I would think that for melted synthetics to be an issue, you'd have to be in direct contact with flame to ignite them. If it's hot enough for them to spontaneously combust without flame contact, well, you will probably have already spontaneously combusted yourself. And if you're in an aircraft disaster where you're in direct contact with flames, that doesn't bode well for your survival in the first place. Most of the folks in aircraft disasters die from the smoke or blunt trauma from the impact. For the unlucky ones who who had not only smoke and physical trauma to deal with, but flames too, I dare say they were probably toast from the get-go. Even if they were wearing cotton.

But I wear cotton most of the time anyway. Not because I fear melting, but because it's more comfortable.