The firewall is the edge of the passenger cabin. There should be no crumpling going on because the perimeter of it is the structural component that ties together the A-pillars and rocker panel structure. If you crumple the firwall you are pushing the dash structure back at the occupants and removing support from the A-pillars allowing them to more easily deform. Getting it to deform 6" or so (I recall that was the number they stated, I'd need to check to be sure though) in a crash that was well within the range of speeds vehicles are crash tested at is noteworthy.

Edit: Is the firewall is structurally irrelevant in modern cars or is reality not convenient today? The inability for people to disagree without trying to silence each other is why people complain that HN is turning into Reddit.