Re: What is error code:sec_error_unknown_issuer? I am getting this from my bank's billpay printout for electronic bills.
You can check who issued that certificate.
You can retrieve the certificate to check who issued the certificate.
You should always do that to see if that helps to identify the problem (missing intermediate certificate or something else).
Click the link at the bottom of the error page: "I Understand the Risks"
Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".
Click the "View..." button to inspect the certificate and check who is the issuer.
If you can't add an exception, but get a pop-up with the error message then you can check the pref browser.xul.error_pages.enabled on the about:config page and make sure that the value is set to the true (default).