Add-on bar or navigation toolbar icon showing same status across windows

	I want to make an app which shows a status (let's say ON and OFF) by 
changing an icon image in the add-on bar or the navigation toolbar.

	To simplify the concept, let's say I have two PNG images, on.png and 
off.png and I need to show one of them according to a certain value.  I 
have gone through XUL tutorial 
(https://developer.mozilla.org/en-US/docs/XUL/School_tutorial) but I 
still have problem finding tutorials for my app.

	In particular, here are my problems:
* How am I supposed to change the image file (from on.png to off.png)?

* I have found another app which uses the trick of changing class for 
the above problem, but this leads to another problem: when there are 
more than one window, only the current window's icon image is changed.

	If someone has some tutorial on this, it would be a great help.  Thanks 
in advance.