There used to be these sysctls in older versions of FreeBSD, viz:
net.inet.ip.fw.dyn_ack_lifetime=3600
net.inet.ip.fw.dyn_udp_lifetime=15
now on FreeBSD 12 sysctl reports that these don't exist.
How can I tune the lifetime for TCP/UDP for in kernel NAT?
ipfw_load="YES"