If Firefox is backdoored, as per GP's hypothetical situation, your data can be sent to a sync server other than the one you configured. The only realistic way

If Firefox is backdoored, as per GP's hypothetical situation, your data can be sent to a sync server other than the one you configured. The only realistic way to stop it would be to only use a specific audited version of FF. If that's the case then there's no need to use your own sync server because you already know it's not backdoored.