If there are bedrooms above the garage, all structural walls must be firestopped, even the exterior ones, to prevent structural failure.

Also, properly packed fiberglass insulation is an approved firestop per IRC:

Originally Posted by IRC 2006
R602.8.1 Materials. Except as provided in Section R602.8,
Item 4, fireblocking shall consist of 2-inch (51 mm) nominal
lumber, or two thicknesses of 1-inch (25.4 mm) nominal
lumber with broken lap joints, or one thickness of 23/32-inch
(19.8 mm) wood structural panels with joints backed by
23/32-inch (19.8 mm)wood structural panels or one thickness
of 3/4-inch (19.1 mm) particleboard with joints backed by
3/4-inch (19.1 mm) particleboard, 1/2-inch (12.7 mm) gypsum
board, or 1/4-inch (6.4 mm) cement-based millboard.
Batts or blankets of mineral wool or glass fiber or other
approved materials installed in such a manner as to be
securely retained in place shall be permitted as an acceptable
fire block. Batts or blankets of mineral or glass fiber or
other approved nonrigid materials shall be permitted for
compliance with the 10 foot horizontal fireblocking inwalls
constructed using parallel rows of studs or staggered studs.
Loose-fill insulation material shall not be used as a fire
block unless specifically tested in the form and manner
intended for use to demonstrate its ability to remain in place
and to retard the spread of fire and hot gases.
R602.8.1.1 Unfaced fiberglass. Unfaced fiberglass batt
insulation used as fireblocking shall fill the entire cross
section of the wall cavity to a minimum height of 16
inches (406 mm) measured vertically. When piping, conduit
or similar obstructions are encountered, the insulation
shall be packed tightly around the obstruction.
R602.8.1.2 Fireblocking integrity. The integrity of all
fireblocks shall be maintained.
Proper insulation is required in IBC for the fire rating of walls, too, but I believe it's more for thermal insulation purposes. IBC is pretty specific about it in the one table. Also, fiberglass does not burn. It's glass! It's rated to 1000F, also. The ill-conceived tuft-test that's always referenced is bogus; repeat the experiment with a full batt and you'll see that the lighter does nothing but leave a little carbon residue.