Steve ... you're continuing to mix apples with oranges. Firestops and fire rated walls are two entirely different topics.
"Firestopping" is simply closing a cavity. Period. We can discuss different ways this is done - but it has nothing at all to do with how well a wall will perform preventing a fire from passing through the wall - either directly, or by getting hot enough to ignite what's on the other side.
Fiberglass doesn't burn? While is is usually classed as 'non combustible' in surface burning tests, I assure you I can light a piece with a torch. I can also make steel wool burn vigorously. But, we're talking about 'fire walls' here, not parlor tricks.
I doubt the IRC, or any other code, would require a particular wall construction. I would expect them to want you to use a 'rated assembly.' That is, assemblies that have been subjected to full scale, industry standard, fire testing.
UL publishedsa 'fire resistance directory' that is one of the more common atlases of rated designs. There are others, as well. A review of many different designs, comparing the same design with fiberglass batts to one without, shows absolutely no difference in the rating achieved. Insulate if you will, but don't do it to achieve greater fire resistance!
