I don't see a problem with fiberglass insulation- properly installed so it stops air flow and don't fall out. I'm from the days when we used to use asbestos cement to "firestop" and when the drywall guys were nailing drywall you could count on the dried out cement falling out so you'd end up with no blocking what so ever. Using fire caulk in a combustible structure is like killing a fly with a 12 gauge shotgun.
