I have code similar to the following:

<!-------START EXAMPLE-------->

<!--------parent.html--------->

<script type='text/javascript'>

function DoStuff()

{

$.post("dosomething.html");/*post is just an example, you can change post to 'get' or 'load' and it does the same thing*/

}

</script>

<iframe src="child.html"></iframe>

<!------------child.html--------->

<script type='text/javascript'>

parent.DoStuff();

</script>

<!-------END EXAMPLE------------->

The ajax call fails every time in Firefox (2.0.0.9). It works fine in IE 7 (haven't tested it in IE 6). The "child.html" does not have to be an iframe; it can simply be a popup window.