"Enabling the adapter, even with no clients connecting, leads to very long DNS lookup times, or faiilure to resolve, and much slower page loads for browsers behind the firewall."
Huh?? What is having a hard time to resolve? You do understand even if you query via IPv4 for a fqdn if there is AAAA record you most likely get that returned as well since many dns clients default to query both..
If you get back a AAAA (ipv6 address for a fqdn) and your client prefers and has ipv6 it will try to use that.. But what does this have to do with pfsense having a link local address? I use ipv6 on some interfaces in pfsense and none on other interfaces that I am not using IPv6 in that network.. Yes those interfaces still get link local as shown above..
Your posting of this
inet6 2001:5<foo>9:4125:5501 prefixlen 128
and
inet6 2601:248:<foo>:44c6 prefixlen 64
This is NOT a none setting on the interface.. Where are you saying this is coming from?? If you have an interface set to NONE for ipv6 it sure and the hell is not going to get a global ipv6 address on it.. 2000::/3
So you bring up openvpn.. I route ipv6 over one of my vpn servers connections, and then on another one I do not - so as you can see from attached one has a global ipv6 address, the other does not but both of them have link local addresses on them for ipv6..
If you are not ready to use ipv6, then make sure all your interfaces in pfsense have none set for ipv6 this is all that should have to be done..
openvpnipv6.png
openvpnipv6.png_thumb</foo></foo>