Have you updated your security software recently? Kaspersky, AVAST 2015, ESET, Microsoft Family Security, and BitDefender include a feature which scans your web traffic for threats. For HTTP this is seamless, but for HTTPS, the software needs to present a fake certificate to Firefox. Unfortunately, the same symptoms can be caused by malware. To help sort it out, you can inspect the issuer information for the certificate. ''If you added an exception for a site:'' While viewing the site, go ahead and click the padlock icon (or gray ! triangle, if there is mixed content) in the address bar, then More Information, then View Certificate. What do you see under "Issued by"? ''If you have not added an exception but the error page shows an "I understand the risks" section:'' You can expand that section and use the Add Exception dialog to view the certificate. What do you see under "Issued by"? You don't actually need to go through with adding an exception; you can cancel after getting the information.