I have struggled with same this past day. It seems to matter how you access the iframe. If you use document.getElementById() you get an Iframe object, which has no onload event. However, if you access through window.frames[] array, for example

var iframeWindow = top.frames['iframeID'],

you get a window object, which does have onload event.

(ie uses the frame id attribute , but ff uses name attribute. So use both and make the same)

You can then assign

iframeWindow.onload=function(){iframeContent=iframeWindow.document.body.innerHTML;};

Notice, since iframeWindow is a window object, you use window syntax to access content.