I used a spray adhesive by Wurth, purchased from Gernold at SL Tech. My firewall pad has the horsehair-type backing. Sprayed one coat on the pad, then the firewall, then the pad again. It's been on for 3 years with no signs of deterioration. When I got my car, it had no existing pad or any indication it ever had one, so there was no old glue to remove. I hope this helps.
Tom
Early 1967 250SL
Euro version
Los Angeles