firefox中div+css外层背景色不显示怎么办？

<div class="header">
<div class="header_left">header_left</div>
<div class="header_right">header_right</div>
</div>
.header{width:1000px;background:red;}
.header .header_left{width:600px;background:green;float:left;}
.header .header_right{width:400px;background:pink;float:right;}
.clear{clear:both;}