This is my experience:

1. I have used epoxy thinned with rubbing alchohol. Works well but with time, the epoxy peels off in the engine compartment.
2. If the surface of the firewall is large enough, I cover it with monokote. Works very well.
3. For small spaces or small firewalls, I am using balsarite. However, I haven't use it for long time for giving you a fair assessment about it.