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
JC Custom WoodWorks
For best results, try not to do anything stupid.
"So this is how liberty dies...with thunderous applause." - Padmé Amidala "Star Wars III: The Revenge of the Sith"