<script >
var video = document.querySelector('video');
var canvas = document.querySelector('canvas');
var img = document.querySelector('img');
var ctx = canvas.getContext('2d');
var localMediaStream = null;



var snapshot = function () {
    alert('开始拍照');
    console.log(localMediaStream);
    if (localMediaStream) {
        alert('拍照了');
        ctx.drawImage(video, 0, 0);
        img.src = canvas.toDataURL('image/png');
        console.log(canvas.toDataURL('image/webp'));
        console.log(img.src);
    }
};

var sizeCanvas = function () {
    setTimeout(function () {
        canvas.width = video.videoWidth;
        canvas.height = video.videoHeight;
        img.width = video.videoWidth;
        img.height = video.videoHeight;
    }, 100);
};

var btnCapture = document.getElementById('capture');
btnCapture.addEventListener('click', snapshot, false);



navigator.mozGetUserMedia(
    {video: true},
    function (stream) {
        console.log(window.URL);
        video.src = window.URL.createObjectURL(stream);
        localMediaStream = stream;
        sizeCanvas();
    },
    function () {
        alert('your browser does not support getUserMedia');
    }
);

</script>

各位大神好！这是从网上找来的代码，在ff里面运行之后，点击snapshot之后，img的url是“data ,”，能不能说明一下是什么原因？
另外，其实之前在ff里成功了，但是不知道为什么重启ff之后又失败了，小白求指教，希望各位大大勿喷。