I remember that years later when he went on an anti-nukes stance that had a lot of people afraid of him, Firestorm formed a baseball bat made out of Kryptonite to hit an attacking Captain Atom with - 'just in case' Atom had the same weakness. I guess he either didn't have the fine control over his powers to do that at this stage, or he didn't know the formula of Kryptonite (perhaps he learnt it while flipping through the internal databases when on all that monitor duty as a Justice Leaguer?)