Serial Number is not populated for Next Gen Juniper Firewall Devices. In case of Juniper device,

JNXBox MIB( OID 1.3.6.1.4.1.2636.3.1.3 ) is the one that we have used to get the serial number for Juniper devices

Firewall discovery flow goes like below.

1.Shazzam will probe snmp port (161) and fetches device details

2.After that SNMP-Classify port will be triggered. Add SNMP. Filed 1.3.6.1.4.1.2636.3.1.3 in SNMP - Classify probe and changes parsing logic into SnmpIdentityInfoParser script include.