IPv6 LAN to IPv4 with PFsense
6to4 needs to know your extern IPv4 address that it should use. This will determine your IPv6 prefix by embedding the IPv4 address in your IPv6 prefix (basically your IPv6 subnet has the information necessary for other IPv6 hosts to find their way back to your IPv4 address via another 6to4 router.
Your ISP must support 6to4, or you must use a public 6to4 router.
PFsense is based on FreeBSD, I don't know PFsense's interface all that well, but I have an Intro to 6to4 for the FreeBSD CLI on my blog.