They aren't exactly linux firewalls, even if they run linux as management OS. AFAIK the forwarding engine is custom(ized), and on physical devices some of them

They aren't exactly linux firewalls, even if they run linux as management OS. AFAIK the forwarding engine is custom(ized), and on physical devices some of them offload to FPGA - or at least they used to when I administrated some 2014~2016. The management UI was in PHP :P