To enable it, run
chmod +x /etc/rc.d/rc.firewalld

and add it to

/etc/rc.d/rc.local like so:

if [ -x /etc/rc.d/rc.firewalld ]; then
        /etc/rc.d/rc.firewalld start
fi

and the following to /etc/rc.d/rc.local_shutdown:

if [ -x /etc/rc.d/rc.firewalld ]; then
        /etc/rc.d/rc.firewalld stop
fi