Hey there! I have an iMac (Retina 4K, 21.5-inch, 2017) running under macOS bigsur and I have no issues! I think this issue has been fixed. Well I guess if you still have this problem why not restart your mac? You got the 500 error response code because the server encountered an unexpected condition that prevented it from fulfilling the request. Hope I helped! Kind regards, Bithiah

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 [https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors?redirectlocale=en-US&redirectslug=Error+loading+web+sites Websites don't load - troubleshoot and fix error messages] http://kb.mozillazine.org/Error_loading_websites [https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean What do the security warning codes mean]