I can confirm that it's possible to get IPV6 on the LB1120 in bridge mode with AT&T working in PFsense, but it's a VERY non-optimal configuration.
It appears that I get a single /64 via SLAAC (as mentioned above). The default route for internet isn't fe80::1 It appears to be randomly generated and locally advertised. Here's where things get weird - although I can see the router adverts, the router won't actually pass the packets if I boot it connected to PFsense.
Here's what did work:
Hook the LB1120 (unpowered) up to a computer running windows.
Turn on the LB1120 and let it boot
Query the ethernet port in windows with 'ipconfig'. Record the IP address received by windows, the GW address assigned, and the ethernet address of the windows machine's ethernet port.
Unplug the LB1120 from the win10 computer (don't power it off).
Configure PFsense to spoof the win10 computer's HW address, set static IPV6 using the assigned address (though you can actually change it slightly, too). I'm also assigning it as a /126 (/128 might be possible), and set a static gw recorded above.
The mac spoof is necessary to get both a DHCPv4 IP and working IPV6. Yes, this is incredibly hackish. Ideally, I'd like to figure out what magic is happening with windows that isn't happening with PFsense, so I can set this thing to autoconfig.
So far, I see only 2 differences in the packet captures:
Windows uses an AT&T-advertised nameserver on a private local address:
fc00:a:a::300
I tried hard-coding that nameserver in the config, but it did not help.
2. Windows sends a bunch of broadcasts on ff02::16. This is multicast listener discovery. I'm not sure how to make PFsense send these, and only a few search hits for mld with pfsense. Any ideas?
Now, I'm having some trouble getting ipv6 packets to pass the wireless WAN link when the router is set to prefer the wired IPV6. But that's a multi-WAN issue, so I'll probably start a new thread on that.