SirHumphreyAppleby:Aredwood:Check your firewall settings. It might be blocking ICMP packet too large notification packets.I'm not sure how the system detects if large MTU (RFC4638) support is available on the WAN, but the documentation for pfSense indicates it falls back to a lower value if not supported.
Setting the MTU to 1500 for PPPoE correctly bumps the values up to 1508 for the Ethernet and VLAN interfaces (em driver), but the MTU remains fixed at 1492, suggesting there isn't end-to-end support. If that's the case, that may explain why the ERL misbehaves.
@Taubin... I'd not risk rebooting, just in case. Back in the early days with Bigpipe, I did once in a while get an address via PPPoE (this is why I was looking in to issues such as race conditions and the order interfaces were brought up in pfSense). I didn't get 'reliable' IPv6 until I switched to IPoE, and then suddenly, it was gone.
The system detects large packet support by just sending out packets as normal. In IPv6, if a packet is too large for a router to forward onwards to the destination server. Instead of fragmenting the large packet into smaller ones. It drops the packet and sends a "packet is too large" ICMP reply. To tell your system what MTU to use for just that connection.
Even if your connection supports 1500MTU, there would probably be servers out there on the internet that are connected via tunnels or other connections that only support lower MTU values. So you would definitely have problems connecting to such servers if all of your packets are being dropped. And your computer is not getting the ICMP replies to tell it to use a lower MTU for connections to that server.



