Hi, do you mean between the firewall, bottom of the cowl, and top of the cowl (which have the metal vents and windshield wiper pivot holes) where they all come together to form a metal lip which Ford spot welded together or the goop Ford tried to throw up on the seam between the firewall and the bottom of the cowl (when viewing from inside the Mustang with the heater box assy removed).
I haven't figured out what Ford used before they spot welded the three panels together. As for the firewal/cowl seam, I've used a flexible seam sealer 3M makes. They come in a caulking type tube and is called seam sealer - for body panels. Wish I had a part number. Another good choice is 3M's Fast and firm, it also comes in a caulking tube.