The Firestorm Matrix is actually capable of manipulating Organic Matter but:
The issue is complexity. We see that an untrained Firestorm has trouble manipulating even basic elements. It clearly requires training and intelligence to operate Firestorm's powers at his normal level. Organic molecules are based on carbon chains which allow for considerably more complex molecule (which is why all known life is carbon based). I believe that if an entity of superhuman intelligence were to take control of the Firestorm matrix, they could manipulate carbon molecules.
The only problem is that he has been able to manipulate Silicon based life in the past, but silicon still does not support the same complexity at the molecular level as carbon does even if life could be made out of it.
Zigzagged: in his early adventures Jason Rusch could, and did manipulate organic matters. After a serious mishap however, with Stein's counselling, he elected to put a permanent veto on organic transmutation, on the account of its intrinsec complexity.