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

It is possible that there is mixed content on the web page that is retrieved via an open HTTP connection. You can check the Web Console for more detail. *"3-bar" menu button or Tools -> Web Developer *https://developer.mozilla.org/en/Tools/Debugger See also: *https://support.mozilla.org/en-US/kb/mixed-content-blocking-firefox *https://support.mozilla.org/en-US/kb/control-center-site-privacy-and-security-firefox *https://developer.mozilla.org/Security/MixedContent