A broadcast by definition will reach all the hosts on the broadcast domain, this will include the switch and the SRX. If both are configured for relaying the DHCP Discover messages to the server, they will do it accordingly. The difference between the SRX and the switch sending the packets to the server, is that the packets from the switch could be dropped on the SRX if there is not a security-policy allowing these unicast packets. On the other hand, the unicast mesages sent from the SRX are generated from the firewall itself hence they dont require a security-policy permitting this traffic.
Hope this helps you.