diff --git a/dom/media/test/dynamic_redirect.sjs b/dom/media/test/dynamic_redirect.sjs
index b78272c33073..4e43dfae2d5f 100644
--- a/dom/media/test/dynamic_redirect.sjs
+++ b/dom/media/test/dynamic_redirect.sjs
@@ -1,34 +1,37 @@
-function parseQuery(request, key) {
-  var params = request.queryString.split('&');
-  for (var j = 0; j < params.length; ++j) {
-    var p = params[j];
-	if (p == key)
-	  return true;
-    if (p.indexOf(key + "=") == 0)
-	  return p.substring(key.length + 1);
-	if (p.indexOf("=") < 0 && key == "")
-	  return p;
+function parseQuery(query, key) {
+  for (let p of query.split('&')) {
+    if (p == key) {
+      return true;
+    }
+    if (p.startsWith(key + "=")) {
+      return p.substring(key.length + 1);
+    }
   }
-  return false;
 }
 
 // Return seek.ogv file content for the first request with a given key.
 // All subsequent requests return a redirect to a different-origin resource.
 function handleRequest(request, response)
 {
-  var key = parseQuery(request, "key");
-  var resource = parseQuery(request, "res");
+  var query = request.queryString;
+  var key = parseQuery(query, "key");
+  var resource = parseQuery(query, "res");
+  var nested = parseQuery(query, "nested") || false;
 
-  if (getState(key) == "redirect") {
-    var origin = request.host == "mochi.test" ? "example.org" : "mochi.test:8888";
-    response.setStatusLine(request.httpVersion, 303, "See Other");
-    response.setHeader("Location", "http://" + origin + "/tests/dom/media/test/" + resource);
-    response.setHeader("Content-Type", "text/html");
-    return;
+  dump("Received request for key = "+ key +"\n");
+  if (!nested) {
+    if (getState(key) == "redirect") {
+      var origin = request.host == "mochi.test" ? "example.org" : "mochi.test:8888";
+      response.setStatusLine(request.httpVersion, 303, "See Other");
+      let url = "http://" + origin +
+                "/tests/dom/media/test/dynamic_redirect.sjs?nested&" + query;
+      dump("Redirecting to "+ url + "\n");
+      response.setHeader("Location", url);
+      response.setHeader("Content-Type", "text/html");
+      return;
+    }
+    setState(key, "redirect");
   }
-
-  setState(key, "redirect");
-
   var file = Components.classes["@mozilla.org/file/directory_service;1"].
                         getService(Components.interfaces.nsIProperties).
                         get("CurWorkD", Components.interfaces.nsILocalFile);
@@ -38,17 +41,27 @@ function handleRequest(request, response)
                         createInstance(Components.interfaces.nsIBinaryInputStream);
   var paths = "tests/dom/media/test/" + resource;
   var split = paths.split("/");
-  for(var i = 0; i < split.length; ++i) {
+  for (var i = 0; i < split.length; ++i) {
     file.append(split[i]);
   }
   fis.init(file, -1, -1, false);
-  dump("file=" + file + "\n");
+
   bis.setInputStream(fis);
   var bytes = bis.readBytes(bis.available());
+  let [from, to] = request.getHeader("range").split("=")[1].split("-").map(s => parseInt(s));
+  to = to || Math.max(from, bytes.length - 1);
+  byterange = bytes.substring(from, to + 1);
+
+  let contentRange = "bytes "+ from +"-"+ to +"/"+ bytes.length;
+  let contentLength = (to - from + 1).toString();
+  dump("Response Content-Range = "+ contentRange +"\n");
+  dump("Response Content-Length = "+ contentLength +"\n");
+
   response.setStatusLine(request.httpVersion, 206, "Partial Content");
-  response.setHeader("Content-Range", "bytes 0-" + (bytes.length - 1) + "/" + bytes.length);
-  response.setHeader("Content-Length", ""+bytes.length, false);
+  response.setHeader("Content-Range", contentRange);
+  response.setHeader("Content-Length", contentLength, false);
   response.setHeader("Content-Type", "video/ogg", false);
-  response.write(bytes, bytes.length);
+  response.setHeader("Accept-Ranges", "bytes", false);
+  response.write(byterange, byterange.length);
   bis.close();
 }
diff --git a/dom/media/test/mochitest.ini b/dom/media/test/mochitest.ini
index ec06bceb5df0..795f2c152e8e 100644
--- a/dom/media/test/mochitest.ini
+++ b/dom/media/test/mochitest.ini
@@ -693,6 +693,8 @@ tags=msg capturestream
 tags=msg
 [test_mediarecorder_getencodeddata.html]
 tags=msg
+[test_mediarecorder_principals.html]
+tags=msg
 [test_mediarecorder_record_4ch_audiocontext.html]
 tags=msg
 [test_mediarecorder_record_audiocontext.html]
diff --git a/dom/media/test/test_mediarecorder_principals.html b/dom/media/test/test_mediarecorder_principals.html
new file mode 100644
index 000000000000..d60f70f5438b
--- /dev/null
+++ b/dom/media/test/test_mediarecorder_principals.html
@@ -0,0 +1,119 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=489415
+-->
+<head>
+  <title>Test for MediaRecorder Reaction to Principal Change</title>
+  <script type="application/javascript" src="/MochiKit/MochiKit.js"></script>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+  <script type="text/javascript" src="manifest.js"></script>
+</head>
+<body>
+<div>
+  <a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1018299">Test for MediaRecorder Principal Handling</a>
+</div>
+
+<video id="v1" preload="auto"></video>
+<video id="v2" preload="auto"></video>
+
+<pre id="test">
+<script type="text/javascript">
+SimpleTest.waitForExplicitFinish();
+
+var pushPrefs = (...p) => new Promise(r => SpecialPowers.pushPrefEnv({set: p}, r));
+var throwOutside = e => setTimeout(() => { throw e; });
+
+// Generate a random key. The first load with that key will return
+// data, the second and subsequent loads with that key will return a redirect
+// to a different origin ('localhost:8888' will be redirected to 'example.org',
+// and 'example.org' will be redirected to 'localhost:8888'). We rely on the
+// fact that Ogg will do a seek to the end of the resource, triggering a new
+// load with the same key which will return a same-origin resource.
