Javascript ทำงานบน Google Chrome แต่ไม่ทำงานใน Firefox

window.onload = function(){
	var AttachDragTo = (function () {
		var _AttachDragTo = function (el) {
			this.el = el;
			this.mouse_is_down = false;
			
			this.init();
		};
		
		_AttachDragTo.prototype = {
			onMousemove: function (e) {
				if ( !this.mouse_is_down ) return;
				var tg = e.target,
					x = e.clientX,
					y = e.clientY;
				
				tg.style.backgroundPositionY = y - this.origin_y + this.origin_bg_pos_y + 'px';
				document.getElementById("position_y").value = tg.style.backgroundPositionY;
			},
		
			onMousedown: function(e) {
				this.mouse_is_down = true;
				this.origin_x = e.clientX;
				this.origin_y = e.clientY;
			},
		
			onMouseup: function(e) {
				var tg = e.target,
					styles = getComputedStyle(tg);
		
				this.mouse_is_down = false;
				this.origin_bg_pos_y = parseInt(styles.getPropertyValue('background-position-y'), 10 );
			},
			
			init: function () {
				var styles = getComputedStyle(this.el);
				this.origin_bg_pos_y = parseInt(styles.getPropertyValue('background-position-y'), 10);

				this.el.addEventListener('mousedown', this.onMousedown.bind(this), false);
				this.el.addEventListener('mouseup', this.onMouseup.bind(this), false);
				this.el.addEventListener('mousemove', this.onMousemove.bind(this), false);
			}
		};
		
		return function ( el ) {
			new _AttachDragTo(el);
		};
	})();
	
	var dropzone = document.getElementById('drop_zone');
	AttachDragTo(dropzone);
	
}