Standard UK mil issue green flying suit passes the 4-second flash fire test in a flame chamber, and passes it well. After the 4 seconds of fire, the suit does not support continued combustion, remains intact and unburnt, if perhaps a touch crispy in places. I have seen the tests. (And the one on the original CS95, which was just scary).
Nomex is a fibre inherent in the fabric; it cannot be washed out. The fire retardancy of the garment can, however, be degraded by impregnation with dirt or oil, damage (such as ironing f*cking creases into the sleeves) and washing them with too much powder and/or inadequate rinsing, all of which can provide a 'flame path' through the fabric and allow the fire to penetrate.
However, if you are not wearing a minimum of one other layer under it, no matter how good the Nomex is (other fire retarding systems are probably available), you WILL burn.