Not sure what you call the rubber like barrier that is mounted on the firewall in the passenger compartment . . .

Mine has some terrible cracks . Can it be repaired? Should it be discarded and replaced with something?Click to view attachment

I would call it a rust magnet and toss it. If you want to re-insullate the firewall, buy some DynaMat or similar product and remove the backed and install it on the inside.

To me this looks like the one that is behind the interior back pad. How thick is it, and is there any felt like material attached?
If it’s all a hard rubber it is definitely interior, I say interior as the engine version we make doesn’t have a lower shelf, like the one in the picture.

Sorry reread, yes it’s interior.
Used some metal duct tape and tape it together. It is a necessity part to reduce noise in the cockpit