Thanks for your insightful comment. It turns our you were right. I had set up a bridge to get my 5 static IP addresses from Comcast onto a private VLAN. Somewhere–and I'm still not sure where, the RA packets were leaking onto my LAN. RA packets are IP6 packets, and I would think they could be filtered by PFSense even on a bridge, but apparently that is not the case.
My work-around is to plug all of the interfaces that need a public IP directly into the Comcast router, and leave all of the others on my switch. It's a little disappointing because I can't watch the traffic with PFSense, but it is working, and I'm not able to set my own nameservers.