+// Loading data from two different origins should be detected by the media
+// cache and result in a null principal so that the MediaRecorder usages below
+// fail.
+let key = Math.floor(Math.random()*100000000);
+let interval;
+
+function testPrincipals(resource) {
+  if (!resource) {
+    todo(false, "No types supported");
+    return;
+  }
+  // Reduce cache size and cache-ahead to make HTMLMediaElement do partial requests.
+  return pushPrefs(['media.cache_readahead_limit', 2],
+                   ['media.cache_size', 192])
+  .then(() => {
+    // First test: Load file from same-origin first, then get redirected to
+    // another origin before attempting to record stream.
+    let video = document.getElementById("v1");
+    video.src =
+        "http://mochi.test:8888/tests/dom/media/test/dynamic_redirect.sjs?key=v1_" +
+        key + "&res=" + resource.name;
+    video.load();
+    // To limit readahead, avoid racing with playback and "catching up" mode.
+    return new Promise(resolve => video.oncanplaythrough = resolve).then(() => {
+      video.play();
+      interval = setInterval(() => info("video.currentTime = "+ video.currentTime), 1000);
+
+      let msg = "mediaRecorder.start() must throw SecurityError";
+      return new Promise(resolve => video.onplaying = resolve)
+      .then(() => waitUntil(() => video.currentTime > resource.duration / 2))
+      // Test failure of the next step only, so "catch-bypass" any errors above.
+      .then(() => Promise.resolve()
+        .then(() => new MediaRecorder(video.mozCaptureStreamUntilEnded()).start())
+        .then(() => ok(false, msg), e => is(e.name, "SecurityError", msg)), 0)
+      .then(() => clearInterval(interval));
+    });
+  })
+  .then(() => {
+    // Second test: Load file from same-origin first, but record ASAP, before
+    // getting redirected to another origin.
+    let video = document.getElementById("v2");
+    video.src =
+        "http://mochi.test:8888/tests/dom/media/test/dynamic_redirect.sjs?key=v2_" +
+        key + "&res=" + resource.name;
+    video.load();
+    let rec, hasStopped, hasEnded = new Promise(r => video.onended = r);
+    let data = [];
+
+    let msgNoThrow = "mediaRecorder.start() should not throw here";
+    let msgSecErr = "mediaRecorder.onerror must fire SecurityError";
+    let msgOnStop = "mediaRecorder.onstop must also have fired";
+    return new Promise(resolve => video.onloadedmetadata = resolve).then(() => {
+      rec = new MediaRecorder(video.mozCaptureStreamUntilEnded());
+      rec.ondataavailable = e => data.push(e.data);
+      rec.start();
+      hasStopped = new Promise(resolve => rec.onstop = resolve);
+      video.play();
+    })
+    .then(() => ok(true, msgNoThrow), e => is(e.name, null, msgNoThrow))
+    .then(() => Promise.race([
+      new Promise((_, reject) => rec.onerror = e => reject(new DOMException("", e.name))),
+      hasEnded
+    ]))
+    .then(() => ok(false, msgSecErr), e => is(e.name, "SecurityError", msgSecErr))
+    .then(() => Promise.race([hasStopped, hasEnded.then(() => Promise.reject())]))
+    .then(() => ok(true, msgOnStop), e => ok(false, msgOnStop))
+    .then(() => clearInterval(interval));
+  });
+}
+
+testPrincipals(getPlayableVideo(gSeekTests))
+.catch(e => throwOutside(e))
+.then(() => SimpleTest.finish())
+.catch(e => throwOutside(e));
+
+var stop = stream => stream.getTracks().forEach(track => track.stop());
+var wait = ms => new Promise(resolve => setTimeout(resolve, ms));
+var waitUntil = f => new Promise(resolve => {
+  var ival = setInterval(() => f() && resolve(clearInterval(ival)), 100);
+});
+
+</script>
+</pre>
+
+</body>
+</html>