Pfsense is, in turn, a fork of m0n0wall. It's also important to note that pretty much all of the functionality of all aforementioned firewalls is part of FreeBS

Pfsense is, in turn, a fork of m0n0wall. It's also important to note that pretty much all of the functionality of all aforementioned firewalls is part of FreeBSD or from packages installed behind the scenes. The configuration UX is the most visible differentiator but they also have different strategies when it comes to pushing updates and testing/validation. But the actual heavy lifting of these router/firewall systems is done by code that ships with FreeBSD.