Take a look at the following code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head></head>
<body>
<div style="float:left; overflow:auto; width:400px; height:300px;">
<table cellspacing="0" cellpadding="0" border="0" style="width:600px; border:solid 1px #999999; border-collapse:collapse;" ><tr><td>testing</td></tr></table>
</div>
</body>
</html>
The top border of the table disappears in Firefox.
What's the problem?