Subpanel in garage firewall

I would like to add a subpanel in my garage. The best place is in the firewall. Before making a large hole in the wall, I want to make sure this is OK. I will of course patch the drywall when I'm done using the same drywall as I remove. The house was build in 1994 and appears to be to code. The drywall is firerated and it goes all the way to the attic. There are already two outlets and a three gang switch in the wall.

My assumption is that provided I don't have any gaps or holes, I can put just about any switch, outlet or subpanel in the firewall. And if I end up with any gaps, I can fill them with fire rated sealant (I read about it in another post).

Please correct me if I am wrong. Thank you!

 

Your theory is right on, but complete seal is necessary to accommodate the "firewall" specifications, even where your wires enter and exit. Without being a concrete wall, it is merely a "fire rated" wall, which inhibits the spread of fire rather than preventing it. Check with your local code authority prior to doing this, as they may have either suggestions or prohibitions on it.
I had a problem with our authorities once on a retirement home with common hallways. I couldn't install peep holes in the metal doors since it would denigrate the fire rating of the doors.
A true firewall you can't break into, but fire rated with proper seal is another story, as you can see with the existing receptacles.