There is security software like Avast and Kaspersky and BitDefender and ESET that intercept secure connections and send their own certificate. http://www.ehow.com/how_11385212_troubleshoot-reset-connection-firefox.html https://support.mozilla.org/en-US/kb/server-not-found-connection-problem https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can 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''' '''This Connection is Untrusted''' is sometimes caused because the computer system clock is wrong. Check the time / date / time zone settings.

thanks for your answer, however this doesn't solve the problem.

You need to install the root certificate from that proxy. If it works in other browser then check the certificate chain there and export the root certificate. Then you can import the root certificate in the Firefox Certificate Manager and set the appropriate trust bit to recognize the certificate for websites.