Mike Kimberley posted on Facebook to say that the firewall is structural. I know he argued with ACBC about the choice of material. Chunky won the argument and they used LDF, low density fiberboard, or something very similar. Apart from its tendency to disintegrate when wet it actually has some nice properties for the job. It's cheap, light, it's good insulation and it's somewhat stiff. It'll even take a little while for a fire to burn through it.
There is little to no fiberglass over the LDF, except at the edges.
I will replace the firewall in my car with a sandwich panel of some sort, to keep the weight down. I'll look for a compromise between price, weight, noise and heat insulation, fire resistance and structural value.