IPv6 should be supported; 10.4+ supports it, and we haven't done anything specific to disable it. If you have changed any settings in about:config, make sure network.dns.disableIPv6 is set to false (the default).
Assuming that's not the case, it may also be a factor of the network between you and the IPv6-based site.
I'm not sure what to say about that. Unfortunately my internal network is strictly IPv4 so this is not something I can investigate or work on further right now. I'll leave this open in case other users have other data points.