I see quite a few differences between IE and FF, and even more in Chrome. Chances are that means there is something(s) that are triggering more than one error.
If I were you, I'd make sure to look in the .vbmenu_control_row2 area just as much as the tm_forumnav_head area.
Also, I'm not sure why you have the _head at the end of the tm_forumnav, because if you look here, there is no _head there, or anywhere else
HTML Code:
<tr class="forumnav">
<td id="forumnav_info" class="vbmenu_control_row2">
<a href="http://forum.vbulletinsetup.com/f18/#">Information</a>
<script type="text/javascript">
vbmenu_register("forumnav_info");
</script> So first:
I would try to remove the _head from the tm_forumnav, save it then see if it fixes that problem. if not, check to make sure that the more specific ruleset in "vbmenu_control_row2" is not interfering.