Opening a script from within an iframe

I have a xul application with several files
y.xul
x.xml
z.js

'y.xul' contains the following fragment...
...
 <groupbox id="widget_y" flex="1">
	    <caption label="Picture Widget"/>
            <iframe src="y.xml" id="frame_y" flex="1"/>
 </groupbox>
...

the file 'y.xml' is an svg file that contains the following
fragment...
...
<svg xmlns="http://www.w3.org/2000/svg"
     xmlns:xlink="http://www.w3.org/1999/xlink"
     xmlns:html="http://www.w3.org/1999/xhtml"
     onload="init(evt);">

  <script type="text/javascript" src="context/z.js"/>
...

the file 'z.js' contains...

function jsdump(str)
{
  Components.classes['@mozilla.org/consoleservice;1']
            .getService(Components.interfaces.nsIConsoleService)
            .logStringMessage(str);
}

The problem is that the script 'z.js' is not loaded.
Apparently the <script> element does not have the correct path for src
attribute?
If the line...
  <script type="text/javascript" src="context/z.js"/>
...is replaced with...
 <script type="text/javascript">
<![CDATA[
function jsdump(str)
{
  Components.classes['@mozilla.org/consoleservice;1']
            .getService(Components.interfaces.nsIConsoleService)
            .logStringMessage(str);
}
// ]]>
</script>
...the jsdump function is properly defined.
The three files all live in the same 'myapp/chrome/context/'
directory.
I have tried the following variations but to no avail.
<script type="text/javascript" src="z.js"/>
<script type="text/javascript" src="context/z.js"/>
<script type="text/javascript" src="chrome://myapp/content/z.js"/>
<script type="text/javascript" src="file://full-path/content/z.js"/>