*MOZILLA_PKIX_ERROR_MITM_DETECTED *uses an invalid security certificate SSL_ERROR_BAD_CERT_DOMAIN *configured their website improperly How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER 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]

Firefox caches intermediate certificates send by a server for future use and can use a stored intermediate certificate in case the server sends an incomplete certificate chain like in this case.<br>If you have never visited a server that has send this specific certificate (i.e. it isn't cached) then you get the SEC_ERROR_UNKNOWN_ISSUER error, otherwise the page will load without problems.<br>So on Windows you have this intermediate certificate cached and on Linux you haven't yet cached this intermediate certificate. Cached intermediate certificates show as "Software Security Device" in the Certificate Manager. *Options/Preferences -> Privacy & Security<br>Certificates: View Certificates -> Authorities