Are you running this in a home environment? I have no experience with m0n0wall but since it seems you are just switching, I thought I would throw out trying Astaro Security Gateway. I am using it currently with a SiXXs tunnel with no issues and it appears, from what I have read, to have no issues with native IPv6.
They provide a free license for home users with some connection restrictions, but other than that, it is a fully fledged business class firewall.
This link lists the essentials and home editions.
»www.sophos.com/en-us/products/fr ··· ols.aspx
Also looks like they have a business essentials version also for free.
»www.astaro.org/astaro-gateway-pr ··· asg.html