I'm with Dan on this one. If there's a real security threat, please point me to the source of that information, someone reporting on it, anything. All I could find from Googling is some other people saying that this could be a security risk. None of them seemed to go any deeper than that. FWIW, it's apparently enough of a non-issue that a Google search for
Firefox prefetch security turns this thread up on the first page. I realize that there may indeed be some risk by keeping leaving it enabled, but until I hear some reputable security firm echo this sentiment, I like my speed surfing enough to leave prefetch turned on.