What they mean is that in Windows 10 a process can opt-in to some security features which mitigate this attack. Firefox/others will still be affected if they d

What they mean is that in Windows 10 a process can opt-in to some security features which mitigate this attack. Firefox/others will still be affected if they don't opt-in. Just in case anybody wonders, this mitigation is opt-in because what it does is it disables all win32k.sys calls (GDI, ...).