Since Ember deals fire damage with all her abilities, she becomes very effective on Infested levels or defense with her area of effect skills.
The Phoenix Helmet can allow you to forget the use of Flow if you choose to equip it, especially if using Streamline. Ember's base Shields, Health and Armor are average to poor; a loss of 5% Shields does not affect her dramatically.
As the residual flames of multiple Fireballs will damage enemies independently, casting several on an enemy that is slowed or crowd controlled (excluding stagger which causes humanoid targets to stumble out of the flames) will cause a high rate of damage over time. This is especially useful on bosses that are held stationary by Rhino's, Frost's or Vauban's hard disabling abilities.
The persistent effect of Fire Blast is extremely effective against hordes of incoming Infested, though weaker when already surrounded. This makes it a strong ability on Infested Defense missions.
If you move outside the ring after casting Fireblast it can help avoid being caught in the center with those enemies that sometimes manage to pass through the flames or where already within it. This will damage or stun them again as they try to pass out of the ring and reach you.
World on Fire
Unlike Ember's other abilities, World on Fire is mitigated by armor, making it ineffective against Infested Ancients and higher level Grineer.
World on Fire is currently being reviewed by the developers for re-working/fixing.