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 SSL_ERROR_NO_CYPHER_OVERLAP Your server apparently doesn't offer any cipher suites necessary to establish a secure https connection that are supported in Firefox. You can check what kind of cipher suites Firefox can make use of by visiting; https://www.ssllabs.com/ssltest/viewMyClient.html