There is security software like Avast and Kaspersky and BitDefender and ESET that intercept secure connections and send their own certificate.

http://www.ehow.com/how_11385212_troubleshoot-reset-connection-firefox.html

https://support.mozilla.org/en-US/kb/server-not-found-connection-problem

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

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://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

This Connection is Untrusted is sometimes caused because the computer system clock is wrong. Check the time / date / time zone settings.

What security software do you have?

Check out why the site is untrusted and click the "Advanced" button to expand this section to see extra details. If the certificate is not trusted because no issuer chain was provided (sec_error_unknown_issuer) then see if you can install this intermediate certificate from another source. Click the "Add Exception" button to inspect the certificate in the Certificate Viewer.

If you can't inspect the certificate via "Add Exception" then try this:

Open the "Add Security Exception" window by pasting this chrome URL in the Firefox location/address bar and check the certificate:

• chrome://pippki/content/exceptionDialog.xul

In the location field of this window type or paste the URL of the website with the https:// protocol prefix (https://xxx.xxx).

• retrieve the certificate via the "Get certificate" button
• click the "View..." button to inspect the certificate in the Certificate Viewer

You can inspect details like the issuer and the certificate chain in the Details tab of the Certificate Viewer. Check who is the issuer of the certificate. If necessary then please attach a screenshot that shows the Certificate Viewer with the issuer.