In some conditions when two Web Gateways are used with a Load Balancer, for example, a PAC script, Firefox can report that duplicate serial numbers are being used for HTTPS certificates. For HTTPS content scanning to work correctly, one of the Web Gateways must be designated as the HTTPS Certificate Master Server. This server will then be responsible for generating all the certificates for all the servers.
See Configuring the HTTPS Settings for details of how to designate an HTTPS Certificate Master Server.