Please provide a link to the problem site.

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

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

Please do not create exceptions for sites that handle sensitive information. By saying that the certificate does not check out, Firefox is warning you that the site has not proven itself to be the actual site you want to visit and could be an impostor. It's important to investigate that problem first.

If you have already given this website your username and password, consider changing that on the "real" Schwab site here using a different browser or a different device in case your entire system is compromised, or phone Schwab and ask for your account to be locked temporarily as a precaution:

https://client.schwab.com/Login/SignOn/CustomerCenterLogin.aspx

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• click "Advanced" to expand the error message
• click "Add Exception" to open "Add Security Exception"
• alternatively you can open "Add Security Exception" by pasting this URL in the location/address bar:
  chrome://pippki/content/exceptionDialog.xul
  paste the URL of the website (https://xxx.xxx) in it's location field
• let Firefox retrieve the certificate -> "Get Certificate"
• click the "View" button and inspect the certificate
  check who is the issuer of the certificate

You can see details like intermediate certificates that are used in the Detail tab.

Who is the issuer of the certificate?

Can you attach a screenshot of the Certificate Viewer window?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB