--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -973,17 +973,17 @@ void HTMLMediaElement::AbortExistingLoad
   mSuspendedAfterFirstFrame = false;
   mAllowSuspendAfterFirstFrame = true;
   mHaveQueuedSelectResource = false;
   mSuspendedForPreloadNone = false;
   mDownloadSuspendedByCache = false;
   mMediaInfo = MediaInfo();
   mIsEncrypted = false;
   mPendingEncryptedInitData.mInitDatas.Clear();
-  mWaitingForKey = false;
+  mWaitingForKey = NOT_WAITING_FOR_KEY;
   mSourcePointer = nullptr;
 
   mTags = nullptr;
 
   if (mNetworkState != nsIDOMHTMLMediaElement::NETWORK_EMPTY) {
     NS_ASSERTION(!mDecoder && !mSrcStream, "How did someone setup a new stream/decoder already?");
     // ChangeNetworkState() will call UpdateAudioChannelPlayingState()
     // indirectly which depends on mPaused. So we need to update mPaused first.
@@ -2886,17 +2886,17 @@ HTMLMediaElement::HTMLMediaElement(alrea
     mHasPlayedOrSeeked(false),
     mHasSelfReference(false),
     mShuttingDown(false),
     mSuspendedForPreloadNone(false),
     mSrcStreamIsPlaying(false),
     mMediaSecurityVerified(false),
     mCORSMode(CORS_NONE),
     mIsEncrypted(false),
-    mWaitingForKey(false),
+    mWaitingForKey(NOT_WAITING_FOR_KEY),
     mDownloadSuspendedByCache(false, "HTMLMediaElement::mDownloadSuspendedByCache"),
     mAudioChannelVolume(1.0),
     mPlayingThroughTheAudioChannel(false),
     mDisableVideo(false),
     mHasUserInteraction(false),
     mFirstFrameLoaded(false),
     mDefaultPlaybackStartPosition(0.0),
     mIsAudioTrackAudible(false),
@@ -4349,16 +4349,19 @@ void HTMLMediaElement::MetadataLoaded(co
         }
       }
     }
   }
 }
 
 void HTMLMediaElement::FirstFrameLoaded()
 {
+  LOG(LogLevel::Debug, ("%p, FirstFrameLoaded() mFirstFrameLoaded=%d mWaitingForKey=%d",
+      this, mFirstFrameLoaded, mWaitingForKey));
+
   NS_ASSERTION(!mSuspendedAfterFirstFrame, "Should not have already suspended");
 
   if (!mFirstFrameLoaded) {
     mFirstFrameLoaded = true;
     UpdateReadyStateInternal();
   }
 
   ChangeDelayLoadStatus(false);
@@ -4704,18 +4707,36 @@ HTMLMediaElement::UpdateReadyStateIntern
     }
     if (hasVideoTracks) {
       mediaInfo.EnableVideo();
     }
     MetadataLoaded(&mediaInfo, nsAutoPtr<const MetadataTags>(nullptr));
   }
 
   enum NextFrameStatus nextFrameStatus = NextFrameStatus();
-  if (mDecoder && nextFrameStatus == NEXT_FRAME_UNAVAILABLE) {
-    nextFrameStatus = mDecoder->NextFrameBufferedStatus();
+  if (nextFrameStatus == NEXT_FRAME_UNAVAILABLE) {
+    if (mWaitingForKey != NOT_WAITING_FOR_KEY) {
+      // http://w3c.github.io/encrypted-media/#wait-for-key
+      // Continuing 7.3.4 Queue a "waitingforkey" Event
+      // 4. Queue a task to fire a simple event named waitingforkey
+      // at the media element.
+      if (mWaitingForKey == WAITING_FOR_KEY) {
+        mWaitingForKey = WAITING_FOR_KEY_DISPATCHED;
+        DispatchAsyncEvent(NS_LITERAL_STRING("waitingforkey"));
+      }
+      // 5. Set the readyState of media element to HAVE_METADATA.
+      // NOTE: We'll change to HAVE_CURRENT_DATA or HAVE_METADATA
+      // depending on whether we've loaded the first frame or not
+      // below.
+      // 6. Suspend playback.
+      // Note: Playback will already be stalled, as the next frame is
+      // unavailable.
+    } else if (mDecoder) {
+      nextFrameStatus = mDecoder->NextFrameBufferedStatus();
+    }
   }
 
