Steve, I almost hate to comment, but.....
The insulator looks like Cereblanket or a similar aluminosilicate wool refractory fiber.
Cereblanket is an excellent insulator, and a good choice for the cabin side of a firewall for two reasons. It will not ignite when in contact with a red hot firewall, and it does not contain any significant quantity of binder material, meaning it will not generate much smoke.
However, loose refractory fibers in the air are considered a health risk (fibrosis, lung cancer and mesothelioma). So, if you install refractory fiber against a vibrating firewall, you really should do whatever is necessary to fully encapsulate the pads so that no loose fiber can escape into the cabin. Spending 1000 hours in a little box full of airborne ceramic fiber is a lot like the lab rats mentioned in the MSDS.
I don't see a fastening method. Remember, any glue or tape has the potential to negate the "no fire or smoke" advantages of refractory wool.
Much burn testing says insulating the engine side of the firewall is superior in every way. The inside of my own firewall is shiny unpainted stainless.