--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -1267,56 +1267,64 @@ MediaDecoder::SetSuspendTaint(bool aTain
   UpdateVideoDecodeMode();
 }
 
 void
 MediaDecoder::UpdateVideoDecodeMode()
 {
   // The MDSM may yet be set.
   if (!mDecoderStateMachine) {
+    LOG("UpdateVideoDecodeMode(), early return because we don't have MDSM.");
     return;
   }
 
   // If an element is in-tree with UNTRACKED visibility, the visibility is
   // incomplete and don't update the video decode mode.
   if (mIsElementInTree && mElementVisibility == Visibility::UNTRACKED) {
+    LOG("UpdateVideoDecodeMode(), early return because we have incomplete visibility states.");
     return;
   }
 
   // If mHasSuspendTaint is set, never suspend the video decoder.
   if (mHasSuspendTaint) {
+    LOG("UpdateVideoDecodeMode(), set Normal because the element has been tainted.");
     mDecoderStateMachine->SetVideoDecodeMode(VideoDecodeMode::Normal);
     return;
   }
 
   // Don't suspend elements that is not in tree.
   if (!mIsElementInTree) {
+    LOG("UpdateVideoDecodeMode(), set Normal because the element is not in tree.");
     mDecoderStateMachine->SetVideoDecodeMode(VideoDecodeMode::Normal);
     return;
   }
 
   // If mForcedHidden is set, suspend the video decoder anyway.
   if (mForcedHidden) {
+    LOG("UpdateVideoDecodeMode(), set Suspend because the element is forced to be suspended.");
     mDecoderStateMachine->SetVideoDecodeMode(VideoDecodeMode::Suspend);
     return;
   }
 
   // Resume decoding in the advance, even the element is in the background.
   if (mIsBackgroundVideoDecodingAllowed) {
+    LOG("UpdateVideoDecodeMode(), set Normal because the tab is in background and hovered.");
     mDecoderStateMachine->SetVideoDecodeMode(VideoDecodeMode::Normal);
     return;
   }
 
   // Otherwise, depends on the owner's visibility state.
   // A element is visible only if its document is visible and the element
   // itself is visible.
   if (mIsDocumentVisible &&
       mElementVisibility == Visibility::APPROXIMATELY_VISIBLE) {
+    LOG("UpdateVideoDecodeMode(), set Normal because the element visible.");
     mDecoderStateMachine->SetVideoDecodeMode(VideoDecodeMode::Normal);
   } else {
+    LOG("UpdateVideoDecodeMode(), set Suspend because the element is not visible.");
     mDecoderStateMachine->SetVideoDecodeMode(VideoDecodeMode::Suspend);
   }
 }
 
 bool
 MediaDecoder::HasSuspendTaint() const
 {
   MOZ_ASSERT(NS_IsMainThread());

--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -3161,31 +3161,34 @@ void MediaDecoderStateMachine::SetVideoD
     &MediaDecoderStateMachine::SetVideoDecodeModeInternal,
     aMode);
   OwnerThread()->DispatchStateChange(r.forget());
 }
 
 void MediaDecoderStateMachine::SetVideoDecodeModeInternal(VideoDecodeMode aMode)
 {
   MOZ_ASSERT(OnTaskQueue());
-  LOG("VideoDecodeModeChanged: VideoDecodeMode=(%s->%s), mVideoDecodeSuspended=%c",
-      mVideoDecodeMode == VideoDecodeMode::Normal ? "Normal" : "Suspend",
-      aMode == VideoDecodeMode::Normal ? "Normal" : "Suspend",
-      mVideoDecodeSuspended ? 'T' : 'F');
 
   // Should not suspend decoding if we don't turn on the pref.
   if (!MediaPrefs::MDSMSuspendBackgroundVideoEnabled() &&
       aMode == VideoDecodeMode::Suspend) {
+    LOG("SetVideoDecodeModeInternal(), early return because preference off and set to Suspend");
     return;
   }
 
   if (aMode == mVideoDecodeMode) {
+    LOG("SetVideoDecodeModeInternal(), early return because the mode does not change");
     return;
   }
 
+  LOG("SetVideoDecodeModeInternal(), VideoDecodeMode=(%s->%s), mVideoDecodeSuspended=%c",
+      mVideoDecodeMode == VideoDecodeMode::Normal ? "Normal" : "Suspend",
+      aMode == VideoDecodeMode::Normal ? "Normal" : "Suspend",
+      mVideoDecodeSuspended ? 'T' : 'F');
+
   // Set new video decode mode.
   mVideoDecodeMode = aMode;
 
   // Start timer to trigger suspended video decoding.
   if (mVideoDecodeMode == VideoDecodeMode::Suspend) {
     TimeStamp target = TimeStamp::Now() + SuspendBackgroundVideoDelay();
 
     RefPtr<MediaDecoderStateMachine> self = this;