Why my horizontal menu becomes vertical in IE?

html-----
<div id="header">
    <h1>Armina La Manna's portfolio</h1>
              
 
    <ul> 
    
    <li><a href="index.htm" id ="first" class="current">HOME</a></li> 
    <li><a href="curr_act.htm" id ="second" >CURRENT ACTIVITIES</a></li>
    <li><a href="gallery.htm" id="third">GALLERY</a></li>
    <li><a href="theatre.htm" id="fouth">THEATRE</a></li> 
    <li><a href="teaching.htm" id = "fifth">TEACHING</a></li>
    <li><a href="contact.htm" id = "sixth">CONTACT</a></li> 
    </ul>
  </div> 
 
---------css-------------------
 
/* ---------------- header specific styles ----------------*/
 
#header h1 {
	text-indent: -1000em;
	height: 150px;
	width: 800px;
	position: absolute;
	background: url(imgs/name.png) no-repeat right center;
	display: block;
	padding: 1px;
}
 
#header {
	height:180px;
	width:800px;
	position: relative;
	padding: 0;
	margin: 0;
	
		}
 
/* -------------------main navigation menu -----------------------*/
 
#header ul {
	list-style: none;
	position: relative;
	width:700px;
	font-size: 16px;
	padding: 0;
	bottom: 0px;
	right: 0px;
	margin-top: 150px;
	float:right;
				}
 
#header li {display:inline;
	}
	
 
#header li a {display:block;
	}
	
#header li a:link, #header li a:visited {
	background: #FFFFFF;
	/*padding: .2em 12px;*/
	text-decoration: none;
	color:#000000;
	font-family: Arial, Helvetica, sans-serif;
	float: left;
	}		
 
	
#header li a:hover, #header li a:active {
	color:#666666;
	background:#FFFFFF;
		}
	
#header li a.current, #header li a.current:hover, #header li a.current:active {
	color:#999999;
	background: #FFFFFF;
	cursor: default;
	font-weight: bold;	
   }
   
 #container #header #first {
	width: 90px;
}
#container #header #second {
	width: 202px;
}
#container #header #third {
	width: 112px;
}
#container #header #fouth {
	width: 112px;
}
 
#container #header #fifth {
	width: 92px;
}
#container #header #sixth {
	width: 92px;
}
 
#container #header ul li #sixth {
	text-align: right;
	}

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