I've been researching what to use for sealing the firewall (any holes, gaps where wires come through, etc.) and came upon what Paul Dye (Editor in Chief of Kitplanes Magazine) recommends for this.

It is 3M Fire Barrier Silicone Sealant 2000+ and it's available on Amazon (and probably your local Home Depot or equivalent).

He says it's far superior to high-temp RTV and other options that people have used.