Granted it doesn't light up and burn like paper but it is easier to start on fire than say holding a lighter to a 2x4, for example.
I found a couple of demonstrations of how it burns. My experience of using a lighter to this stuff was very similar to the DAP fireblock video.
DAP fireblock
Dow Great Stuff Pro Gaps and Cracks
Comparison of one firestop foam and two fireblock foams