Window.content.mozInnerScreenY value not working in Firefox 33.1

I am writing a Mozilla extension to compute Y-screen using this "window.content.mozInnerScreenY" API. After updating my firefox version to version 33.1, I cannot view Mozilla in the maximized window, but choosing to view in the Maximized window reverts to the smaller window size. Just to check I commented this line of code in the extension logic then the window size works well.

Why is the above value not working for me. kindly help me to fix this problem. the code is given below:

**var  appcontent = document.getElementById("appcontent");
var myExtension = {  
    init: function() {  
        // The event can be DOMContentLoaded, pageshow, pagehide, load or unload.           
          if(appcontent)
           { 
              appcontent.addEventListener("resize", this.onmyPageResize, false);
           }               
      },  
    onmyPageResize: function(aEvent) {  

        screenY= window.content.mozInnerScreenY;
        //Process screenY
    }  
}

 window.addEventListener("load", function load(event){  
    window.removeEventListener("load", load, false); //remove listener, no longer needed  
   myExtension.init();    
},false);**