I think there's a lot of options to beating firespout. First, I think you should be playing 4 knight of the reliquary in the main. This gives you more big creatures for them to deal with, and 3cc is easier to sneak around balance. You could also consider Elspeth, but I'm not convinced she's necessary. There are also board options that I think I very reasonable to consider to help beat CB-firespout decks:
1)
Choke: I think that this can really catch the counterbalancence player off guard, and many are bringing out force of will for straight creature removal. Even if they firespout this gives them so little mana to work off it's pretty strong anything. I think this is the best strategy against them.
2)
Burrenton Forge-Tender: This card is a little more narrow, but it's also pretty useful in a lot of respects. First and foremost it says "counter target firespout." Or, if they have to use removal like swords on it, then they're not using In the mirror you can counter bolts/chains (though I'm not sure if you want to run this in the mirror...). It's great against goblins, and gets rid of
bridge from below against dredge.
3)
Duergar Hedge-mage: Now, why would this be particularly good against CB/firespout decks? Well, it gives you an answer to shackles or counterbalance or any other random things they have, while also giving you a creature to beat them down with. He can be countered, but he can also be useful to overwhelm the counterbalance player with creatures.