[XulRunner] Problem with browser element

Hi,

I already asked my question there https://forums.mozilla.org/addons/viewtopic.php?f=7&t=9930
, but someone advised me to ask my question on this group.
I hope it's the right place. My post will be mostly a copy/paste of
the post in the mozilla forums.

I am trying to build an application based on XulRunner. This
application will need to display some html pages so I want to use the
browser element (https://developer.mozilla.org/en/XUL/browser)
However it does not seem to work. Here is my very simple code :

main.xul :

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window id="main" title="This is my app" width="500" height="500"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
	<browser src="http://www.mozilla.org" type="content-primary" />
</window>

When I try to start the application (using Firefox as XulRunner), I
get this :

Gmo:MyApp gmo$ /Applications/Firefox.app/Contents/MacOS/firefox -app /
Users/gmo/code/MyApp/application.ini -purgecaches
2012-05-16 20:37:12.445 firefox[5798:903] invalid drawable

I'm running with Firefox 13 on Mac OS X Snow Leopard. I have the same
result on Firefox 13 with WIndows XP (albeit no error message, just a
blank window)
Note that if I replace the browser by an iframe, the page is correctly
dislayed.

Any ideas ?

Thanks