The firewall does not trust intermediate CAs by default because intermediate CAs are not a part of the chain of trust between the firewall and the trusted root CA. You must manually add any intermediate CAs that you want the firewall to trust, along with any additional trusted enterprise CAs that your organization requires (Device > Certificate Management > Certificates > Device Certificates).
If you disabled a CA, you can re-Enable it. Select the CA and Disable it. You might use this option to trust only specific CAs or to disable all other CAs and trust only your local CA. Select and Export the CA certificate. You can import into another system or view the certificate offline.