I doubt this will matter to the most vehement, but I emailed the author of RFC 2979 and asked if a basic NAT Router (with filtering, SPI, or such) qualifies as a firewall. (The RFC notes that NAT and firewalls are not the same thing.) He stated that, in his opinion, the presence of filtering makes it a firewall.
Again, the issue is one of quality. There are low-end firewalls than can do very little, and there are high-end firewalls that can do virtually everything. But all such devices are used in the process of firewalling, by definition.
Oh well. It goes without saying that the vendors of home firewalls are in no legal jeopardy for calling them firewalls.
RFC 2979- Behavior of and Requirements for Internet Firewalls