Firewalls play a critical role in modern networks, and their importance is increasing as organizations recognize the vulnerabilities of internetworking. We can no longer be satisfied merely to have accomplished communications. The ability to communicate is now a given and the challenge is to do so safely and efficiently. It's possible and practical to configure redundant firewalls to provide continued operation despite router or firewall failure, with minimal impact on security.
Chapter 9 of my book, High Availability Networking with Cisco (Addison-Wesley, 2001, ISBN 0-201-70455-2), is devoted to the topic of configuring redundant firewalls. There you'll find more detailed discussion of the alternatives and tradeoffs involved, along with examples of Cisco router configurations to support automated failover between both router-mode and end-system mode firewalls.