Checking certificates requires that either a) The user have a complete set of CT Logs to check against or b) The user makes a request of a third-party server to verify the certificate. "a" is disk space and download uneconomical, "b" is a privacy concern. Mozilla has decided that it's value is not worth the privacy risk of yet. Time will tell if that's the correct answer; Chrome and Safari are likely enough to keep the CAs honest.