You can enable this now in Firefox 62 (latest stable)
> In advance of removing all trust for Symantec-issued certificates in Firefox 63, a preference was added that allows users to distrust certificates issued by Symantec. To use this preference, go to about:config in the address bar and set the preference "security.pki.distrust_ca_policy" to 2.
> In advance of removing all trust for Symantec-issued certificates in Firefox 63, a preference was added that allows users to distrust certificates issued by Symantec. To use this preference, go to about:config in the address bar and set the preference "security.pki.distrust_ca_policy" to 2.