Engine that leaks oil, firewall insulation that would sag and sit on the exhaust manifold, and leaves/debris can easily fall and get stick between the exhaust manifold and firewall. Add all of this up and you have a fire waiting to happen from the factory.
All of these problems are also easy to fix. Ask anyone one who is running a Small Block Chevy, NorthStar, or 3800S/C in their Fiero.