The firewall/toeboard factory mounted pins are for the rubber floor mat. The ABS plastic/jute firewall cover is held in place with metal "alligator" fasteners (factory). The fasteners are simply pushed through the cover and into factory holes in the firewall. My cover came with the holes/slots already in the cover and they lined up close enough once the cover was trimmed to fit (width). Some of the common vendors sell the clips.
Cover retainer
