You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates. *Click the link at the bottom of the error page: "I Understand the Risks" *Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate" *Click the "View" button and inspect the certificate and check who is the issuer. You can see more details like the intermediate certificates that are used in the Details tab. If this didn't work then open this chrome URI by pasting or typing this URI in the location/address bar to open the "Add Security Exception" window and check the certificate: * chrome://pippki/content/exceptionDialog.xul In the location field type/paste the URL of the website * retrieve the certificate via the "Get certificate" button * inspect the certificate via the "View..." button ---- You can check the connection settings. *Tools > Options > Advanced > Network : Connection > Settings *https://support.mozilla.org/kb/Options+window+-+Advanced+panel If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly. See "Firefox connection settings": *https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can Boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps. *http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/