diff --git a/browser/base/content/pictureInPictureContent.js b/browser/base/content/pictureInPictureContent.js
index 074415b598..a2735082fd 100644
--- a/browser/base/content/pictureInPictureContent.js
+++ b/browser/base/content/pictureInPictureContent.js
@@ -19,6 +19,11 @@ function update() {
   if (!source) {
     return;
   }
+  // Gate already-buffered audio here rather than waiting for source mute
+  // changes to travel through the decoder's capture queue.
+  if (output) {
+    output.muted = !!source.srcObject || source.muted;
+  }
   // Capture streams do not produce frames while their source is paused.
   // Keep a snapshot for that case, without changing the source's play state.
   still.hidden = !source.paused && !source.ended;
@@ -116,14 +121,14 @@ addMessageListener("PictureInPicture:Init", message => {
                              "max-width:none;max-height:none;object-fit:contain";
       doc.body.appendChild(element);
     }
-    // Decoder capture redirects audio to the captured stream. Its samples
-    // already include the source's volume/mute settings. MediaStream sources
-    // keep their own audio output, so mute only that case to avoid an echo.
-    output.muted = !!source.srcObject;
+    // Decoder capture redirects audio to this output. PiP content mute is
+    // applied here; other volume/mute policies remain in the decoder.
+    // MediaStream sources keep their own output, so avoid doubling it.
+    output.muted = !!source.srcObject || source.muted;
     output.volume = 1;
-    source.mozPictureInPicture = true;
     stream = source.mozCaptureStream();
     output.srcObject = stream;
+    source.mozPictureInPicture = true;
     output.play().catch(error => {
       Components.utils.reportError(error);
       closePlayer();
@@ -161,6 +166,7 @@ addMessageListener("PictureInPicture:Command", message => {
       break;
     case "mute":
       source.muted = !source.muted;
+      update();
       break;
     case "seek":
       seekTo(message.data.time);
diff --git a/dom/html/HTMLMediaElement.cpp b/dom/html/HTMLMediaElement.cpp
index 0a1ecceeb5..269f134d39 100644
--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -2404,6 +2404,12 @@ void HTMLMediaElement::SetVolumeInternal()
   float effectiveVolume = ComputedVolume();
 
   if (mDecoder) {
+    // PiP applies content mute at its playback element, after the capture
+    // queue. Keep those samples available for immediate unmute, while still
+    // honoring audio-channel, playback-rate and disabled-track muting.
+    if (mPictureInPicture && !(mMuted & ~MUTED_BY_CONTENT)) {
+      effectiveVolume = float(mVolume * mAudioChannelVolume);
+    }
     mDecoder->SetVolume(effectiveVolume);
   } else if (MediaStream* stream = GetSrcMediaStream()) {
     if (mSrcStreamIsPlaying) {
@@ -5188,6 +5194,7 @@ void HTMLMediaElement::SetMozPictureInPicture(bool aEnabled)
   MOZ_ASSERT(NS_IsMainThread());
   mPictureInPicture = aEnabled;
   if (mDecoder && !mShuttingDown) {
+    SetVolumeInternal();
     mDecoder->NotifyOwnerActivityChanged(!IsHidden());
   }
 }
diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index eb20b94282..232443fc66 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -3207,6 +3207,12 @@ NS_IMETHODIMP nsWindow::HideWindowChrome(bool aShouldHide)
   ::SetWindowLongPtrW(hwnd, GWL_STYLE, style);
   ::SetWindowLongPtrW(hwnd, GWL_EXSTYLE, exStyle);
 
+  // Apply the new non-client metrics immediately. Otherwise the client area
+  // keeps its old titlebar/border dimensions until the first move or resize.
+  ::SetWindowPos(hwnd, nullptr, 0, 0, 0, 0,
+                 SWP_FRAMECHANGED | SWP_NOMOVE | SWP_NOSIZE |
+                 SWP_NOZORDER | SWP_NOACTIVATE);
+
   return NS_OK;
 }