I found the same HTML tag on a customer's computer which was inserted right at the beginning of the <body>
tag:
<div id="_GPL_e6a00_parent_div" style="position: absolute; top: 0px; left: 0px; width: 1px; height: 1px; z-index: 2147483647;"><object data="http://cdncache-a.akamaihd.net/items/e6a00/storage.swf?r=1" id="_GPL_e6a00_swf" type="application/x-shockwave-flash" height="1" width="1"><param value="transparent" name="wmode"><param value="always" name="allowscriptaccess"><param value="logfn=_GPL.items.e6a00.log&onload=_GPL.items.e6a00.onload&onerror=_GPL.items.e6a00.onerror&LSOName=gpl" name="flashvars"></object></div>
Since most computers did not have this, and since it was not there in another browser on the same computer, I thought it had to be related to some plugins, and it turns out that by disabling them, the tag was no longer inserted there.
I am still trying to figure out which plugin caused this, since there were so many installed, but disabling Shockwave Flash plugin from Firefox solved the problem for this computer.