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.
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www.sophos.com/en-us/pro ··· ols.aspxAlso looks like they have a business essentials version also for free.
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www.astaro.org/astaro-ga ··· asg.html