Upstream the question was 'what is the point of ZFS on a firewall? it just complicates things.'
That's certainly the case for many sorts of firewalls. Consider firewalls that do lots of caching of client side http downloads, that run postfix mail exchangers and other 'big storage' packages. Restoring from an XML backup doesn't preserve the mail queue, doesn't preserve potentially gigabytes of cached downloads. ZFS snapshots and rollbacks and remote sends can do that.
There's an even better reason: The ability to run pfsense with ZFS on the 'bare metal' with direct access to several hardware nics, while running other 'close to the network' type servers (web, mail, cloud, voip/pbx, etc) in virtual machines running on a 'big iron' style pfsense install. All those can use zvols maintained by ZFS on the bare metal with all the sys-admin and error catching advantages zfs brings.
Currently to use ZFS and pfsense on a 'big iron' system, pfsense has to run in a virtual machine and the network interface card 'plumbing' is a pain.