Where's pemerton when you need him?
At your service!
A 4e fireball didn't even set matches on fire, since it couldn't damage objects with fire damage.
Prove it, then.
Show the power from 4e, and copy+paste the spell description of Fireball from every edition.
Here is the fireball text from Moldvay Basic:
This spell creates a missile of fire that explodes into a ball of fire 40' diamter when it strikes a target. The fire ball will case 1-6 (1d6) points of fire damage per evel of the caster to all creatures within this area. If a victim of a fireball spell saves vs Spells, the spell will only do 1/2 damage.
Here is the fireball text from the 4e PHB:
Standard Action
Area burst 3 within 20 squares
Target: Each creature in burst
Attack: Intelligence vs. Reflex
Hit: 3d6 + Intelligence modifier fire damage.
Miss: Half damage.
That is basically indistinguishable from the Moldvay text, other than format. Neither talks about damage to objects. Both assume that the participants at the table can work out that a great honking ball of fire (40' d in Moldvay Basic, 7sq x 7 sq in 4e) is capable of setting flammable material alight.
so if you didn't buy beyond the 3 main books then this wasn't actually a rule
From the 4e DMG pp 65-66:
Damaging Objects
Like characters, objects have hit points and defense scores (except for Will defense; see Object Immunities and Vulnerabilities, below). . .
Object Immunities and Vulnerabilities
Usually, it doesn’t matter what kind of attack you make against an object: Damage is damage. However, there are a few exceptions.
All objects are immune to poison damage, psychic damage, and necrotic damage.
Objects don’t have a Will defense and are immune to attacks that target Will defense.
Some unusual materials might be particularly resistant to some or all kinds of damage. In addition, you might rule that some kinds of damage are particularly effective against certain objects and grant the object vulnerability to that damage type. For example, a gauzy curtain or a pile of dry papers might have vulnerability 5 to fire because any spark is likely to destroy it.
I think this is pretty clear.
Gasp! It's Mother May I, the bane of all gaming!
What is the GM being asked to adjudicate? I think it's pretty clear that it is the fictional positioning
of the objects, and of the PC in relation to the objects if that is in doubt. That a [fire] power can set things alight is pretty unambiguous. That's what fire does:
(From PHB p 55): Fire: Explosive bursts, fiery rays, or simple ignition.
The player doesn't need the GM's permission to establish that, in the game, when fireball is cast, the area is filled with an explosive burst!