I know this doesn't fix your problem, but you might be able to hide any apparent discrepancy by using background color instead of border highlighting to mark the hover state:
For instance, (menu.css line 98)
ul#cssmw ul.level-1 {
left: 0px;
top: 100%;
}and (menu.css line 108)
ul#cssmw ul.level-1 > li > a {
background-color: #040499;
background-image: none;
border-top: 1px solid #FFFFFF;
border-bottom: solid 1px #FFFFFF;
color: #FFFFFF;
display: block;
font: small-caps bold 12px Arial,Helvetica,sans-serif;
margin: 0;
padding: 5px 10px 5px 15px;
text-align: left;
text-decoration: none;
text-transform: capitalize;
width: 100px;
}and (menu.css line 130)
ul#cssmw ul.level-1 > li:hover > a {
background-color: #D22E09;
background-image: url("../cssmw_images/navstar.gif");
background-position: 2px center;
background-repeat: no-repeat;
border-top: 1px solid #D22E09;
border-bottom: 1px solid #D22E09;
color: #FFFFFF;
font-weight: bold;
text-decoration: none;
}might not be what you had in mind, but it may not appear to be 'faulty' to your clients. I can't test this on MAC (Firefox 3.6), though (on a Windows machine) - so it may have its own flaws.
