In my expereince ( I have worked for 3 different carriers), most airlines get around having non-fire resistant parts of the uniform (such as ties, scarves, stockings) by having procedures in fire-fighting drills to remove such articles that may be a fire hazard before fighting a fire. It only takes seconds to rip off a pair of stockings or a tie/ scarf.
But any airline that has non-natural fibre suits/ jackets/ skirts/ trousers etc (in my opinion) is being reckless.
I would guess most airlines have wool-blend suits, with cotton-blend shirts/dresses. These are relatively fire-resistant(that is, they wont immediately burst into flames and melt onto the wearer - as a lot of synthetic fibres do (eg polyester)).