not really sure you can get any router nowadays that doesn't have nat and IPS of some sort. even the cheapest $10 dsl modems have "IPS firewalls" in that sense, they all can be called firewall.
I guess it comes back to what set of features qualifies as a firewall. nat? routing? intrusion prevention? caching? av?
i guess in traditional sense anything that can intervene with the free flow of traffic qualifies. but in that sense even spyware softwares are firewalls. modifying your host file would be a firewall. or cisco ASA would be a firewall.
