You would be a lot better off if you virtualized everything and ran them as separate VMs, rather than trying to nest one inside the other.
I'm not sure if FreeNAS can run inside a jail, but even if it can, running any service like that on a firewall (even in a jail) is still questionable.
With a VM setup (e.g. ESX, KVM, etc) you get better separation between the hosts. You lose a little performance, but then you could also put other things on ther besides pfSense and FreeNAS, too.