Fiber is interesting. It firms up loose stools by absorbing excess water, but it also softens hard stools by retaining more water in the stool. By tending to form a soft, squishy mass, it helps get stools from either end of the spectrum (hard and dry, or loose and watery) to a soft, squishy consistency, too.
When I used to take fiber and miralax, my doctor suggested that I take them at separate times. He said that the fiber would tend to undermine the effects of the miralax. I tried it both ways, and it didn't seem to make much of a difference to me.
Now that I don't have a colon or rectum, I take fiber to help prevent liquid output, which is otherwise a daily problem. It seems to help.