Can I restore a m0n0wall configuration to pfSense?

From PFSenseDocs

Yes, you can use pfSense's restore functionality to restore m0n0wall configuration backups. Due to significant differences in the traffic shaper, any traffic shaping configuration in a restored m0n0wall configuration will be ignored. Everything else should be converted to a pfSense configuration and work the same as it did with m0n0wall.

If you use a m0n0wall version that supports IPsec filtering, you will need to manually change your config to avoid having a non-working system. Every instance of <interface>ipsec</interface> in the config will need to be replaced with <interface>enc0</interface>. The configuration converter in current releases up to and including 1.2.1 was created before m0n0wall supported IPsec filtering, and m0n0wall implemented it differently than pfSense had already implemented it.