I don't know for sure, but I doubt SF makes the stool firmer. I'd think it gets firmer when most of the water gets absorbed through the walls of the large intestine. That means it should not be evacuated too early...
I think the only idea of SF is to help us tolerate foods. It mixes with them to make them more tolerable. IF is what makes for more stuff in the stool... whether it would make it firmer, I have no idea.