary "thing." That "thing" can be an encrypted envelope, managed by the client[s]. Since it's a closed-loop system, then a key could be shared programmatically, outside the purview of the server.
I don't think Firefox sync makes it very easy for devs to build other encrypted applications. Even if they did however, there are a lot of challenges with encrypted applications that they just don't take care of. Not to mention that they are very we centric and don't have developer APIs in many languages.
