The captive portal in m0n0wall uses ipfw. The method used to bring you to the captive portal, a redirect in the ipfw table, doesn't support ipv6, until freebsd 9.x.
as m0n0wall is based on earlier versions of freebsd, it can't implement this.
however, as most clients will be dual stack, this should be ok, m0n0wall allows ipv6 through the portal, but you have to have working ipv4 to authenticate with the portal.
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