There are a few ICMPv6 types that are required and thus they are allowed by default in pfSense.
/etc/inc/filter.inc
# IPv6 ICMP is not auxilary, it is required for operation
# See man icmp6(4)
# 1 unreach Destination unreachable
# 2 toobig Packet too big
# 128 echoreq Echo service request
# 129 echorep Echo service reply
# 133 routersol Router solicitation
# 134 routeradv Router advertisement
# 135 neighbrsol Neighbor solicitation
# 136 neighbradv Neighbor advertisement
pass quick inet6 proto ipv6-icmp from any to any icmp6-type {1,2,135,136} keep state
# Allow only bare essential icmpv6 packets (NS, NA, and RA, echoreq, echorep)
pass out quick inet6 proto ipv6-icmp from fe80::/10 to fe80::/10 icmp6-type {129,133,134,135,136} keep state
pass out quick inet6 proto ipv6-icmp from fe80::/10 to ff02::/16 icmp6-type {129,133,134,135,136} keep state
pass in quick inet6 proto ipv6-icmp from fe80::/10 to fe80::/10 icmp6-type {128,133,134,135,136} keep state
pass in quick inet6 proto ipv6-icmp from ff02::/16 to fe80::/10 icmp6-type {128,133,134,135,136} keep state
pass in quick inet6 proto ipv6-icmp from fe80::/10 to ff02::/16 icmp6-type {128,133,134,135,136} keep state