KB ID 0001727
Firefox Certificate Settings
So the machine I’m using DOES trust the CA that issued that certificate, (it’s a FortiGate firewall) But the BROWSER does not. (Firefox maintains its own list of certificates, and more importantly which CA certificates it will trust). Essentially the browser is trying to protect you from a MITM attack.
Browse to about:prefernces#privacy > Certificates > View Certificates.
Navigate to the CA certificate for the authority that signed the certificate(s) you are having a problem with, and import it > Select ‘Trust this CA to identify websites” > OK
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