The following selector is causing issues due to position absolute:
/** Top menu >> Fusion alignleft **/
.fusion-alignleft {
margin-top: 24px !important;
position: absolute !important;
}
I’m not sure exactly why IE messes up the layout completely, I haven’t tried to support any version of IE for a long time. Perhaps if you set position:relative on the parent of the menu container it might help.