Forum response - Some websites won't load after updating to version 51
Browsers look at a website's certificates so that your information remains safe when it connects you to that website. Recently, an issue has been discovered with two types of certificates: WoSign and StartCom. Websites that use these certificates will no longer work with major browsers, including Firefox. This is to ensure your information doesn't get stolen when you visit these websites.
For more information, see Distrusting New WoSign and StartCom Certificates.