In fact using a router before (and behind) a firewall is strongly recommended. A firewall is a device which has to be protected itself by forcing packages to be routed at least once between the recipient/originator and the firewall. This way IP spoofing is much more complicated (and most methods doomed to fail).

Generally all those puny packet filters which are sold as "personal firewalls" are no real firewalls at all and just give users a (misleading) sense of security.

I hope this helps.

bakunin