recorder.js

var script = 'data:text/plain,' + encodeURIComponent(`
	var cursors = {
		pointer: 'red',
		text: 'blue'
	};
	var lastEvent;
	var left = 200;
	var top = 0;
	var canvas = content.document.createElementNS('http://www.w3.org/1999/xhtml', 'canvas');
	canvas.width = 800;
	canvas.height = 500;
	var context = canvas.getContext('2d');
	var interval = content.setInterval(function() {
		context.drawWindow(content, left, top, canvas.width, canvas.height, '#fff');
		if (lastEvent) {
			var cursor = content.getComputedStyle(lastEvent.target).cursor;
			context.fillStyle = cursor in cursors ? cursors[cursor] : 'black';
			context.fillRect(lastEvent.clientX - left, lastEvent.clientY - top, 20, 20);
		}
	}, 40);
	content.addEventListener('mousemove', el);
	var stream = canvas.captureStream(25);
	var recorder = new content.MediaRecorder(stream);
	recorder.ondataavailable = function(event) {
		sendAsyncMessage('foobar2', event.data);
	};
	recorder.start();
	content.setTimeout(function() {
		recorder.stop();
		content.clearInterval(interval);
		content.removeEventListener('mousemove', el);
	}, 5000);

	function el(event) {
		lastEvent = event;
	}
`);
gBrowser.selectedBrowser.messageManager.addMessageListener('foobar2', function removeMe(message) {
	gBrowser.selectedBrowser.messageManager.removeMessageListener('foobar2', removeMe);
	let destFile = OS.Path.join(OS.Constants.Path.tmpDir, 'test.webm');
	let reader = new FileReader();
	reader.onloadend = function() {
		OS.File.openUnique(destFile).then(function({file}) {
			return file.write(new Uint8Array(reader.result));
		}).then(console.log.bind(console), console.error.bind(console));
	};
	reader.readAsArrayBuffer(message.data);
});
gBrowser.selectedBrowser.messageManager.loadFrameScript(script, false);