I've had it happen. Lost both dividing lines and, on a few occasions, all
the non-text elements on the main forum index page. (On gHacks too come to think of it.)
It seemed to be a problem with something in my profile or Firefox's settings. But I'm a professional network administrator, so I opted for a simple "joott maneuver" which "cured" the problem each time. As a result, I never bothered to try to get to the absolute bottom of it. And it doesn't seem to be happening any more so it's hardly worth the time to me. (I just started sitting down with O'Reilly's Java programming books so I have much bigger headaches right now.)
@SB - some of us don't have that
much free time on our hands...
Gotta love that Joott Maneuver!