I just started DMing a campaign with everyone starting out at Level 5. In this campaign I have an Oracle with a Flame foci so I thought I'd look through the class and get a good feel for it. Most of the revelations don't seem too bad, but I really think that Firestorm looks extremely overpowered. When a character could get it they could cover 110ft of ground with 11d6 damage! I realize this is a once per day thing, but with a good roll this could wipe out an entire combat in one turn.
While I've heard it compared to a fireball, fireballs have the disadvantage of a massive blast radius that can be hard to work around rather than freeform 10ft cubes that can snake all over the place if they are touching. Am I making too much of this? I don't see any other foci having such a powerful ability.
Not as much as it may sound.
Remember these things:
1) One of the cubes of fire has to start adjacent to you.
2) The cubes have to also be adjacent to one another.
Firestorm is a really nice spell...but unless you run out into the middle of a mob of monsters, its less devastating that what you're worried about. Since the Oracle gets access to this at level 11th level, you are already going to be competing with Chain Lightning from a party Wizard or will be in 1 level from a party Sorcerer.
Something else to remember, that while a Sorcerer/Wizard might not take Chain Lightning because the spell might just not be their style...
You've chosen an Oracle of Flame. Flame is an aggressive and tempestuous force (if you think Elemental Fire), so it makes sense that the Oracle of Flame would have better Offensive powers than just about all the other types of Oracles.
The Oracle of Battles, for example, fits more the mold of a battle leader and commander than someone who blasts people with destructive magics. On the other hand, blasting people with Fire is part of what the Oracle of Flame does.
It's not that powerful. You're doing the same damage as the wizard's cone of cold spell over a larger, shapeable area. Yes, it lasts longer as well, but how often will someone stay in the confines of the firestorm? 11d6 damage against foes you'll face at 11th level is nothing special either. And let's not forget that even though it's a shapeable effect, that persistence can cause problems as people try to manuever around the battlefield without getting hit by the fire. I think you're worrying about this one a bit too much; it's similar to the actual spell but lasts longer, covers a smaller area, and doesn't specifically catch people on fire. And the spell itself is not one of the great game-breakers.