If you add the certificate as authority then it should be used as authority, i.e. for signing other certificates. If you instead use it as a server certificate (i.e. as end entity and not authority) then it should not be added as authority to firefox but instead as server certificate. This will be automatically done if the certificate is not known and you click through the certificate error messages when connecting to your site and accept the sites certificate permanently.
You should also make sure that your certificates contains the necessary key purpose to be used as a server certificate.
