On all platforms the command fullScreen=true has no effect; this was already true with xulRunner 1.8.*, and there is a well-known work-around:

setTimeout("window.fullScreen=true;",10)

By deferring the command, it works. On Mac OS X this has no immediate effect, you have to open the window a second time.
Moreover, if you have the typical developer preference to reload XUL, it won’t have effect at all.

In any case on Mac OS X, even with such limitation, the titlebar will stay…
Using xulRunner 1.8 it was possible to hide also the titlebar with an openDialog, but this is no longer true in xulRunner 1.9

How-to

The fullscreen window

It’s necessary to open the fullscreen window directly in fullscreen, using another window object as a launcher, because it seems to be impossible to swith a already opened window to fullscreen.

The window to be opened placing/sizing needs be determined at open time only, otherwise it will conflict with window atributes.

This window will be very simple, it won’t report any of sizing attributes:sizemode, width, height, screenX and screenY must be omitted.
Obviously we’ll also omit them from the persist window attribute, if any.

Note: don’t forget to include the hidechrome="true" attribute.

<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<!DOCTYPE window SYSTEM "chrome://main/locale/main.dtd" >

<window
	xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
	id="myFullScreenWindow"
	hidechrome="true"
	onload="myOnloadCode()"
>

The opener code

In the opener window we’ll issue a command like:

var uri = 'chrome://main/content/fullscreen.xul';
var winName = 'fullscreenWindow',
var winFeatures = 'chrome,dependent,top=0,left=0,width=800,height=600';
winFeatures += ",titlebar=no,fullscreen=yes";
window.open(uri,winName,winFeatures );

Note: It’s also possible to use window.openDialog() instead of window.open().

That’s all.