   if (nextFrameStatus == MediaDecoderOwner::NEXT_FRAME_UNAVAILABLE_SEEKING) {
     LOG(LogLevel::Debug, ("MediaElement %p UpdateReadyStateInternal() "
                           "NEXT_FRAME_UNAVAILABLE_SEEKING; Forcing HAVE_METADATA", this));
     ChangeReadyState(nsIDOMHTMLMediaElement::HAVE_METADATA);
     return;
   }
@@ -4840,17 +4861,17 @@ void HTMLMediaElement::ChangeReadyState(
     DispatchAsyncEvent(NS_LITERAL_STRING("loadeddata"));
     mLoadedDataFired = true;
   }
 
   if (oldState < nsIDOMHTMLMediaElement::HAVE_FUTURE_DATA &&
       mReadyState >= nsIDOMHTMLMediaElement::HAVE_FUTURE_DATA) {
     DispatchAsyncEvent(NS_LITERAL_STRING("canplay"));
     if (!mPaused) {
-      mWaitingForKey = false;
+      mWaitingForKey = NOT_WAITING_FOR_KEY;
       DispatchAsyncEvent(NS_LITERAL_STRING("playing"));
     }
   }
 
   CheckAutoplayDataReady();
 
   if (oldState < nsIDOMHTMLMediaElement::HAVE_ENOUGH_DATA &&
       mReadyState >= nsIDOMHTMLMediaElement::HAVE_ENOUGH_DATA) {
@@ -6233,31 +6254,28 @@ HTMLMediaElement::GetTopLevelPrincipal()
   }
   principal = doc->NodePrincipal();
   return principal.forget();
 }
 
 void
 HTMLMediaElement::CannotDecryptWaitingForKey()
 {
-  // See: http://w3c.github.io/encrypted-media/#dom-evt-waitingforkey
-  // Spec: 7.5.4 Queue a "waitingforkey" Event
-  // Spec: 1. Let the media element be the specified HTMLMediaElement object.
-
-  // Note, existing code will handle the ready state of this element, as
-  // such this function does not handle changing or checking mReadyState.
-
-  // Spec: 2. If the media element's waiting for key value is true, abort these steps.
-  if (!mWaitingForKey) {
-    // Spec: 3. Set the media element's waiting for key value to true.
-    // Spec: 4. Queue a task to fire a simple event named waitingforkey at the media element.
-    DispatchAsyncEvent(NS_LITERAL_STRING("waitingforkey"));
-    mWaitingForKey = true;
-    // No need to explicitly suspend playback, it happens automatically when
-    // it's starving for decoded frames.
+  LOG(LogLevel::Debug, ("%p, CannotDecryptWaitingForKey()", this));
+
+  // http://w3c.github.io/encrypted-media/#wait-for-key
+  // 7.3.4 Queue a "waitingforkey" Event
+  // 1. Let the media element be the specified HTMLMediaElement object.
+  // 2. If the media element's waiting for key value is true, abort these steps.
+  if (mWaitingForKey == NOT_WAITING_FOR_KEY) {
+    // 3. Set the media element's waiting for key value to true.
+    // Note: algorithm continues in UpdateReadyStateInternal() when all decoded
+    // data enqueued in the MDSM is consumed.
+    mWaitingForKey = WAITING_FOR_KEY;
+    UpdateReadyStateInternal();
   }
 }
 
 NS_IMETHODIMP HTMLMediaElement::WindowAudioCaptureChanged(bool aCapture)
 {
   MOZ_ASSERT(mAudioChannelAgent);
 
   if (mAudioCapturedByWindow != aCapture) {

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -1584,20 +1584,26 @@ protected:
   CORSMode mCORSMode;
 
   // Info about the played media.
   MediaInfo mMediaInfo;
 
   // True if the media has encryption information.
   bool mIsEncrypted;
 
-  // True when the CDM cannot decrypt the current block, and the
-  // waitingforkey event has been fired. Back to false when keys have become
-  // available and we can advance the current playback position.
-  bool mWaitingForKey;
+  enum WaitingForKeyState {
+    NOT_WAITING_FOR_KEY = 0,
+    WAITING_FOR_KEY = 1,
+    WAITING_FOR_KEY_DISPATCHED = 2
+  };
+
+  // True when the CDM cannot decrypt the current block due to lacking a key.
+  // Note: the "waitingforkey" event is not dispatched until all decoded data
+  // has been rendered.
+  WaitingForKeyState mWaitingForKey;
 
   // Listens for waitingForKey events from the owned decoder.
   MediaEventListener mWaitingForKeyListener;
 
   // Init Data that needs to be sent in 'encrypted' events in MetadataLoaded().
   EncryptionInfo mPendingEncryptedInitData;
 
   // True if the media's channel's download has been suspended.