[firefox] Programmatically Install Certificate into Mozilla

Recent versions of Firefox support a policies.json file that will be applied to all Firefox profiles.

For CA certificates, you have some options, here's one example, tested with Linux/Ubuntu where I already have system-wide CA certs in /usr/local/share/ca-certificates:

In /usr/lib/firefox/distribution/policies.json

{
    "policies": {
        "Certificates": {
            "Install": [
                "/usr/local/share/ca-certificates/my-custom-root-ca.crt"
            ]
        }
    }
}

Support for Thunderbird is on its way.