What site do these questions really belong to and can I have them moved to that site?
From my point of view they potentially belong on all sites. These days firewalls exist at almost every level, and so it is reasonable for questions about them to come from both end-users and professional system administrators.
The difference in my mind is the context of the question. If someone is asking how to setup a host based firewall for their personal system, or how to adjust the firewall on their broadband router to permit their TF2 server to work, then it almost certainly belongs on superuser.
If they are asking about how to configure a network firewall on a Linux-based router for their business then it belongs on serverfault.
The particular question you linked to is sort of an usual case where it appears that someone is trying to use NAT and SSH to kludge a solution together. I can easily see how that may be migrated to superuser. I am not sure that the question as written fits here. What this guy seems to be trying to do, doesn't really give me a professional system-admin vibe.
Several users from serverfault where just complaining that we didn't want junk coming to serverfault, so perhaps it got moved to superuser to avoid us.
Anyway my point is that if you interested in answering questions about a specific topic at both the professional level, and the home-user level you will probably need to accept that you need to monitor serverfault, superuser, unix.se, ubuntu.se, and even security.se. I can see that this may be frustrating and having your rep distributed between the sites is a bit annoying, but the sites are split based on audience instead of just based on the specific topic.