There is security software like Avast/AVG, Kaspersky, BitDefender, ESET and some times McAfee that intercept secure connections and send their own certificate or block with their scanners. If you are running any of the above software please check their community forum for a work around to apply settings. Note : below is now a example *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/secure-connection-failed-error-message *https://support.mozilla.org/en-US/kb/connection-untrusted-error-message