Can I Stifle Heritage Druid's ability?
You cannot Stifle Heritage Druid's ability since it is what is called a mana ability


  605.1a An activated ability is a mana ability if it meets all of the following criteria: it doesn’t have a target, it could put mana into a player’s mana pool when it resolves, and it’s not a loyalty ability
  
  605.3b An activated mana ability doesn’t go on the stack, so it can’t be targeted, countered, or otherwise responded to. Rather, it resolves immediately after it is activated.


If instead of the Heritage Druid's ability if your opponent was trying to Stifle Imperious Perfect's activated ability your Perfect and lets say the Forest you tapped for the {G} would remain tapped.


  701.5a To counter a spell or ability means to cancel it, removing it from the stack. It doesn’t resolve and none of its effects occur. A countered spell is put into its owner’s graveyard.
  
  701.5b The player who cast a countered spell or activated a countered ability doesn’t get a “refund” of any costs that were paid.