Brake booster
 

I didn't notice while replacing it but should there be a firewall gasket?

If there ever was one on mine, I don't remember. I've had mine on and off about 5 times over two decades and I can't remember there ever being one. I dont think there is an actual need for one, so I doubt there would be one. Metal booster to metal firewall

There's a boot that is usually inserted in the back on the booster, at least I've seen them on 2nd-4th gen boosters