you see Mike Holmes use up there in Canada. I XPSd my sill area and this insulation would add some fire protection. It was yellowish and more ridged looking insulation than fg. Is it sold in the states?
Rock wool. I found it at a fire protection supplier.
Plain old fiberglass is not flammable.
I do not think code says fg is good enough against xps...firewise...
I doubt any officials will accept fibrous materials as a fireproofing material over foam. Typically, foam is covered with gypsum wall board. Also acceptable, depending on the jurisdiction, are sprayed cementitious coatings, certain fireproof paints, sprayed plaster and various thicknesses of sheet metal.
Here is an example of a cementitious spray.
..the old sill area delimma...