If IPv6 is not needed, then optionally disable it to simplify network management.
Navigate to System → Advanced → Networking:
NOTE: This sets up some firewall rules behind the scenes, but it does not stop pfSense from generating IPv6 traffic.
Navigate to Services → DHCPv6 Relay:
In DHCP6 Relay Configuration:
Disable IPv6 on each of the interfaces:
Navigate to Interfaces → WAN (pppoe0).
In General Configuration:
Navigate to Interfaces → LAN (em1).
In General Configuration:
Navigate to System → Routing → Gateways.
In Default Gateway:
Firewall rules to block IPv6.
NOTE: This is not necessarily needed, but a precaution if something is not working or gets misconfigured.
Navigate to Firewall → Rules → LAN.
NOTE: Do not delete the IPv6 rule, as this may be wanted later, and easier then to just enable.