Mozart's tuning fork (or perhaps one of them) is still in existence, and reveals that he tuned a whole semitone down from A440. His arias must have been a lot easier for the sopranos, if perhaps harder for the basses.

Or perhaps the other way round. Mozart would have had to live with whatever pitch standard an opera house used. He may have written The Queen of the Night's aria for a pitch a semitone lower than what the soprano was subsequently required to sing at.