How do you change the MSS on an interface? Did you mean MTU?
I'd suggest trying again with all ICMP for IPv6 allowed to/from LAN & WAN in the firewall.
If you have a Linux machine, try the tracepath6 command to a hostname that's giving you trouble. Usually it will tell you where the MTU on the link changes. Ideally run the command from the otherside to you as well for even more information on the PMTU.
And maybe increase the MTU of wan_stf to 1480 (but only if you're not using PPPoE). FreeBSD also seems to have MTUs per route.
For example, these commands can help you see what routes there are along with the diagnostics->routes page:
netstat -r -n
route -6 get default
BTW the 1232 I think comes from the (1280B MTU - 20B IPv4 6rd header - 28B tcp header)?