I do not know the background of this quote, but I can offer a basic explanation that may help a bit.
It will not hurt you to carry cooled ashes. However, carrying fire would burn your hands. (incendiary's punishment = burn) So, Nietzsche is asking the person that carried the ashes if he will also carry fire or is he afraid of getting burned. I am not sure what the significance of the mountain and valley are.
Hope this can help a bit. ^-^