alt Hacker NewsNAT gateways that utilize connection tracking are effectively stateful firewalls. Whether a separate set of ‘firewall’ rules does much good because most SNAT implementations by necessity duplicate this functionality is a bit ignorant, IMO.
Meanwhile, an IPv6 network behind your average Linux-based home router is 2-3 nftables rules to lock down in a similar fashion.
It's also trivial to roll your own version of dropbox. With IPv6 it's possible to fail to configure those nftables rules. The firewall could be turned off.
In theory you could turn off IPv4 NAT as well but in practice most ISPs will only give you a single address. That makes it functionally impossible to misconfigure. I inadvertently plugged the WAN cable directly into my LAN one time and my ISP's DHCP server promptly banned my ONT entirely.