The Mozilla Foundation Security Advisory's description of this flaw: The error page for sites with invalid TLS certificates was missing the activation-delay Firefox uses to protect prompts and permission dialogs from attacks that exploit human response time delays. If a malicious page elicited user clicks in precise locations immediately before navigating to a site with a certificate error and made the renderer extremely busy at the same time, it could create a gap between when the error page was loaded and when the display actually refreshed. With the right timing the elicited clicks could land in that gap and activate the button that overrides the certificate error for that site.