Wrong value for window.innerWidth during onload event in Firefox for Android?
The problem (innerWidth === 980) persists for Firefox 40.0 under Android 4.4.4. A 1 msec wait is a circumvention. Replace
window.onload = myProgram;by
window.onload = function() {setTimeout(myProgram, 1)};
In the meantime I encountered this problem while adapting a fairly elaborate site to small screens. Firefox obeys the CSS following "@media only screen and (max-width: 768px)". However, when one tries to set event handlers depending on device widths, Firefox fails miserably. I needed the above trick with 0.5 second wait at all spots where I picked up the device width. This wait time was necessary for Nexus 7 (2012), but who knows what is needed for other devices?
Make sure your measurement is done when whole document is loaded and resized.
window.onload = showViewport;
window.onresize = showViewport;
function showViewport() {
var output=document.getElementById("output");
var width = Math.max(document.documentElement.clientWidth, window.innerWidth || 0);
var height= Math.max(document.documentElement.clientHeight, window.innerHeight || 0)
output.innerHTML = "Viewport size is " + width + "x" + height;
}<body>
<p id="output">Default Output</p>
</body>