He cambiado lo de HTMLElement y pues sigue funcionando en Firefox pero no en IE...
Código:
<html>
<head>
<style>
.tunedtextbox{
border:0;
background-color:transparent;
background-image:url('http://img85.imageshack.us/img85/7228/boxbkgrhn4.gif');
background-repeat:no-repeat;
color:white;
}
.pathfield{
position:absolute;
top:0px;
left:0px;
width:153px;
height:30px;
}
.filetextbox{
position:absolute;
top:5px;
left:5px;
}
.filefield{
position:absolute;
top:5px;
left:5px;
width:243px;
height:20px;
}
.searchbutton{
position:absolute;
top:0px;
left:148px;
}
.fileinput{
position:absolute;
top:0px;
left:0px;
-moz-opacity:0;
filter:alpha(opacity: 0);
}
#fileform1{
position:absolute;
top:50px;
left:100px;
}
</style>
<script type='text/javascript'>
function copyvalue(target,source){
document.getElementById(target).value=document.getElementById(source).value;
}
</script>
</head>
<body>
<form>
<div class="atorigin allsize" id="fileform1">
<div class="filefield">
<img class="searchbutton" src="http://img158.imageshack.us/img158/7820/fakebuscarzo3.jpg"/>
<div class="fileinput">
<input type="file" id="file" name="file" onclick="copyvalue('pathbox','file')"/>
</div>
</div>
<div class="pathfield">
<input class="tunedtextbox filetextbox" id="pathbox" name="pathbox" type="text" onchange="copyvalue('file','pathbox')"/>
</div>
</div>
</form>
<font style="color:red;" onClick="alert(document.getElementById('file').value);">Haz click aquí para ver el contenido del File
Input!!</font>
</body>
</html>