bsduck So I think I do have proper IPv6.
dhcpcd_flags="-4" is what I was looking for.
Everything works fine that way, and not only in Firefox.
Cool. Though, it's a shame to not use IPv6 when you clearly have it (ICMPv6 bi-dir is OK; routing works).
I was going to suggest checking your firewall(s) (both on the ISP router and elsewhere--if you have it) to see if the other sorts of IPv6 traffic was being blocked...but, this is your show...