--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -557,26 +557,31 @@ private:
  *   SHUTDOWN if any decode error.
  *   BUFFERING if playback can't continue due to lack of decoded data.
  *   COMPLETED when having decoded all audio/video data.
  */
 class MediaDecoderStateMachine::DecodingState
   : public MediaDecoderStateMachine::StateObject
 {
 public:
-  explicit DecodingState(Master* aPtr) : StateObject(aPtr) {}
+  explicit DecodingState(Master* aPtr)
+    : StateObject(aPtr)
+    , mDormantTimer(OwnerThread())
+  {
+  }
 
   void Enter();
 
   void Exit() override
   {
     if (!mDecodeStartTime.IsNull()) {
       TimeDuration decodeDuration = TimeStamp::Now() - mDecodeStartTime;
       SLOG("Exiting DECODING, decoded for %.3lfs", decodeDuration.ToSeconds());
     }
+    mDormantTimer.Reset();
   }
 
   void Step() override
   {
     if (mMaster->mPlayState != MediaDecoder::PLAY_STATE_PLAYING &&
         mMaster->IsPlaying()) {
       // We're playing, but the element/decoder is in paused state. Stop
       // playing!
@@ -645,16 +650,22 @@ public:
   }
 
   void HandlePlayStateChanged(MediaDecoder::PlayState aPlayState) override
   {
     if (aPlayState == MediaDecoder::PLAY_STATE_PLAYING) {
       // Schedule Step() to check if we can start playback.
       mMaster->ScheduleStateMachine();
     }
+
+    if (aPlayState == MediaDecoder::PLAY_STATE_PAUSED) {
+      StartDormantTimer();
+    } else {
+      mDormantTimer.Reset();
+    }
   }
 
   void DumpDebugInfo() override
   {
     SDUMP("mIsPrerolling=%d", mIsPrerolling);
   }
 
 private:
@@ -711,27 +722,54 @@ private:
         (DonePrerollingAudio() || Reader()->IsWaitingAudioData()) &&
         (DonePrerollingVideo() || Reader()->IsWaitingVideoData())) {
       mIsPrerolling = false;
       // Check if we can start playback.
       mMaster->ScheduleStateMachine();
     }
   }
 
+  void StartDormantTimer()
+  {
+    auto timeout = MediaPrefs::DormantOnPauseTimeout();
+    if (timeout < 0) {
+      // Disabled when timeout is negative.
+      return;
+    } else if (timeout == 0) {
+      // Enter dormant immediately without scheduling a timer.
+      HandleDormant(true);
+      return;
+    }
+
+    TimeStamp target = TimeStamp::Now() +
+      TimeDuration::FromMilliseconds(timeout);
+
+    mDormantTimer.Ensure(target,
+      [this] () {
+        mDormantTimer.CompleteRequest();
+        HandleDormant(true);
+      }, [this] () {
+        mDormantTimer.CompleteRequest();
+      });
+  }
+
   // Time at which we started decoding.
   TimeStamp mDecodeStartTime;
 
   // When we start decoding (either for the first time, or after a pause)
   // we may be low on decoded data. We don't want our "low data" logic to
   // kick in and decide that we're low on decoded data because the download
   // can't keep up with the decode, and cause us to pause playback. So we
   // have a "preroll" stage, where we ignore the results of our "low data"
   // logic during the first few frames of our decode. This occurs during
   // playback.
   bool mIsPrerolling = true;
+
+  // Fired when playback is paused for a while to enter dormant.
+  DelayedScheduler mDormantTimer;
 };
 
 /**
  * Purpose: seek to a particular new playback position.
  *
  * Transition to:
  *   DORMANT if any dormant request.
  *   SEEKING if any new seek request.
@@ -1437,16 +1475,21 @@ DecodingState::Enter()
   mDecodeStartTime = TimeStamp::Now();
 
   MaybeStopPrerolling();
 
   // Ensure that we've got tasks enqueued to decode data if we need to.
   mMaster->DispatchDecodeTasksIfNeeded();
 
   mMaster->ScheduleStateMachine();
+
+  // Will enter dormant when playback is paused for a while.
+  if (mMaster->mPlayState == MediaDecoder::PLAY_STATE_PAUSED) {
+    StartDormantTimer();
+  }
 }
 
 RefPtr<MediaDecoder::SeekPromise>
 MediaDecoderStateMachine::
 DecodingState::HandleSeek(SeekTarget aTarget)
 {
   SLOG("Changed state to SEEKING (to %lld)", aTarget.GetTime().ToMicroseconds());
   SeekJob seekJob;

--- a/dom/media/MediaPrefs.h
+++ b/dom/media/MediaPrefs.h
@@ -131,16 +131,17 @@ private:
   DECL_MEDIA_PREF("media.decoder.fuzzing.dont-delay-inputexhausted", PDMFuzzingDelayInputExhausted, bool, true);
   DECL_MEDIA_PREF("media.gmp.decoder.enabled",                PDMGMPEnabled, bool, true);
   DECL_MEDIA_PREF("media.gmp.decoder.aac",                    GMPAACPreferred, uint32_t, 0);
   DECL_MEDIA_PREF("media.gmp.decoder.h264",                   GMPH264Preferred, uint32_t, 0);
 
   // MediaDecoderStateMachine
   DECL_MEDIA_PREF("media.suspend-bkgnd-video.enabled",        MDSMSuspendBackgroundVideoEnabled, bool, false);
   DECL_MEDIA_PREF("media.suspend-bkgnd-video.delay-ms",       MDSMSuspendBackgroundVideoDelay, AtomicUint32, SUSPEND_BACKGROUND_VIDEO_DELAY_MS);
+  DECL_MEDIA_PREF("media.dormant-on-pause-timeout-ms",        DormantOnPauseTimeout, int32_t, 5000);
 
   // WebSpeech
   DECL_MEDIA_PREF("media.webspeech.synth.force_global_queue", WebSpeechForceGlobal, bool, false);
   DECL_MEDIA_PREF("media.webspeech.test.enable",              WebSpeechTestEnabled, bool, false);
   DECL_MEDIA_PREF("media.webspeech.test.fake_fsm_events",     WebSpeechFakeFSMEvents, bool, false);
   DECL_MEDIA_PREF(TEST_PREFERENCE_FAKE_RECOGNITION_SERVICE,   WebSpeechFakeRecognitionService, bool, false);
   DECL_MEDIA_PREF("media.webspeech.recognition.enable",       WebSpeechRecognitionEnabled, bool, false);
   DECL_MEDIA_PREF("media.webspeech.recognition.force_enable", WebSpeechRecognitionForceEnabled, bool, false);

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -306,16 +306,23 @@ pref("mathml.scale_stretchy_operators.en
 
 // Disable MediaError.message.
 #ifdef RELEASE_OR_BETA
 pref("dom.MediaError.message.enabled", false);
 #else
 pref("dom.MediaError.message.enabled", true);
 #endif
 
+// Enabled on nightly only until we fix mochitest failures.
+#ifdef NIGHTLY_BUILD
+pref("media.dormant-on-pause-timeout-ms", 5000);
+#else
+pref("media.dormant-on-pause-timeout-ms", -1);
+#endif
+
 // Media cache size in kilobytes
 pref("media.cache_size", 512000);
 // When a network connection is suspended, don't resume it until the
 // amount of buffered data falls below this threshold (in seconds).
 pref("media.cache_resume_threshold", 999999);
 // Stop reading ahead when our buffered data is this many seconds ahead
 // of the current playback position. This limit can stop us from using arbitrary
 // amounts of network bandwidth prefetching huge videos.