There is security software like Avast/AVG, Kaspersky, BitDefender and ESET that intercept secure connections and send their own certificate. If you are running any of the above software please check their community forum for a work around to apply settings. *https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message#w_avast-and-avg-security-products These pages are also for the errors you may have besides relating to the above. *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 *https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER Please let us know if this solved your issue or if need further assistance.