Hi, I have a similar problem with horizontal solpart menu, but can resolve it.
In Firefox there is only 1 subitem but in IE all is OK and subitems are more. 
Here is link: http://bgcomfort.com/BGCOMFORTWebandSoftwareDesign/tabid/36/portalid/0/language/en-US/Default.aspx
You can see in horizontal menu - News for example...
And here is code:
/* Menu for Skin Comp - skin.css*/
.ActiveRootTab TD { width: 120px; color: #FFFFFF; font-family: Trebuchet MS, Tahoma, Verdana; font-size: 9pt; font-weight: bold; background-color: transparent; text-align: center; }
.RootTab TD { width: 120px; height: 25px; font-family: Trebuchet MS, Tahoma, Verdana; font-size: 10pt; font-weight: bold; color: #FFFFFF; background: transparent; text-align: center; }
.SelectedRootTab TD { width: 120px; color: #FFFFFF; height: 25px; font-family: Trebuchet MS, Tahoma, Verdana; font-size: 9pt; font-weight: bold; border-left: #ffffff 0px solid; border-top: #ffffff 0px solid; background-color: transparent; text-align: center; }
.BreadcrumbRootTab TD { width: 120px; font-family: Trebuchet MS, Tahoma, Verdana; font-size: 9pt; font-weight: bold; color: #ffffff; height: 25px; text-align: center; }
.ActiveSubTab { background-color: #FEA203; #333333; color: #ffffff; border-top: 0px solid; font-family: Tahoma, Verdana; font-size: 10pt; font-weight: bold; padding-left: 2px; padding-right: 2px; padding-top: 2px; padding-bottom: 2px; }
.SubTab { background-color: #ffffff; color: #000000; font-family: Trebuchet MS, Tahoma, Verdana; font-size: 9pt; font-weight: bold; padding-left: 2px; padding-right: 2px; padding-top: 2px; padding-bottom: 2px; }
.SelectedSubTab { background-color: #C14337; color: #000000; font-family: Trebuchet MS, Tahoma, Verdana; font-size: 9pt; font-weight: bold; padding-left: 2px; padding-right: 2px; padding-top: 2px; padding-bottom: 2px; height: 25px; }
.BreadcrumbSubTab { background-color: transparent; /*#000000;*/ color: #ffffff; font-family: Trebuchet MS, Tahoma, Verdana; font-size: 9pt; font-weight: bold; border-top: 0px solid; padding-left: 2px; padding-right: 2px; padding-top: 2px; padding-bottom: 2px; }
/* Main Menu */
.MainMenu_MenuContainer
{
background-color: transparent;
}
.MainMenu_MenuBar
{
cursor: pointer;
height: 16px;
background-color: Transparent;
}
.MainMenu_MenuItem
{
cursor: pointer;
color: black;
font-family: Tahoma, Arial, Helvetica;
font-size: 9pt;
font-weight: bold;
font-style: normal;
border-left: white 0px solid;
border-bottom: white 1px solid;
border-top: white 1px solid;
border-right: white 0px solid;
background-color: Transparent;
}
.MainMenu_MenuIcon
{
cursor: pointer;
background-color: #EEEEEE;
border-left: #EEEEEE 1px solid;
border-bottom: #EEEEEE 1px solid;
border-top: #EEEEEE 1px solid;
text-align: center;
width: 15px;
height: 21px;
}
.MainMenu_SubMenu
{
z-index: 1000;
cursor: pointer;
background-color: #FFFFFF;
border-bottom: #FFFFFF 0px solid;
border-left: #FFFFFF 0px solid;
border-top: #FFFFFF 0px solid;
border-right: #FFFFFF 0px solid;
}
.MainMenu_MenuBreak
{
border-bottom: #EEEEEE 1px solid;
border-left: #EEEEEE 0px solid;
border-top: #EEEEEE 1px solid;
border-right: #EEEEEE 0px solid;
background-color: #EEEEEE;
height: 1px;
}
.MainMenu_MenuItemSel
{
cursor: pointer;
color: black;
font-family: Tahoma, Arial, Helvetica;
font-size: 9pt;
font-weight: bold;
font-style: normal;
/*background-color: #C1D2EE; work only in Firefox*/
}
.MainMenu_MenuArrow
{
font-family: webdings;
font-size: 10pt;
cursor: pointer;
border-right: #FFFFFF 1px solid;
border-bottom: #FFFFFF 1px solid;
border-top: #FFFFFF 0px solid;
}
.MainMenu_RootMenuArrow
{
font-family: webdings;
font-size: 10pt;
cursor: pointer;
}