--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -5739,37 +5739,43 @@ HTMLMediaElement::UpdateReadyStateIntern
 
   if (mMediaSource) {
     // readyState has changed, assuming it's following the pending mediasource
     // operations. Notify the Mediasource that the operations have completed.
     mMediaSource->CompletePendingTransactions();
   }
 
   enum NextFrameStatus nextFrameStatus = NextFrameStatus();
-  if (nextFrameStatus == NEXT_FRAME_UNAVAILABLE ||
-      (nextFrameStatus == NEXT_FRAME_UNAVAILABLE_BUFFERING &&
-       mWaitingForKey == WAITING_FOR_KEY)) {
-    if (mWaitingForKey != NOT_WAITING_FOR_KEY) {
+  if (mWaitingForKey == NOT_WAITING_FOR_KEY) {
+    if (nextFrameStatus == NEXT_FRAME_UNAVAILABLE &&
+        mDecoder && !mDecoder->IsEnded()) {
+      nextFrameStatus = mDecoder->NextFrameBufferedStatus();
+    }
+  } else if (mWaitingForKey == WAITING_FOR_KEY) {
+    if (nextFrameStatus == NEXT_FRAME_UNAVAILABLE ||
+        nextFrameStatus == NEXT_FRAME_UNAVAILABLE_BUFFERING) {
       // 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 && !mDecoder->IsEnded()) {
-      nextFrameStatus = mDecoder->NextFrameBufferedStatus();
+      mWaitingForKey = WAITING_FOR_KEY_DISPATCHED;
+      DispatchAsyncEvent(NS_LITERAL_STRING("waitingforkey"));
+    }
+  } else { // mWaitingForKey == WAITING_FOR_KEY_DISPATCHED
+    if (nextFrameStatus == NEXT_FRAME_AVAILABLE) {
+      // We have new frames after dispatching "waitingforkey".
+      // This means we've got the key and can reset mWaitingForKey now.
+      mWaitingForKey = NOT_WAITING_FOR_KEY;
     }
   }
 
   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;
@@ -5905,17 +5911,16 @@ 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 = NOT_WAITING_FOR_KEY;
       NotifyAboutPlaying();
     }
   }
 
   CheckAutoplayDataReady();
 
   if (oldState < nsIDOMHTMLMediaElement::HAVE_ENOUGH_DATA &&
       mReadyState >= nsIDOMHTMLMediaElement::HAVE_ENOUGH_DATA) {