? Is 100% cotton flame resistant

No. In most cases, 100% cotton is flammable. It can ignite and continue to burn after being exposed to an ignition source. Many people understand the burning and melting hazards of nylon and polyester fabrics, but fail to recognize the danger of wearing 100% cotton when exposed to an ignition source. However, cotton can be a good fabric to wear underneath proper FR clothing.
One recommended combination is wearing a 100% cotton T-shirt and underwear underneath FR clothing. Layering garments in this manner can increase the thermal protection of the clothing system by adding “air gaps” that provide excellent thermal insulation.