<video autoplay></video> <img src=""> <canvas style=“display: none;”></canvas> <button id=“capture”>snapshot</button>

<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。