You will receive this error in Firefox if the SSL certificate has not been issued for your domain yet. Cloudflare offers two SSL options that activate in different ways that are outlined here:
Free Universal SSL: Activation of free SSL can take up to 24 hours. You will see different SSL error messages in different browsers until the certificate has been successfully issued by our vendors.
Standard SSL: The Standard SSL option is when you activate SSL through a paid Cloudflare plan. The activation process for this SSL option will typically take 10-15 minutes or so through our vendors.
Please see this KB article for more details.