Fire blocks are usually for balloon framed buildings but sometimes are in other styles as well.

The easiest way to patch a hole that big is to secure a wood strip behind it with screws on each side then secure a patch on the wood. It is still a real pain and going with the trim is a very very good idea.

Make sure there aren't any other holes (other side) as I know someone who once pumped a lot of insulation into a bathroom where he removed a fan and had an opening into a bay he was pumping cellulose into.

I though it was funny, he didn't.

Joe