Iframes always draw on top of other content, even when they shouldn't. One example is if you use absolute positioning to place a div on top of an iframe. The iframe should in this case be clipped by the div, but it is drawn on top. It should respect the same ordering rules as any other element, that is, later elements in the html go on top. Z-index does not work to fix this ordering, and setting -moz-opacitiy below 1.0 does not allow you to see below the iframe. This is on Mozilla build 2002021108.
Created attachment 69214 [details] simple testcase Attaching simple testcase. Right should match left. The green box should be on top.
This problem seems that is going to be fixed (let's hope so) quite soon in bug 91516 "IFRAMEs do not respect z-index of other, non-iframe content" *** This bug has been marked as a duplicate of 91516 ***