With FW and Monterey, I had observed the following.
- when wanting to boot from FW, OCLP needs the according flag in Settings / Misc Settings / FireWire Boot and then boots fine from a drive connected to FW. But this I've used this only for booting an installer, not for booting Monterey
- however only without this setting, I can connect a drive to FW in an already booted system... otherwise system freezes as you describe.
As far as I remember. Didn't use it since a while now. See post
#4,171 .
Worth mentioning that booting from and using USB3 (on a nanoTECH expresscard, FL1100 chipset) with a storage medium, I see similar behaviour with Monterey, while OK with Big Sur.