Okay, I've found out what was causing it, and now that it's past my five days probation, i can share it with everyone. It's a HUGE pain in the ass, and it came down to something extremely simple. I wish I had found it sooner.

I had in my sites CSS the following line:

#header {
  height:71px;
}

If the admin's see this and want a demo of this friggen glitch, I'd be glad to show them what happens. Contact me!

Again, the problem seems to be with Firefox on the Mac. It's fine everywhere else.

Whatever it is that's generating your pop-up is giving you problems.  If you right click on it and open in a new window it seems to be working fine.  Is that some kind of autoslideshow pop-up? If so, try disabling it.