Tony
1 - 'fire proofing' is inherent to the fabric and not a coating that is aplied to it so no amount of washing will wear it off.
2 - Just checking, you do realise that there is no such thing as fire proof when we are talking about clothing? The best we can hope for is flame retardent but even this will burn once it gets hot enough.
3 - The level of protection Nomex or cotton clothing will offer in anything other than the smallest of fires is negligible. Even in Nomex or cotton the heat generated by a fire will severely burn you.
I strongly support the wearing of helmets as, in the event of any impact, they may well save your life and I absolutely agree that looking like a div is neither here nor there.
Ultimately I would rather fly in shorts, t-shirt and a helmet than a Nomex flying suit with no helmet.
Like you said, danger is very much down to the perception of the individual so as long as the individual is comfortable and able to concentrate on their flying...