изменение overflow в hidden перезапускает/перезагружает флешку в ff как минимум с версии 3.0:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>Untitled-1</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style type="text/css" media="screen">
html, body { height:100%; background-color: #ffffff;}
body { margin:0; padding:0; }
</style>
</head>
<body>
<div id="flashContent">
<object classid="clsid:d27cdb6e-ae6d-11cf-96b8-444553540000" width="50" height="50" id="Untitled-1" align="middle">
<param name="movie" value="Untitled-1.swf" />
<param name="quality" value="high" />
<param name="bgcolor" value="#ffffff" />
<param name="play" value="true" />
<param name="loop" value="true" />
<param name="wmode" value="transparent" />
<param name="scale" value="showall" />
<param name="menu" value="true" />
<param name="devicefont" value="false" />
<param name="salign" value="" />
<param name="allowScriptAccess" value="always" />
<!--[if !IE]>-->
<object type="application/x-shockwave-flash" data="Untitled-1.swf" width="50" height="50">
<param name="movie" value="Untitled-1.swf" />
<param name="quality" value="high" />
<param name="bgcolor" value="#ffffff" />
<param name="play" value="true" />
<param name="loop" value="true" />
<param name="wmode" value="transparent" />
<param name="scale" value="showall" />
<param name="menu" value="true" />
<param name="devicefont" value="false" />
<param name="salign" value="" />
<param name="allowScriptAccess" value="always" />
<!--<![endif]-->
<a href="http://www.adobe.com/go/getflash">
<img src="http://www.adobe.com/images/shared/download_buttons/get_flash_player.gif" alt="Get Adobe Flash player" />
</a>
<!--[if !IE]>-->
</object>
<!--<![endif]-->
</object>
</div>
<a href="#" onclick="
document.body.style.overflow = document.body.style.overflow == 'hidden' ? '' : 'hidden';
return false;
">overflow: hidden;</a>
<script>
function f(){
document.body.appendChild(
document.createTextNode('x')
);
}
</script>
</body>
</html>
swf, fla