How to dynamically set "src" for browser or iframe element (Firefox extension)

Question

I am trying to create a Firefox extension that uses a flex application. I have tried to wrap it in XUL types ( and ) and I have no preference as to which one I use... whichever works.

The problem is that whenever I use a relative path (access through chrome:// or mySWF.html) the flash fails to load.

I have a method to search for the absolute path (it's posted below) but I cannot for the life of me figure out a way to dynamically change the src of either an iframe or browser.

 <script type="text/javascript">
 function loadSWF() {
  alert("loadSWF!");
  var fullPath = "file:///" + extensionPath.path.replace(/\\/g,"/") +  "/chrome/content/HelloWorld.html";
  top.document.getElementById('AppFrame').setAttribute("src",fullPath);
 }
 </script>

Below are my 2 methods of calling the flex app:

 <iframe
  type="content"
  src=??????
  flex="1"
  id="AppFrame"
  name="AppFrame"
  onLoad="loadSWF();"/>

 <browser 
  id="browserid"
  type="content"
  src=??????
  flex="1"/>

How can I call my function to set the src attribute???