Most likely IE is having a problem with the path you've specified to the external file, in this case, the code in red:
Code:
div id="mysubmenuarea" class="tabsmenucontentclass">
<!--1st link within submenu container should point to the external submenu contents file-->
<a href="submenucontents.htm" style="visibility:hidden">Sub Menu contents</a>
</div>
What's the path you're using? You won't have a problem if the path is just the file name itself, but if it consists of directory paths as well, it may trip up some browsers.
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