Heat and open flame are two different animals. Cotton may serve as an insulator from a heat source, while at the same an open flame could ignite the same cotton. A nomex outer garment such as a flight suit protects the wearer from open flame (for a finite period). The clothing an individual is wearing under the nomex provides additional protection from the heat -- cotton and wool perform well in this role because they don't melt. Polyester fabrics would be a poor choice.

While nomex will not support flame (burn) nor melt, what it will do is char and then it just disintegrates. I've had one of my flight suits for over 30 years, but it's never been subjected to flames.