You can download and save the certificate, then import the certificate in the Firefox Certificate Manager and set the trust bit to identify websites when prompted to make the Federal Common Policy CA certificate act as a root certificate.

Firefox/Tools > Options > Advanced > Certificates/Encryption: View Certificates

This should install the required certificates.

Is this about a page with mixed content or about blocking plugins?

If content is missing or otherwise not working when a secure https connection is used then check if there is a shield icon to the left of the "Site Identity Button" (globe/padlock) on the location bar indicating that content is blocked.

• https://support.mozilla.org/kb/how-does-content-isnt-secure-affect-my-safety
• https://developer.mozilla.org/en/Security/MixedContent

• Toggle Mixed Active Content: https://addons.mozilla.org/firefox/addon/toggle-mixed-active-content/