火狐瀏覽器不支援e.offsetX，解決方法

function getOffsetX(e){
    var e =e||window.event;
    var srcObj = e.target || e.srcElement;
    if (e.offsetX){
        return e.offsetX;
    }else{
        var rect = srcObj.getBoundingClientRect();
        var clientx = e.clientX;
        return clientx - rect.left;
    }
}

或者
 var srcObj = e.target
 
 || e.srcElement;
return e.offsetX || (e.clientX - srcObj.getBoundingClientRect().left);

srcObj.getBoundingClientRect().left =>元素左邊距離頁面左邊的距離
e.clientX =>滑鼠相對於瀏覽器視窗可視區域的X，Y座標的距離