In side some coding compile with FreeBSD only.
On the OS/Security topic...
The old Secure Computing Sidewinder G2's were based of some flavor of BSD(BSDi...?) and then they moved to FreeBSD. Lot's of good reading here about how and why things were done:
So Secure Computing had a bit of a marketing fetish for the OS security, Administrative and Operational kernels and as they used FreeBSD I guess it would be plausible, but if I was supporting/building anything today - I'd likely use CEntOS. I'm with WDF - to get proof you'd want to get on the CLI or the filesystem and you'd probably find out pretty quickly, but at that point you have to ask yourself why you care?
If it's really so important that the OS is secure you should hang around with the guys at https://www.openbsd.org, but you should know I'm only going to attack your firewall if I can't get in anywhere else - I'm reasonably sure it's had a lot of work done on it, and it's probably pumping all it's logs back to your SOC, and it's listening for attacks, and the traffic going through it is probably SSL/TLS the most part. Just sounds like anything from Palo alto Networks or Checkpoint would be very crunchy.