OK, finally got to the bottom of the annoying white line.

I deleted the following red text from my stylesheet_header_menu.css:

div#dropMenu li {
position:relative;
list-style:none;
margin:0;
float:left;
line-height:1em;
width:16.5%;
*width:16.5%;
}

.clearBoth {
clear:both;
height:0;
font-size:0;
font-style:normal;
line-height:0;
}


Finally, 100% cross browser display :-)

Thanks all who helped here!! Pixel perfect now.