Thunderbird progressmeter update

Thunderbird Add-ons
#1

I’m trying to update the progress-meter XUL element in javascript code. However when I try setting the value property, there is no visual update. Here is my code:

var statusBar = document.getElementById("status-bar");
statusBar.value = parseInt(percentage);

And here is part of my XUL file:

<statusbar>
	<statusbarpanel>
		...
		<progressmeter 
			id="status-bar"
			mode="determined" 
			max="100"/>
	</statusbarpanel>
  </statusbar>

Why does not my progress-meter see the value updates ?