get real screen size on FireFox

In firefox screen size change with page zoom

HTML

<body>
my screen resolution : <p id="demo"></p>
</body>

JavaScript

/*
following function work fine in chrome, even page zoom this gives same values, but in firefox screen size change with page zoom, I need real screen size even on page zoom. 
*/
function getScreenSize(){
  document.getElementById("demo").innerHTML =  screen.width + " : " + screen.height;
}

getScreenSize();