I understand where you are coming from. but the word technically doesn't fit to your description. but if you add few words it does. all firewalls have an OS. as long as the OS is dedicated to running the firewall software and other duties as firewall, it just technically doesn't fit. I know I am splitting hairs, but the whole discussion is about splitting hairs. in many cases firewalls running on full linux distro or windows do better job than ones deployed as "real firewall" the form of the chassis might look cool when it's 1U rackmountable but doesn't... shouldn't bring any confidence in the security or performance.
