A combination actually.
The content part of the page is a construction of div's with are positioned through css. Mostly with absolute positioning.
The whole content part, however, is wrapped in a table to center it on the screen.
Something along the lines of:
<html>
...
<body>
<table class="centerScreen">
<tr><td>
<div id="placeHolder">
...
<div id="menuHolder">
<ca:Menu id="menuMain" ... >
</div>
...
</div>
</td></tr>
</table>
</body>
</html>
html and body have 100% height and width set through css, as does the centerScreen table, tr and td. The td also has vertical and horizontal centering set.
The placeHolder div has a fixed height and width and other styling information.
The menuHolder div has an absolute position, fixed height and width and other styling information.
If we use javascript to position the placeHolder div then the menu component works fine in both IE and Firefox. (But then there's an anoying jump in the page as it is first rendered and then repositioned.) The problem with the pulldown menu's started when I added the table for positioning.