What is the exact error message? 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 https://www.bing.com/search?q=web+site+access+denied

If it's urgent, and you do not think it's a malware problem on your computer, I suggest using a different browser for that site.

Are you sure you need to use a secure HTTPS connection and not an open HTTP connection to access that website (or at least its home page)? See also: *https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER