It's not a competitive deck type, in either T1 or T1.5. Dragon and Landstill are strong in T1.5 and both of those can crush this. However, that doesn't make this a bad idea for casual play. In fact, this type of deck is hella fun to play casually. It's just not going to beat competitive decks, which is why I suggest using 1.5 construction. Since no one builds a casual T1 deck (other than young and new players who toss in a Sol Ring or a Demonic Tutor and call it T1), building 1.5 is more reasonable. I suppose the inclusion of Mox Diamonds and Chrome Moxes won't upset too many people if you just want to have fun. In that case, you probably don't need to limit yourself to 1 of each, but speed isn't really important in casual play, and there's a bunch of stuff you could put in that would be a lot more fun.
You're underestimating Firestorm too, especially in a deck based on drawing an extra three cards a turn (which is where it was first popularized if I'm not mistaken, at least the first time I saw it was in Lauerpotence).
The problem with firestorm is that it needs a bunch of targets to be effective, and even then it costs you a bunch of cards from your hand.To do 3 damage requires your opponent to have 2 creatures unless you want to hit yourself or your own critter. Seismic Assault is a better choice. Firestorm can be better replaced with something to get those basic lands back from your 'yard, like Groundskeeper.
Saying "This sucks, but you can play it in 1.5" doesn't make you seem very esteemed to me.
That's not what I said. I said this deck lacks direction, and even casual decks need to have one direction if you're playing against decent opponents with decent deck-building skills. I tried to imply that casual decks should lean more toward 1.5, since that format allows all the funky old cards but disallows anything that's broken (none of the restricted cards should appear in casual decks, save maybe stuff like the new moxes, voltaic key, maybe earthcraft...stuff that you can have fun with but doesn't ruin the game).