解決方法

<link href="http://cdn.bootcss.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" rel="stylesheet">
    <style>
        @-moz-document url-prefix() {
            select.form-control {
                -moz-appearance: none;
                appearance: none;
                background-image: url("http://221.228.109.114:8083/manage/more/firefox_select_icon.png");
                background-repeat: no-repeat;
                background-position: calc(100% - 7px) 50%;
                background-size: 2% auto;
                border-radius:3px;
                padding:0;
            }
        }
    </style>