I used high-temp RTV to seal the firewall recess along its mating flanges and in all its little nooks and crannies. Once it's cured I'll touch up any missed spots and rivet it in place for good.
The flavor of RTV I used was Permatex Ultra Copper, which has apparently replaced the red kind as the non-corrosive, high-temperature goo of choice. At least that's what the guy at the auto store said when I went in to pick up another tube to finish up the firewall. I decided not to use Biotherm here, because there were a lot of gaps to fill and Biotherm seems to be less rubbery and more saggy than RTV.