The firebat's attack is optimized against groups of small-sized units, as it deals concussivesplash damage. Firebats are most useful in the early game, as all of the base units for the three races are small. Supported by medics, firebats can stand up to zealots despite the large difference in health between them. With stims and medics, firebats can easily defeat swarms of zerglings, especially when positioned at choke points. Unlike marines, firebats cannot attack air targets. However, firebats are less effective against more durable targets, and do not work well in large numbers, due to their limited range of attack.
The firebat's attack is ranged, but it still works under the defiler's dark swarm. The attack also deals full damage against protossshields, regardless of the size of the unit.
Firebats were developed from the flamethrower weapon initially given to the goliath in the StarCraft alpha. It was decided the goliath had too many weapons for one unit, so the flamethrower weapon was developed into its own unit, creating the firebat. Firebats could originally set doodads (like trees) on fire.
The firebat in StarCraft: Remastered
Development firebat renders for StarCraft: Remastered