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 *uses an invalid security certificate SSL_ERROR_BAD_CERT_DOMAIN *configured their website improperly How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER