help with javascript - offsetheight different in i.e. and FF

[ArchAnGeL]

I am using the code below to set Div's to equal height

<script type="text/javascript">
var DivHeight = document.getElementById("content1").offsetHeight;
document.write(DivHeight)
document.getElementById("leftcol").style.height = DivHeight+"px";
</script>

My problem is ie is returning 649px which is correct yet FF ony 120?

The leftcol Div starts life as 1200px as declared in the css file

Thanks for any help