To be fair, the answer does depend slightly on which version of the army lists that you use. I will assume that you are using the
latest compendium in this forum.
As Andrew says, many / most people field at least one and usually two Firestorms in each Falcon formation if you anticipate facing any air-power. The reduction in firepower is more than compensated for by the AA and the increased AP range. Fielding 6x units in the SoV formation also makes the Firestorms less vulnerable to shooting or suppression and gives more bite generally.
However, you will note that the Fire-Prisms have lost their AA in favour of greater AT capabilities. If you need more AA, then you need to use a titan or Cobra(s) - which have their own strategies.
Given you have three Firestorms, I would invest in another three (conveniently a single GW pack if you can find it), and use three or four formations of 5 / 6 units depending on your other list choices to match your strategy.