Problem with HTML IFrame

Hi,

I have this css:

#popup-background
{
    position: fixed;
    top: 0%;
    left: 0%;
    width: 100%;
    height: 100%;
    background-color: #000;
    z-index:1001;
    -moz-opacity: 0.4;
    opacity: 0.4;
    display: none;
}

#popup
{
    width: 600px;
    height: 400px;
    z-index: 1002;
    display: none;
    position: absolute;
    top:0;
    bottom: 0;
    left: 0;
    right: 0;
    margin: auto;
    background: white;
}

#frame
{
    width: 580px;
    height: 390px;
}

And this iframe:

        <div id="popup-background" onclick="$('#popup-background').hide();$('#popup').hide();$('#frame').attr('src','');"></div>
        <div id="popup">
            <div id="loading2" class="loading"></div>
            <iframe id="frame" frameborder="0"></iframe>
        </div>

The popup-background is the gray background for the whole page, the popup is the div, which is showing and which has the iframe in it.

As you see, the popup div is a bit larger as the iframe.

In this iframe I load an aspx.page. On this page, the main div has the size 550x380.

But after loading in the iframe, a vertical scrollbar appears:

I figured out, that it's because the latest textbox.

And curiosly it works on ie, but not on firefox (12).

I have already tried to set the body and form size of the loaded page, but no success.

It is very fustrating, because I can't unterstand what can be here the problem.

Can you help me?

Thanks.