Is the intent of this site to stick to a core Unix functionality and not go into topics like firewalls? It seems like there is some cross-over with serverfault.com when you start getting into some topics.
This site technically has 100% crossover with SU (it's a subset of the questions allowed there), so I wouldn't worry about potential SF crossover.
What exactly do you mean by "topics like firewalls"? A question like this I would definitely consider on-topic here:
How do I write an iptables rule to do X
Um... depends obviously it should be related to a firewall on a unix/linux system (PF and iptables comes to mind. no cisco questions) I'd think it should be more syntax than anything. Though personally I think the Network guys deserve there own Q&A site like this area51 proposal. Because networking isn't really Servers. IMO SF and SU are TOO general.
My feeling would be that questions specifically relating to Linux and Unix systems would be on topic, so I'd be comfortable with questions on configuring Unix/Linux systems. Once the questions leave the box, I'd consider them off-topic.