Screenshot of the error.

There is security software like Avast and Kaspersky and BitDefender and ESET that intercepts secure connections and sends their own certificate or that incorporates special web shielding features that can block content.

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?

cor-el said

There is security software like Avast and Kaspersky and BitDefender and ESET that intercepts secure connections and sends their own certificate or that incorporates special web shielding features that can block content. 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?

Since I am using a Mac, it was unnecessary for me to have an antivirus (I was using Avast at the time), and it's Web Shield technology could have hindered the security certificates Firefox uses in order to validate the authenticity of an https page. However, I will do some more experimenting and come to you guys with a reply for those of you that do need help. Thank you for the reminder that my antivirus could be the problem though!

kelvint said

Since I am using a Mac, it was unnecessary for me to have an antivirus...

That's a common misconception.

Of course I know it's not COMPLETELY unnecessary, but being a cautious computer user, since OS X is a Unix based operating system, I don't need to worry much about having an antivirus. I have Malwarebytes Anti-Malware in those rare occasions that I need to clean a virus.