-moz-border-xxx-colors: Mozilla Rahmenfarben

<style type="text/css">
div{
 height:200px;
 width:300px;
 margin:10px;
 background-color:#f7f7f7;
 border-top:4px solid blue;
 -moz-border-top-colors:red blue yellow green;
}
</style>
...

<body>
<div>&nbsp;</div>