There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

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

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

Is there way to solve the issue? In other browser, there is a pop up window to install security software which I installed. Is it because the security software is not compatible with the firefox? FYI, I can visit the same website with the firefox mobile.

Thank you.

When I try the link, I get;

Secure Connection Failed

An error occurred during a connection to project.seoul.go.kr. Cannot communicate securely with peer: no common encryption algorithm(s). Error code: SSL_ERROR_NO_CYPHER_OVERLAP

I did a web search. None of the links I was shown connect.

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

Firefox will just show the SSL_ERROR_NO_CYPHER_OVERLAP error when encountered any site using the RC4 cipher.

While SSL_ERROR_NO_CYPHER_OVERLAP could be caused by restrictions in Firefox, it more often seems to be caused by changes to server settings that were not tested with Firefox.

In this case, it seem the server is poorly configured (grade "F") and definitely needs an update. For now, you'll need to use a different browser to access it. https://www.ssllabs.com/ssltest/analyze.html?d=project.seoul.go.kr