I don't think it's anything to do with Neftis' hair, except that the hair is very detailed and therefore large. I suspect the problem is that the Firefly renderer uses a ton of system resources. I have a fairly decent computer - Pentium 2.8 GHz, 1.5 GB RAM, Windows XP - but Firefly simply won't render some images. Things I've found that help: 1. Reduce the size of the rendered image. 2. Eliminate the options you don't need. I.e., texture filtering isn't needed for most images. 3. Reduce the "bucket size." The render will be slower, but IME is more likely to complete. 4. Increase the shading rate. 5. Reduce the pixel sampling. 6. Render in draft mode.