You can check the date and time and time zone in the clock on your computer: (double) click the clock icon on the Windows Taskbar. *https://support.mozilla.org/kb/Secure+Connection+Failed 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<br>Check who is the issuer of the certificate You can see details like intermediate certificates that are used in the Detail tab. Who is the issuer of the certificate? There is security software like Avast and Kaspersky that intercept secure connections and send their own certificate. ---- If you can't inspect the certificate via "I Understand the Risks" then try this: Open the "Add Security Exception" window by pasting this chrome URL in the Firefox location/address bar and check the certificate: *chrome://pippki/content/exceptionDialog.xul In the location field of this window type or paste the URL of the website. *retrieve the certificate via the "Get certificate" button *click the "View..." button to inspect the certificate in the Certificate Viewer If necessary then you can attach a screenshot that shows the certificate viewer.