It's appears all the same for me and I've just checked in FF, IE and GC. The menu stays above all the content when you scroll down, in all those browsers.
It sounds like a z-index issue to me. You've only declared z-index on one element. I'm not sure maybe try explicitly declaring z-indexes for the elements you are having issues with, but make sure the z-index is less than the value of the menu's z-index.
Last edited by LearningCoder; 01-04-2013 at 06:05 AM..