--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -3249,17 +3249,20 @@ nsCSSFrameConstructor::ConstructSelectFr
     // initialized as absolutely positioned.
     nsContainerFrame* scrolledFrame =
       NS_NewSelectsAreaFrame(mPresShell, styleContext, flags);
 
     InitializeSelectFrame(aState, listFrame, scrolledFrame, content,
                           comboboxFrame, listStyle, true,
                           aItem.mPendingBinding, childItems);
 
-    NS_ASSERTION(listFrame->GetView(), "ListFrame's view is nullptr");
+    if (!nsLayoutUtils::IsContentSelectEnabled()) {
+      // TODO(kuoe0) Remove this assertion when content-select is shipped.
+      NS_ASSERTION(listFrame->GetView(), "ListFrame's view is nullptr");
+    }
 
     // Create display and button frames from the combobox's anonymous content.
     // The anonymous content is appended to existing anonymous content for this
     // element (the scrollbars).
     //
     // nsComboboxControlFrame needs special frame creation behavior for its first
     // piece of anonymous content, which means that we can't take the normal
     // ProcessChildren path.

--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -7429,17 +7429,18 @@ nsLayoutUtils::GetFrameTransparency(nsIF
     return eTransparencyTransparent;
   return eTransparencyOpaque;
 }
 
 static bool IsPopupFrame(nsIFrame* aFrame)
 {
   // aFrame is a popup it's the list control frame dropdown for a combobox.
   LayoutFrameType frameType = aFrame->Type();
-  if (frameType == LayoutFrameType::ListControl) {
+  if (!nsLayoutUtils::IsContentSelectEnabled() &&
+      frameType == LayoutFrameType::ListControl) {
     nsListControlFrame* lcf = static_cast<nsListControlFrame*>(aFrame);
     return lcf->IsInDropDownMode();
   }
 
   // ... or if it's a XUL menupopup frame.
   return frameType == LayoutFrameType::MenuPopup;
 }
 

--- a/layout/forms/nsComboboxControlFrame.cpp
+++ b/layout/forms/nsComboboxControlFrame.cpp
@@ -302,16 +302,18 @@ nsComboboxControlFrame::SetFocus(bool aO
   // rect to be drawn. This is much faster than ReResolvingStyle
   // Bug 32920
   InvalidateFrame();
 }
 
 void
 nsComboboxControlFrame::ShowPopup(bool aShowPopup)
 {
+  // TODO(kuoe0) Remove this function when content-select is enabled.
+
   nsView* view = mDropdownFrame->GetView();
   nsViewManager* viewManager = view->GetViewManager();
 
   if (aShowPopup) {
     nsRect rect = mDropdownFrame->GetRect();
     rect.x = rect.y = 0;
     viewManager->ResizeView(view, rect);
     viewManager->SetViewVisibility(view, nsViewVisibility_kShow);
@@ -330,16 +332,26 @@ nsComboboxControlFrame::ShowPopup(bool a
 
     shell->HandleDOMEventWithTarget(mContent, &event, &status);
   }
 }
 
 bool
 nsComboboxControlFrame::ShowList(bool aShowList)
 {
+
+  // TODO(kuoe0) Remove this function when content-select is enabled.
+  //
+  // This function is used to handle the widget/view stuff, so we just return
+  // when content-select is enabled. And the following callee, ShowPopup(), will
+  // also be ignored, it is only used to show and hide the widget.
+  if (nsLayoutUtils::IsContentSelectEnabled()) {
+    return true;
+  }
+
   nsView* view = mDropdownFrame->GetView();
   if (aShowList) {
     NS_ASSERTION(!view->HasWidget(),
                  "We shouldn't have a widget before we need to display the popup");
 
     // Create the widget for the drop-down list
     view->GetViewManager()->SetViewFloating(view, true);
 
@@ -447,17 +459,18 @@ nsComboboxControlFrame::ReflowDropdown(n
   // both sets of mComputedBorderPadding.
   nscoord forcedISize = aReflowInput.ComputedISize() +
     aReflowInput.ComputedLogicalBorderPadding().IStartEnd(wm) -
     kidReflowInput.ComputedLogicalBorderPadding().IStartEnd(wm);
   kidReflowInput.SetComputedISize(std::max(kidReflowInput.ComputedISize(),
                                          forcedISize));
 
   // ensure we start off hidden
-  if (!mDroppedDown && GetStateBits() & NS_FRAME_FIRST_REFLOW) {
+  if (!nsLayoutUtils::IsContentSelectEnabled() &&
+      !mDroppedDown && GetStateBits() & NS_FRAME_FIRST_REFLOW) {
     nsView* view = mDropdownFrame->GetView();
     nsViewManager* viewManager = view->GetViewManager();
     viewManager->SetViewVisibility(view, nsViewVisibility_kHide);
     nsRect emptyRect(0, 0, 0, 0);
     viewManager->ResizeView(view, emptyRect);
   }
 
   // Allow the child to move/size/change-visibility its view if it's currently
@@ -650,17 +663,17 @@ nsComboboxControlFrame::AbsolutelyPositi
   }
 
