WYSIWYG xml editor cursor position Iframe FireFox problem

Hi all,
I'm trying to do an WYSIWYG xml editor using Iframe.
I want that this editor to autocolor and autoclose xml tags in real time when user type xml code in iframe.To do that i attache keyup,keydown events to the iframe.When an event is fired i copy the iframe content into textarea then i edit the content(color the xml tags add colse tags...) and then set the edit content back to the iframe. The problem is that in FireFox after i set the new content in iframe cursor position is always at the start of iframe,and i want to be where user want...
This is the code(i found on this forum and he had the same problem ) that work's for IE:

function color1()
{
if(document.selection)
{
//here I save the cursor position
var rng = document.getElementById('iframe').document.selecti on.createRange();
var position = { left : rng.offsetLeft, top : rng.offsetTop};
// get the content(another function set the content of //textarea with iframe content )
var txt = document.getElementById("ta");
var content=txt.value;
//edit content
content=strip(content);
content=colorTags(content);
// now I restore the updated content
document.getElementById('iframe').contentWindow.do cument.body.innerHTML=content;
// here I restore cursor position
rng.moveToPoint(position.left, position.top);
rng.select();
}
else if(document.getSelection) //FF detection
{ //THIS IS THE PROBLEM I don't know what to do







}

Or any ideas how to do this in other way?
Thank you!