Nothing will happen–you've enabled surge, but not much else. The relevant rule here is CR609.4:
Originally posted by The Comprehensive Rules:
609.4. Some effects state that a player may do something “as though” some condition were true or a creature can do something “as though” some condition were true. This applies only to the stated effect. For purposes of that effect, treat the game exactly as if the stated condition were true. For all other purposes, treat the game normally.
So for the purposes of spending white mana under the effects of
Celestial Dawn, you treat that mana as though it were any color you like. However, for all other purposes, including the check
Firespout performs upon resolution, you treat the game normally. Upon resolution,
Firespout sees that it was cast for {W}{W}{W}, and since neither {R} nor {G} was spent to cast it, neither of its effects occur.