   WritingMode wm = GetWritingMode();
   LogicalPoint translation(wm);
   nscoord before, after;
   mLastDropDownAfterScreenBCoord = nscoord_MIN;
   GetAvailableDropdownSpace(wm, &before, &after, &translation);
   if (before <= 0 && after <= 0) {
-    if (IsDroppedDown()) {
+    if (!nsLayoutUtils::IsContentSelectEnabled() && IsDroppedDown()) {
       // Hide the view immediately to minimize flicker.
       nsView* view = mDropdownFrame->GetView();
       view->GetViewManager()->SetViewVisibility(view, nsViewVisibility_kHide);
       NS_DispatchToCurrentThread(new nsAsyncRollup(this));
     }
     return eDropDownPositionSuppressed;
   }
 
@@ -1401,17 +1414,17 @@ nsComboboxControlFrame::DestroyFrom(nsIF
     sFocused = nullptr;
   }
 
   // Revoke any pending RedisplayTextEvent
   mRedisplayTextEvent.Revoke();
 
   nsCheckboxRadioFrame::RegUnRegAccessKey(static_cast<nsIFrame*>(this), false);
 
-  if (mDroppedDown) {
+  if (!nsLayoutUtils::IsContentSelectEnabled() && mDroppedDown) {
     MOZ_ASSERT(mDropdownFrame, "mDroppedDown without frame");
     nsView* view = mDropdownFrame->GetView();
     MOZ_ASSERT(view);
     nsIWidget* widget = view->GetWidget();
     if (widget) {
       widget->CaptureRollupEvents(this, false);
     }
   }
@@ -1480,17 +1493,18 @@ nsComboboxControlFrame::Rollup(uint32_t 
   if (!weakFrame.IsAlive()) {
     return consume;
   }
   ShowDropDown(false); // might destroy us
   if (weakFrame.IsAlive()) {
     mListControlFrame->CaptureMouseEvents(false);
   }
 
-  if (aFlush && weakFrame.IsAlive()) {
+  if (!nsLayoutUtils::IsContentSelectEnabled() &&
+      aFlush && weakFrame.IsAlive()) {
     // The popup's visibility doesn't update until the minimize animation has
     // finished, so call UpdateWidgetGeometry to update it right away.
     nsViewManager* viewManager = mDropdownFrame->GetView()->GetViewManager();
     viewManager->UpdateWidgetGeometry(); // might destroy us
   }
 
   if (!weakFrame.IsAlive()) {
     return consume;
@@ -1500,16 +1514,20 @@ nsComboboxControlFrame::Rollup(uint32_t 
     *aLastRolledUp = GetContent();
   }
   return consume;
 }
 
 nsIWidget*
 nsComboboxControlFrame::GetRollupWidget()
 {
+  if (nsLayoutUtils::IsContentSelectEnabled()) {
+    return nullptr;
+  }
+
   nsView* view = mDropdownFrame->GetView();
   MOZ_ASSERT(view);
   return view->GetWidget();
 }
 
 void
 nsComboboxControlFrame::RollupFromList()
 {

--- a/layout/forms/nsListControlFrame.cpp
+++ b/layout/forms/nsListControlFrame.cpp
@@ -114,16 +114,22 @@ nsListControlFrame::nsListControlFrame(n
 
 //---------------------------------------------------------
 nsListControlFrame::~nsListControlFrame()
 {
   mComboboxFrame = nullptr;
 }
 
 static bool ShouldFireDropDownEvent() {
+  // We don't need to fire the event to SelectContentHelper when content-select
+  // is enabled.
+  if (nsLayoutUtils::IsContentSelectEnabled()) {
+    return false;
+  }
+
   return (XRE_IsContentProcess() &&
           Preferences::GetBool("browser.tabs.remote.desktopbehavior", false)) ||
          Preferences::GetBool("dom.select_popup_in_parent.enabled", false);
 }
 
 // for Bug 47302 (remove this comment later)
 void
 nsListControlFrame::DestroyFrom(nsIFrame* aDestructRoot, PostDestroyData& aPostDestroyData)
@@ -964,17 +970,19 @@ nsListControlFrame::SetInitialChildList(
 //---------------------------------------------------------
 void
 nsListControlFrame::Init(nsIContent*       aContent,
                          nsContainerFrame* aParent,
                          nsIFrame*         aPrevInFlow)
 {
   nsHTMLScrollFrame::Init(aContent, aParent, aPrevInFlow);
 
-  if (IsInDropDownMode()) {
+  if (!nsLayoutUtils::IsContentSelectEnabled() &&
+      IsInDropDownMode()) {
+    // TODO(kuoe0) Remove the following code when content-select is enabled.
     AddStateBits(NS_FRAME_IN_POPUP);
     CreateView();
   }
 
   // we shouldn't have to unregister this listener because when
   // our frame goes away all these content node go away as well
   // because our frame is the only one who references them.
   // we need to hook up our listeners before the editor is initialized