The documentation could use some help to work with Squeeze.

I added this near the top of /etc/init.d/fwbuilder:
### BEGIN INIT INFO
# Provides:          firewall
# Required-Start:    $network
# Required-Stop:     $network
# Default-Start:     S
# Default-Stop:      0 6
# Short-Description: Runs iptables firewall script
### END INIT INFO

I dropped a file with the following into /etc/dhcp3/dhclient-exit-hooks.d/:
RUN="yes"
if [ "$RUN" = "yes" ]; then
    case $reason in
        BOUND|RENEW|REBIND|REBOOT)
            if [ "$interface" = "eth1" ] && [ "$new_ip_address" != "$old_ip_address" ]; then
                invoke-rc.d fwbuilder restart
            fi
            ;;
    esac
fi