jquery中position().top在IE和FF中的取值

============ HTML ================
<body>
<div class="main">
<div class="roll" id="roll">
<div class="caption">aaaaaaaaaaaaa</div>
<div class="caption">bbbbbbbbbbbbb</div>
<div class="caption">ccccccccccccc</div>
<div class="caption">ddddddddddddd</div>
<div class="caption">eeeeeeeeeeeee</div>
<div class="caption">fffffffffffff</div>
<div class="caption">ggggggggggggg</div>
<div class="caption">hhhhhhhhhhhhh</div>
<div class="caption">iiiiiiiiiiiii</div>
<div class="caption">jjjjjjjjjjjjj</div>
<div class="caption">kkkkkkkkkkkkk</div>
<div class="caption">mmmmmmmmmmmmm</div>
<div class="caption">nnnnnnnnnnnnn</div>
</div>
</div>
</body>=========== CSS ================
.main{
width:200px;
height:200px;
overflow:hidden;
border:1px #666 solid;
}.roll{
position:absolute;
}.caption{
height:20px;
line-height:20px;
}=========== JQuery ==============打印: $("#roll").position().top 的值，怎么不一样呢？IE中是16
FF中是0而且：overflow:hidden;在FF中不起作用？——虽然没有出现滚动条，但是也没有隐藏多出的内容。
求解？？？？？