Some people have claimed that they cannot yet sell
IPv6 Internet access because there is no IPv6 firewall support. According to this ICANN study
this is not quite true. At least 30% of the 42 vendors surveyed, had IPv6 support.

According to this talk many open-source and commercial firewalls supporting IPv6 are available.

Commercial firewall support may be lagging behind OS and router support, but not by much.

The Campus IPv6 Wiki has a more detailed rundown of IPv6 firewall testing and hints for their use here.

You can Build an IPv6 Firewall with OpenBSD.

You can also build an IPv6 firewall with Windows Server 2008.

Vyatta and it's open-source core both fully support IPv6

ip6tables is used to build IPv6 firewalls in both openwrt and dd-wrt. Although dd-wrt
and openwrt are built for wireless routers, you can turn off the wireless interface on these routers and use them as ethernet-to-ethernet routers.

DD-WRT supports IPv6 and runs on a lot of hardware, including the Linksys WRT 160N, IPv6 support is discussed here. (IPv6 firewall support not in the GUI, only from command line as of March 2011)

OpenWRT also supports IPv6 with both tunnelling and native, and also runs on a large amount of hardware, including both the Netgear WGR614L and Linksys WRT54GL. In the US, the WGR614L is stocked at www.amazon.com and the WRT54GL is stocked at Frys Electronics and is available at www.frys.com IPv6 support is discussed here.

Read about other people's experience using an Enterprise IPv6 firewall/IDS http://www3.ietf.org/proceedings/07mar/slides/v6ops-6/v6ops-6.ppt