negative margins for iframe showing different in FF and IE

i am trying to display a form from a website inside an iframe

the problem is, this form displays perfectly in IE:




but it displays wrong in FF:




heres the CSS:

#container{
  width: 380px;
  height: 288px;
  overflow: hidden;
  border: 3px solid #4398d4;
}
#container iframe {
  width: 800px;
  height: 600px;
  margin: -305px 0px 0px -40px;
}

Code (markup):

heres the HTML:

<div id="container">
<iframe src="http://wow.eroticagateway.com/signup/signup.php?step=signup&qualify=1&site=226&nats=NzUxNjg6MzoyMjY" scrolling="no" frameborder="0"></iframe>
</div>

Code (markup):

any idea why this is happening?