The spell creates fire and projects it away from you in a cone. The difference between this spell and, say, fireball, is that the fire is no longer magical once it's projected, and thus overcomes Spell Resistance.
Natural fires require fuel to maintain themselves for long. Once the spell is no longer maintaining the flame, it should quickly dissipate, assuming it wasn't cast into a field of dry grass or over a pool of oil. Note that lesser acid orb from Complete Arcane and Spell Compendium is also Conjuration(Creation) with an Instantaneous duration and creates neither an object nor a creature, simply a (non-magical, after leaving the caster) spell effect.
Orb of force is clearly meant to work like orb of acid, although this explanation is going to belong more on Physics.SE than RPG.SE. Force, in real world physics, is transferred between objects when they touch. Ergo, rather than "breaking" or "bouncing off", an orb composed entirely of force should be absorbed by a target (or by the ground or scenery on a miss) on impact. This is what does the damage of the spell - raw kinetic force being transferred to a relatively-stationary target.
Of course, the spell in this case probably shouldn't have the [Force] descriptor, as DnD [Force] has absolutely nothing to do with real-world force, as indicated by the fact that [Force] has full effect on incorporeal creatures and force should have none. If you (or your DM) want to rule orbs of [Force] continue to exist after striking a ghost or object, go for it. (However, both [Force] and force can be assumed to be weightless and noncorporeal, so rather than falling to the ground or filling a ball pit, they should probably keeping bouncing like that one scene in Men in Black until it escapes to outer space...)
Best Answer
Your caster level (CL) is 8. Your maximum spell level is 4th. Fireball is a 3rd-level spell. Spells can be prepared in higher-level slots, although this does nothing special*. You can always cast a spell at a lower CL than normal**, down to the minimum CL needed to cast the spell for your class (in this case, 5th or 6th, depending on whether a Wizard or Sorcerer is casting it — presumably the former, given the mention of preparation). And fireball damage scales only with CL, per the original rules quote.
Therefore, you would cast fireball out of a 3rd-level slot for 8d6 damage normally, or as little as 5d6 damage at your option.
In two more class levels, your CL would be 10 and you would be casting fireball, still out of a 3rd-level slot, for up to 10d6 damage. After that, you don't get any more damage increases from the spell itself, although there are lots of feats and class features and what-not that can increase damage in various ways.
*Preparing in higher slots:
**Casting at lower CL: