With the newest Firefox, some Security Certificates have been disabled because of issues with new mal-ware. Can you check with the site on what Security Certificates they are using? (SSL and TSL encryption protocols) https://developer.mozilla.org/en-US/Firefox/Releases/37/Site_Compatibility#Security https://blog.mozilla.org/security/2014/09/23/phasing-out-certificates-with-sha-1-based-signature-algorithms/ https://blog.mozilla.org/security/2015/01/28/phase-2-phasing-out-certificates-with-1024-bit-rsa-keys/ https://blog.mozilla.org/security/2014/10/14/the-poodle-attack-and-the-end-of-ssl-3-0/ Run your website through https://www.ssllabs.com/ and you can see all security issues that need to be fixed. [https://www.ssllabs.com/projects/index.html Qualys SSL Labs Projects] [https://www.ssllabs.com/ssltest/viewMyClient.html SSL/TLS Capabilities of Your Browser] The SSL client test shows the SSL/TLS capabilities of your browser.