OK... The only real way I know how to describe this to you is simple by showing you. Navigate to the following link in anything but Firefox or IE6 and scroll down on the page.
As you can see, I have a navigation bar that is fixed to the top of the page. But when you scroll the page down, the flash graphic slides over the top of the navigation bar. Firefox is the only browser that the page displays correctly in. IE6 does not display the navigation bar fixed to the top of the page, so the problem is not evident in IE6. But in IE7, Opera, and Safari, this problem occurs. To see the CSS I'm using to build all this, go to the link below.