Three approaches:

A) Reduce the resolution of the hair textures and transmaps. This may seem counterintuitive, but it often counteracts the grainy effect that Firefly tends to produce. You can either resize the maps in a paint program (keep copies of the originals), or change the Max Texture Size in your render settings (which affects all maps in the scene).

B) Lower the Minimum Shading Rate in your render settings. I've found that a value of 0.2 or 0.25 usually works well with most transmapped hair. The downside is that it'll increase your render time. But that's generally the tradeoff for improved quality.

C) Enable Texture Filtering in your render settings. This works great for transmapped hair, but it may have a negative effect on other textures in your scene (seams, particularly); it's also a resource hog.