- Did you disable UPnP on your router? If not, any device behind the router can simply ask the router to open a port, typically without authentication, bypassing this "firewall" completely.

- TURN and STUN trivially bypass this side-effect, and a side effect of that is a third party has to often be involved, which can be collecting data later leaked or used against you.

- The monstrosity of NAT is that it's the core thing that drives centralization - because of NAT any two Internet hosts generally have to involve a third party to communicate, a third party which again, can be collecting data later leaked or used against you.

If you don't care about the security implications of the above, then you don't really care about the "firewall" either.