diff --git a/dom/media/test/eme.js b/dom/media/test/eme.js
index d5fd0a4d2afa..50a66b1642d6 100644
--- a/dom/media/test/eme.js
+++ b/dom/media/test/eme.js
@@ -390,12 +390,13 @@ function SetupEMEPref(callback) {
     [ "media.mediasource.enabled", true ],
     [ "media.mediasource.whitelist", false ],
     [ "media.fragmented-mp4.exposed", true ],
+    [ "media.eme.apiVisible", true ],
   ];
 
-  if (/Linux/.test(navigator.userAgent)) {
+  if (/Linux/.test(manifestNavigator().userAgent)) {
     prefs.push([ "media.fragmented-mp4.ffmpeg.enabled", true ]);
   } else if (SpecialPowers.Services.appinfo.name == "B2G" ||
-             !document.createElement('video').canPlayType("video/mp4")) {
+             !manifestVideo().canPlayType("video/mp4")) {
    // XXX remove once we have mp4 PlatformDecoderModules on all platforms.
    prefs.push([ "media.fragmented-mp4.use-blank-decoder", true ]);
   }
diff --git a/dom/media/test/manifest.js b/dom/media/test/manifest.js
index 5a3fac5aca28..695aa1d7ed7e 100644
--- a/dom/media/test/manifest.js
+++ b/dom/media/test/manifest.js
@@ -2,6 +2,26 @@
 // be ignored. To make sure tests respect that, we include a file of type
 // "bogus/duh" in each list.
 
+// Make sure to not touch navigator in here, since we want to push prefs that
+// will affect the APIs it exposes, but the set of exposed APIs is determined
+// when Navigator.prototype is created.  So if we touch navigator before pushing
+// the prefs, the APIs it exposes will not take those prefs into account.  We
+// work around this by using a navigator object from a different global for our
+// UA string testing.
+var gManifestNavigatorSource = document.documentElement.appendChild(document.createElement("iframe"));
+gManifestNavigatorSource.style.display = "none";
+function manifestNavigator() {
+  return gManifestNavigatorSource.contentWindow.navigator;
+}
+
+// Similarly, use a <video> element from a different global for canPlayType or
+// other feature testing.  If we used one from our global and did so before our
+// prefs are pushed, then we'd instantiate HTMLMediaElement.prototype before the
+// prefs are pushed and APIs we expect to be on that object would not be there.
+function manifestVideo() {
+  return gManifestNavigatorSource.contentDocument.createElement('video');
+}
+
 // These are small test files, good for just seeing if something loads. We
 // really only need one test file per backend here.
 var gSmallTests = [
@@ -334,7 +354,7 @@ var gOggTrackInfoResults = {
 // we've specified.
 function fileUriToSrc(path, mustExist) {
   // android mochitest doesn't support file://
-  if (navigator.appVersion.indexOf("Android") != -1 || SpecialPowers.Services.appinfo.name == "B2G")
+  if (manifestNavigator().appVersion.indexOf("Android") != -1 || SpecialPowers.Services.appinfo.name == "B2G")
     return path;
 
   const Ci = SpecialPowers.Ci;
@@ -458,7 +478,7 @@ var gFastSeekTests = [
 
 function IsWindows8OrLater() {
   var re = /Windows NT (\d.\d)/;
-  var winver = navigator.userAgent.match(re);
+  var winver = manifestNavigator().userAgent.match(re);
   return winver && winver.length == 2 && parseFloat(winver[1]) >= 6.2;
 }
 
@@ -473,7 +493,7 @@ var androidVersion = SpecialPowers.Cc['@mozilla.org/system-info;1']
                                   .getService(SpecialPowers.Ci.nsIPropertyBag2)
                                   .getProperty('version');
 // Fragmented MP4.
-if (navigator.userAgent.indexOf("Mobile") != -1 && androidVersion >= 18) {
+if (manifestNavigator().userAgent.indexOf("Mobile") != -1 && androidVersion >= 18) {
   gUnseekableTests = gUnseekableTests.concat([
     { name:"street.mp4", type:"video/mp4" }
   ]);
@@ -1049,4 +1069,4 @@ function setMediaTestsPrefs(callback, extraPrefs) {
   SpecialPowers.pushPrefEnv({"set": prefs}, callback);
 }
 
-SimpleTest.requestFlakyTimeout("untriaged");
\ No newline at end of file
+SimpleTest.requestFlakyTimeout("untriaged");
diff --git a/dom/tests/mochitest/general/test_interfaces.html b/dom/tests/mochitest/general/test_interfaces.html
index 35a1bfb07b81..277fbd2571f6 100644
--- a/dom/tests/mochitest/general/test_interfaces.html
+++ b/dom/tests/mochitest/general/test_interfaces.html
@@ -665,19 +665,19 @@ var interfaceNamesInGlobalScope =
 // IMPORTANT: Do not change this list without review from a DOM peer!
     "MediaError",
 // IMPORTANT: Do not change this list without review from a DOM peer!
-    {name: "MediaKeyError", pref: "media.eme.apiVisible"},
+    {name: "MediaKeyError", android: false},
 // IMPORTANT: Do not change this list without review from a DOM peer!
-    {name: "MediaEncryptedEvent", pref: "media.eme.apiVisible"},
+    {name: "MediaEncryptedEvent", android: false},
 // IMPORTANT: Do not change this list without review from a DOM peer!
-    {name: "MediaKeys", pref: "media.eme.apiVisible"},
+    {name: "MediaKeys", android: false},
 // IMPORTANT: Do not change this list without review from a DOM peer!
-    {name: "MediaKeySession", pref: "media.eme.apiVisible"},
+    {name: "MediaKeySession", android: false},
 // IMPORTANT: Do not change this list without review from a DOM peer!
-    {name: "MediaKeySystemAccess", pref: "media.eme.apiVisible"},
+    {name: "MediaKeySystemAccess", android: false},
 // IMPORTANT: Do not change this list without review from a DOM peer!
-    {name: "MediaKeyMessageEvent", pref: "media.eme.apiVisible"},
+    {name: "MediaKeyMessageEvent", android: false},
 // IMPORTANT: Do not change this list without review from a DOM peer!
-    {name: "MediaKeyStatusMap", pref: "media.eme.apiVisible"},
+    {name: "MediaKeyStatusMap", android: false},
 // IMPORTANT: Do not change this list without review from a DOM peer!
     "MediaList",
 // IMPORTANT: Do not change this list without review from a DOM peer!
diff --git a/testing/profiles/prefs_general.js b/testing/profiles/prefs_general.js
index 5a01a16c2fd4..c1d22c929ca3 100644
--- a/testing/profiles/prefs_general.js
+++ b/testing/profiles/prefs_general.js
@@ -292,7 +292,6 @@ user_pref("browser.search.countryCode", "US");
 user_pref("browser.selfsupport.url", "https://%(server)s/selfsupport-dummy/");
 
 user_pref("media.eme.enabled", true);
-user_pref("media.eme.apiVisible", true);
 
 #if defined(XP_WIN)
 user_pref("media.decoder.heuristic.dormant.timeout", 0);