Sorry, Steve, but I have to differ. First, though, I'll return to Norcal's question.
It's a very common requirement that there be a firewall in two circumstances: buildings adjoining each other ... and between the garage and living spaces. In both situations, a 1 hour rated wall is usually what's called for.
Now, that 1 hr. rating is pretty easy to achieve. Two layers of 5/8 drywall will do it .... and the layers need not be on the same side. That's why you often find garages with drywall only on the 'house' side, and a double layer on the ceiling (when there are rooms above the garage).
In a duplex building, the two garages usually do NOT need a 1 hr wall between them, as long as the walls between the garages and the units qualify. There are actually some duplexes where there is no wall between the two parts of the garage.
Otherwise, in the industry's standard fire test of wall assemblies. penetrations do matter. That's why you're supposed to place the boxes for either side in different stud bays. (Code language is actually 24" apart). The dividing line for size is 16 sq. inches .... leading to some debate as to whether your typical 4-square box requires you to use the putty pads. (Generally, the answer is 'yes.')
Fiberglass insulation may not burn very well, but it does nothing for the fire rating of a wall assembly. Let's not confuse the rating of the wall assembly with simple draft stopping.
