What hair are you using? If it's Neftis hair, you may have trouble rendering it in Firefly even if there's not much else in the scene. Neftis uses very large textures, and a lot of them, and Firefly often chokes on them, unless you have a very powerful computer indeed. The solution is to render with the P4 renderer, or to reduce the size of the textures. (The hair still looks good with the textures reduced to 1/4 the size.)