Why are firewalls preceded by routers and not the reverse? Are there any scenarios in which it makes sense to reverse the norm?

Ans: Quite simply, routers are faster than firewalls. A router is a relatively simple networking device designed solely to get packets from point A to point B. In terms of unit cost, it's generally much cheaper for a router to handle a packet than for a firewall to analyze it. Additionally, there are a lot of "junk" packets out there on the Internet, as a result of port scanning and other malicious activity.