Firefox extension development: How to find out if window has focus

Question

Hey :) how can i find out if the active window has focus or no focus (means: hidden, minimized or what ever) ? I fetch the active window with "Application.activeWindow" but lets say i minimize the window or change focus to another program than firefox. How do i figure out that the window has no focus ?

Example:

setInterval(function() {
    var sourceURL = Application.activeWindow.activeTab.uri.spec;
    Firebug.Console.log(sourceURL); // This should be printet only if the window has focus and is not minimized.
}, 4000);