That's the classical definition but the meaning is evolving, these days I would say it's more accurate to consider hardware forwarding decisions is switching and software/CPU-based forwarding is routing.
As for the original question, lots of networking kit uses Linux behind the scenes. Checkpoint splat platform is Linux (IPSO is FreeBSD), I think Mcafee Sidewinder is too, Cisco ASA was a Linux kernel with an IOS-like shell stuck on it (not sure about the new ones). Bluecoat SGOS is very Linux-like but not sure how close it is in reality.
The difficulty is the lack of hardware forwarding, Enterprise networking kit doesn't generally use fast busses or big backplanes to shift packets, it uses proprietary ASICs to handle the packet processing and forwarding at line rate. You can't just buy a top end server, stick TCP-offloading 10Gbps NICs in it and expect it to firewall at 10Gbps. Although that said a lot of "enterprise" firewalls that are sold as 1Gbps struggle to hit 200Mbps and they still sell plenty of boxes.