yes, just splat the image on a "real" disk. Just make sure to select the correct disk for installation and for legacy boot you might have to adjust the boot drive in BIOS.
mfsBSD is an in-memory minimal image of FreeBSD. You can either build images yourself [1] or use pre-built images [2].
It is especially useful if you don't have any other means of getting data "into" a machine than its sole internal disk (or flash memory), or no physical access at all. Then you can dd the image onto the storage the machine boots from, either by physically removinge the drive and attach it to another system or via a rescue system (e.g. on rented VPS/root servers).
The mfsBSD image loads an installer-environment into RAM and you can install onto the same disk the image was loaded from.
[1]
https://github.com/mmatuska/mfsbsd/blob/master/BUILD.md
[2]
https://mfsbsd.vx.sk/
edit:
If you want to use this system as a firewall you might want to consider/calculate its power draw. Those old CPUs don't have P/C-states and i'm also not sure if they supported clock rate adjustments - i.e. the CPU will always use its full rated TDP.
Also, depending on what you want to do on that firewall and what your uplink (or local link) speeds are, you might build yourself a bottleneck because this CPU is pretty weak by todays standards. I don't think you'll be able to do any meaningful firewalling or even routing anywhere near GBit line speed with that system, especially if NAT, let alone any en/decryption (e.g. for VPN) is involved..
Even a puny Celeron N5105 outperforms any Athlon 64 X2 by several orders of magnitude, while having a TDP of 10W, and usually drawing only a fraction of that under real-world loads. You can get those N5105 (or various other low-TDP CPUs) based systems with 4+ Gbit (or usually 2.5GBit nowadays) ports for as low as 100-150$.
