I always request that my female passengers wear something cotton, light and airy.
Skirts must always be WELL above the knee too to avoid them getting caught up in the controls......
On a serious note, I have never really paid enough attention to this subject but in the event of a fire on landing I suppose it could suddenly become a rather important issue.
I'm surprised noone has designed some kind of lightweight cotton-like fire proof material for people such as ourselves to wear in the form of shirts etc.