I have flown in everything from barefoot and swimtrunks with earmuff noise suppressers, to exposure suit, nomex, and helmet. I have sat in a fire and had my nomex burned off up to my knees and had the nomex gloves burn off. Nomex is cool stuff...but only delays the inevitable if you are in a real fire...at least when clean...it does not tend to stick to you like a polyester suit does.
For fire protection...you are about as safe in cotton or wool as you are in nomex....operative word....about. The helmet will surely save you from injury or incapacitation that could otherwise be fatal.
Given a choice....I will still fly in my sneaks...shorts...and tee shirt with a collar (keeps the shoulder harness from rubbing)....and with a good headset. Most mishaps are of such a degree that kevlar underwear does you little good...when a blade or something critical lets go.....and you are zipping along at more than a low hover....you pretty well become a statistic with no hope of telling stories.