What is the exact error message? There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connections and send their own certificate. https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message https://support.mozilla.org/en-US/kb/connection-untrusted-error-message http://kb.mozillazine.org/Error_loading_websites

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/en-US/kb/what-does-your-connection-is-not-secure-mean *https://support.mozilla.org/en-US/kb/troubleshoot-time-errors-secure-websites *https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message ---- You can check if there is more detail available about the issuer of the certificate. *click the "Advanced" button show more detail *click the blue SEC_ERROR_UNKNOWN_ISSUER message to show the certificate chain *click "Copy text to clipboard" and paste the base64 certificate chain text in a reply If clicking the SEC_ERROR_UNKNOWN_ISSUER text doesn't provide the certificate chain then try these steps to inspect the certificate. *open the Server tab in the Certificate Manager **Options/Preferences -> Privacy & Security<br>Certificates: View Certificates -> Servers: "Add Exception" *paste the URL of the website (https://xxx.xxx) in it's Location field. Let Firefox retrieve the certificate -> "Get Certificate" *click the "View" button and inspect the certificate You can see detail like the issuer of the certificate and intermediate certificates in the Details tab.