Table with adsense is too widest. You must change cellpadding="10" to "0" and set cellspacing to "6" (maximum). Otherwise IE can't put this table in the right place. In FireFox table has 500 pixels width (including 10 pixels of cellpadding), but Internet Explorer is interpreting it the other way, so the table in IE has 500 pixels + 10 pixels = 510 pixels - but you have only 506 pixels free space there
