They have the right intention. But how is this useful? As far as I can tell, it does not prevent FF from leaking information that helps FB generate a fingerprin

They have the right intention. But how is this useful? As far as I can tell, it does not prevent FF from leaking information that helps FB generate a fingerprint. I'd still reveal the same IP address (would be cool if Mozilla provided a VPN that all FB traffic goes through), the same browser, the same OS, screen resolution etc...