What do you mean, screw over the format? And if there is a flaw in my logic, please point it out -- I honestly can't see it. As to for how long it should be banned, I'm not sure, but I don't see much sense in bringing it back when E-on rotates out.
Think about it this way. Just like, for most of Neon (and, if I'm not mistaken, even a lot of the original Modified), tournament-quality decks were mostly either Gatr or anti-Gatr, now the format is basically either Blaze or anti-Blaze. I don't know how you feel about it, but personally, I don't think any Modified format needs a card that makes the balance of power like that.
This is not to say that every card that becomes the basis for a first-tier deck should be banned -- far from it. If we did that, the game would quickly be ruined. I firmly believe that banning a card is something that should only be done if it is absolutely necessary. Even Gardevoir, which, with Gardevoir ex, was dominant in the time between the change to E-on and the debut of Dragons, is not a card I would support banning.
Yes, banning Firestarter would greatly change the format, but I believe it would be a change for the better. Perhaps with Firestarter gone, the deck variety we had when E-on began would return.