6 power favor tile Town/Culture benefit order
Yes, if a Town would be founded by taking this tile, you get the key immediately, and may use it to reach 10 on the Fire Cult with the 2 Fire influence. 


  we see that founding a town is not an active action you have to execute but happens automatically and instantly when the conditions are met.
  and by taking the favor tile(s) the conditions have been met.
  thus in the same moment you have a town and are allowed to advance to level 10 with the cult bonus shown on your favor tile(s). 


(source: http://boardgamegeek.com/article/10450352#10450352)