not when there is no routing between the nets. So it all depends on what you are using as the firewall. For traffic to pass between networks, something has to perform the routing function. A firewall is just a router that has rules to manage traffic is able to be routed - o.k. that is a very simplistic description.

So there is no way to route between the various SSIDs except for what I want to allow. And my "guest" network cannot route to any of the other SSID nets. My guest network is just as much "behind" a firewall as the other nets.