Yes, JKnott, I do have "do not allow PD Address release" checked. And you're right, there is no control over what the ISP will actually do. I think the addresses had been the same for about 2 months but it seems like a power cycle of the modem is what triggered the IP change. pfSense had little control over it.
I'm actually on the phone with Comcast Xfinity now, it's taken 1h22m to get to a supervisor. Seems I've been talking a foreign language to both reps I've talked to so far. How hard is it to get a static /60 - /48 on an account? :) I'm currently finding out. It's not like I'm asking for a static IPv4, I'm not even bothering with that.
...and after the call, Comcast Xfinity confirmed they still don't hand out/sell IPv6 blocks to Residential customers. So it is what it is.
Would it be a fair (acceptable?) compromise to only run DNS lookups over IPv4? It looks like if I reorder my IPv4 DNS servers System -> General to place my DCs IPv4 addresses at the top of the list (with no outside interface assigned to it), then remove the RA & DHCPv6 DNS servers - the pfSense DHCPv6 server will assign out its own IPv6 per-interface address as a DNS server, and proxy the replies from the servers, in sequence, from Settings -> General. Seems to do away with the need for a DNS forwarder, which also seems to be IPv6-dependent (i.e. only take IPv6 addresses).