This will help me monitor if the firewall service is running or not from a remote monitoring server or a monitoring system like Uptime Robot
Can you please tell me how can i find out what port number; is the firewall process listening ON; inorder to monitor the firewall service from a remote system?
The Firewall isn't a service that listens on a particular port so netstat will not be able to identify if it is running like it can for other services (like apache).
You could use firewall-cmd to enquire on the firewall status like this:
A firewall is not a service, but a software controlling the network operation of a system:
Let us start with a router: a router basically is a system with two network interfaces which stand in different IP-networks and a machinery in between which relays traffice between these two networks from one interface to the other, based on some rules (the "routing table"). Routing takes place on layer 3 of the network and hence the router "operates at layer 3".
A firewall is a similar machinery but operates on (more correctly: on top of) layer 4: again two network interfaces homed in different IP-networks, but the machinery (usually some software - the firewall process) in between operates on layer 4 and relays layer-4-packets (TCP, UDP, and similar protocols) between these two interfaces.
Per default all packets are "not relayed" (blocked) this way. By configuring rules some of these packets are allowed nonetheless and therefore indeed relayed.
There are two fundamental ways such firewalls operate: packet-filtering and stateful inspection.
In packet filtering the rules state which port of a certain IP on one side may communicate with which port on which IP on the other side. No effort is made to monitor the communication other than allowing some IP/port/destination/origin-combinations and blocking everything else.
Stateful inspection is different: packets are interpreted and their content is checked. Packets (or, rather, whole connections) are allowed/disallowed based on these checks. This needs a lot more computing and is a lot more intrusive than packet filtering and therefore real-world firewalls are usually a combination of both packet filters and some stateful inspection.