Consider Ancient Rome. If some proportion of soldiers in Caesar's military had the ability to shoot small jets of fire out of their hands for a maximum duration of 1 second, and perhaps a total limit of 3-4 times (or perhaps the only limit is physical exhaustion), do you think that forming battle lines would become obsolete?
A major part of maintaining a battle line is psychology. The most deaths in warfare occur when the soldiers panic and flee, breaking the line. I think having fire shot at you would be pretty panic inducing.
Could this lead towards trench warfare?
Possible counters include shields wrapped in leather and soaked in vinegar, and also asbestos. Short article about asbestos