While I haven't been working on the plane due to the cold, I have been researching and doing various things outside the garage (and in my much warmer house...).

One thing I've looked into was the heater box, which, unlike the firewall itself, is made of aluminum and as such it's a weak point in keeping an engine fire away from the cabin.  I was alerted to this by builder Jim Pavlick whose solution was to fabricate a steel version of the heater box.  Since I'm definitely less ambitious and way lazier than Jim, I looked for other solutions. :-)

What I found was this product which is basically a thin coating you paint on whatever surface you want to protect.  When it's exposed to fire it foams up into a thick barrier and protects whatever is on the other side.  It's so protective the underlying surface is often reusable!  See here.

See their videos here.  I like "Demo."

I'll definitely use that on the heater box and may also coat the whole firewall with it.  What's the downside to a thick layer of protection between you and an engine fire?...

Since the firewall is galvanized though, you can't just paint on it as it will just flake off.  Here's some info on how to prep a galvanized metal surface for painting.

It's available from Aircraft Spruce here.