Cotton!
Don't wear ordinary cotton either if you really worry about fire: ordinary cotton is a vegetable fibre that burns like a torch! Wear wool or silk. These are animal derived and will merely shrivel and extinguish if burned.
Even in the early Nineteenth Century the Rev Patrick Bronte, father of the three Bronte sisters, used to insist that his wife and daughters all dressed in either wool or silk (,even though as a clergyman he could barely afford the cost of it). This was because he had officiated at the funerals of so many women and girls who had died of burns. Their cotton dresses had become ignited by sparks from the open fires people had in their homes at that time.
Treated cotton, however, is a different kettle of fish. 'Proban' is a special proprietory cotton that has fire resistant properties, and I believe there is a process you can apply yourself to make ordinary cotton somewhat fire resistant, although I don't know how effective this is.
Other than that it has to be Nomex II.
Broomstick.