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Would it be possible to run m0n0wall in a FreeBSD jail? I could always just try it, but if anyone knows how I would appreciate the hint. Thanks.

No.  You can only install it to an entire HD.  Even if you could install it to a jail, jails bind to a single IP on a single interface, they can't use multiple interfaces. Plus all the kernel dependent things would cause numerous issues.

Plus, running in a jail doesn't give you as much protection as running under a true hypervisor--and even that is generally considered a bad idea for production environments from both a security and performance standpoint. (Understandably very tempting, however...)