If they are assigning a /64 to your WAN you cannot use it on LAN.
If they are assigning an IPv6 address on WAN and ROUTING a /64 to that you can use that /64 on LAN.
This is not a pfSense problem. It is an ISP problem
The WAN IP is a /128 assigned to the external interface on the router. ONLY the router uses this address; in fact, it doesn’t even have to be allocated for IPv6 to work on the LAN. I don’t have a WAN /128 on my router (Linux box) as I didn’t configure it to ask for one. (using DHCPv6)
The LAN address is a /64 assigned to your internal network subnet. Your router has an IP on this subnet, as does everything on your network, usually allocated with SLAAC. This is what’s important to have, so that all your devices have IPv6 addresses.
If you want to have multiple subnets, you can get something bigger than a /64, but most people don’t need this.
IPv6 uses link local addresses for routing, so your router DOES NOT need a WAN /128 address. This sort of address is only typically used when you connect a computer directly to your cable modem and get a routable IPv4 address and a WAN /128 for that single computer to use. It’s not a typical configuration.