--- a/dom/media/tests/mochitest/head.js
+++ b/dom/media/tests/mochitest/head.js
@@ -2,53 +2,59 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var Cc = SpecialPowers.Cc;
 var Ci = SpecialPowers.Ci;
 
-// Specifies whether we are using fake streams to run this automation
-var FAKE_ENABLED = true;
-var TEST_AUDIO_FREQ = 1000;
-try {
-  var audioDevice = SpecialPowers.getCharPref('media.audio_loopback_dev');
-  var videoDevice = SpecialPowers.getCharPref('media.video_loopback_dev');
-  dump('TEST DEVICES: Using media devices:\n');
-  dump('audio: ' + audioDevice + '\nvideo: ' + videoDevice + '\n');
-  FAKE_ENABLED = false;
+// Specifies if we want fake audio streams for this run
+let WANT_FAKE_AUDIO = true;
+// Specifies if we want fake video streams for this run
+let WANT_FAKE_VIDEO = true;
+let TEST_AUDIO_FREQ = 1000;
+let audioDevice = SpecialPowers.getCharPref("media.audio_loopback_dev", "");
+if (audioDevice) {
+  WANT_FAKE_AUDIO = false;
   // It will be updated to 440 when/if DefaultLoopbackTone is instantiated.
   TEST_AUDIO_FREQ = -1;
-} catch (e) {
-  dump('TEST DEVICES: No test devices found (in media.{audio,video}_loopback_dev, using fake streams.\n');
-  FAKE_ENABLED = true;
+  dump("TEST DEVICES: Got loopback audio: " + audioDevice + "\n");
+} else {
+  dump("TEST DEVICES: No test device found in media.audio_loopback_dev, using fake audio streams.\n");
+}
+let videoDevice = SpecialPowers.getCharPref("media.video_loopback_dev", "");
+if (videoDevice) {
+  WANT_FAKE_VIDEO = false;
+  dump("TEST DEVICES: Got loopback video: " + videoDevice + "\n");
+} else {
+  dump("TEST DEVICES: No test device found in media.video_loopback_dev, using fake video streams.\n");
 }
 
 /**
  *  Global flag to skip LoopbackTone
  */
-var DISABLE_LOOPBACK_TONE = false
+let DISABLE_LOOPBACK_TONE = false;
 /**
  * Helper class to setup a sine tone of a given frequency.
  */
 class LoopbackTone {
   constructor(audioContext, frequency) {
     if (!audioContext) {
       throw new Error("You must provide a valid AudioContext");
     }
     this.oscNode = audioContext.createOscillator();
     var gainNode = audioContext.createGain();
     gainNode.gain.value = 0.5;
     this.oscNode.connect(gainNode);
     gainNode.connect(audioContext.destination);
     this.changeFrequency(frequency);
   }
 
-  // Method should be used when FAKE_ENABLED is false.
+  // Method should be used when WANT_FAKE_AUDIO is false.
   start() {
     if (!this.oscNode) {
       throw new Error("Attempt to start a stopped LoopbackTone");
     }
     info(`Start loopback tone at ${this.oscNode.frequency.value}`);
     this.oscNode.start();
   }
 
@@ -63,17 +69,17 @@ class LoopbackTone {
 
   stop() {
     if (!this.oscNode) {
       throw new Error("Attempt to stop a stopped LoopbackTone");
     }
     this.oscNode.stop();
     this.oscNode = null;
   }
-};
+}
 // Object that holds the default loopback tone.
 var DefaultLoopbackTone = null;
 
 /**
  * This class provides helpers around analysing the audio content in a stream
  * using WebAudio AnalyserNodes.
  *
  * @constructor
@@ -343,17 +349,17 @@ function createMediaElementForTrack(trac
 /**
  * Wrapper function for mediaDevices.getUserMedia used by some tests. Whether
  * to use fake devices or not is now determined in pref further below instead.
  *
  * @param {Dictionary} constraints
  *        The constraints for this mozGetUserMedia callback
  */
 function getUserMedia(constraints) {
-  if (!FAKE_ENABLED
+  if (!WANT_FAKE_AUDIO
       && !constraints.fake
       && constraints.audio
       && !DISABLE_LOOPBACK_TONE) {
     // Loopback device is configured, start the default loopback tone
     if (!DefaultLoopbackTone) {
       TEST_AUDIO_FREQ = 440;
       DefaultLoopbackTone = new LoopbackTone(new AudioContext, TEST_AUDIO_FREQ);
       DefaultLoopbackTone.start();
@@ -390,24 +396,26 @@ function setupEnvironment() {
       ['media.peerconnection.enabled', true],
       ['media.peerconnection.identity.enabled', true],
       ['media.peerconnection.identity.timeout', 120000],
       ['media.peerconnection.ice.stun_client_maximum_transmits', 14],
       ['media.peerconnection.ice.trickle_grace_period', 30000],
       ['media.peerconnection.remoteTrackId.enabled', true],
       ['media.peerconnection.rtpsourcesapi.enabled', true],
       ['media.navigator.permission.disabled', true],
-      ['media.navigator.streams.fake', FAKE_ENABLED],
+      // If either fake audio or video is desired we enable fake streams.
+      // If loopback devices are set they will be chosen instead of fakes in gecko.
+      ['media.navigator.streams.fake', WANT_FAKE_AUDIO || WANT_FAKE_VIDEO],
       ['media.getusermedia.screensharing.enabled', true],
       ['media.getusermedia.audiocapture.enabled', true],
       ['media.recorder.audio_node.enabled', true]
     ]
   };
 
-  if (!FAKE_ENABLED) {
+  if (!WANT_FAKE_AUDIO) {
     defaultMochitestPrefs.set.push(
       ["media.volume_scale", "1"],
     );
   }
 
   const isAndroid = !!navigator.userAgent.includes("Android");
 
   if (isAndroid) {