Force open select HTML element that does not work in Firefox.

Question

Here is what I did: http://jsfiddle.net/cYHae/4/

Here is the code:

// <select> element displays its options on mousedown, not click.
showDropdown = function (element) {
    var event;
    event = document.createEvent('MouseEvents');
    event.initMouseEvent('mousedown', true, true, window, 0, event.screenX, event.screenY, event.clientX, event.clientY,event.ctrlKey, event.altKey, event.shiftKey, event.metaKey, 0, null);
    element.dispatchEvent(event);
};

// This isn't magic.
window.runThis = function () { 
    var dropdown = document.getElementById('dropdown');
    showDropdown(dropdown);
};

It just does not work in Firefox. Why?