Well, here's an idea, why not allow Firebenders to add more of a martial boost to their attacks? This way, they could exceed the Earthbenders on damage output, but be beaten when it comes to Defence/Battlefield control. For instance, we could allow the Firebenders to add bonus DC to their Firebending level to deal extra damage or advance the die, (The limit for this would be derived from their level somehow) or we could allow them to add their Strength bonus to damage they deal with their fire akin to a Composite Bow, a stronger punch should kind of equal stronger Fire Blast anyway.