Css problem with firefox

1

Hello guys,
i have a problem with this:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "[http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd](http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd)">
<html xmlns="[http://www.w3.org/1999/xhtml](http://www.w3.org/1999/xhtml)">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Untitled Document</title>
<style type="text/css">
<!--
#nueva {
 background-color: #000000;
 width: 480px;
 color: #FFFFFF;
}
#algo {
 float:left;
 background-color: #000000;
 width: 50px;
 color: #FFFFFF;
}
#mas {
 float:left;
 background-color: #000000;
 width: 50px;
 color: #FFFFFF;
 margin-left:25px;
}
-->
</style>
</head>
<body>
<div id="nueva">
  <div id="algo">Content for New Div Tag Goes Here</div><div id="mas">Content for New Div Tag Goes Here</div>
</div>
</body>
</html>

the problem is that when i see this in IE its ok but when i test in firefox the div who contains the others doesnt show the background… am i doing anything bad?

I really need your help

Thanks