--- a/layout/xul/nsMenuFrame.cpp
+++ b/layout/xul/nsMenuFrame.cpp
@@ -1263,16 +1263,23 @@ nsMenuFrame::GetActiveChild(dom::Element
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsMenuFrame::SetActiveChild(dom::Element* aChild) {
   nsMenuPopupFrame* popupFrame = GetPopup();
   if (!popupFrame) return NS_ERROR_FAILURE;
 
+  // Force the child frames within the popup to be generated.
+  AutoWeakFrame weakFrame(popupFrame);
+  popupFrame->GenerateFrames();
+  if (!weakFrame.IsAlive()) {
+    return NS_OK;
+  }
+
   if (!aChild) {
     // Remove the current selection
     popupFrame->ChangeMenuItem(nullptr, false, false);
     return NS_OK;
   }
 
   nsMenuFrame* menu = do_QueryFrame(aChild->GetPrimaryFrame());
   if (menu) popupFrame->ChangeMenuItem(menu, false, false);

--- a/layout/xul/nsMenuPopupFrame.cpp
+++ b/layout/xul/nsMenuPopupFrame.cpp
@@ -1607,16 +1607,28 @@ nsresult nsMenuPopupFrame::SetPopupPosit
     if (aNotify) {
       nsXULPopupPositionedEvent::DispatchIfNeeded(mContent, false, false);
     }
   }
 
   return NS_OK;
 }
 
+void nsMenuPopupFrame::GenerateFrames()
+{
+  const bool generateFrames = IsLeaf();
+  MOZ_ASSERT_IF(generateFrames, !mGeneratedChildren);
+  mGeneratedChildren = true;
+  if (generateFrames) {
+    MOZ_ASSERT(PrincipalChildList().IsEmpty());
+    nsCOMPtr<nsIPresShell> presShell = PresContext()->PresShell();
+    presShell->FrameConstructor()->GenerateChildFrames(this);
+  }
+}
+
 /* virtual */
 nsMenuFrame* nsMenuPopupFrame::GetCurrentMenuItem() { return mCurrentMenu; }
 
 LayoutDeviceIntRect nsMenuPopupFrame::GetConstraintRect(
     const LayoutDeviceIntRect& aAnchorRect,
     const LayoutDeviceIntRect& aRootScreenRect, nsPopupLevel aPopupLevel) {
   LayoutDeviceIntRect screenRectPixels;
 

--- a/layout/xul/nsMenuPopupFrame.h
+++ b/layout/xul/nsMenuPopupFrame.h
@@ -261,18 +261,18 @@ class nsMenuPopupFrame final : public ns
   // (or the frame for mAnchorContent if aAnchorFrame is null), anchored at a
   // rectangle, or at a specific point if a screen position is set. The popup
   // will be adjusted so that it is on screen. If aIsMove is true, then the
   // popup is being moved, and should not be flipped. If aNotify is true, then
   // a popuppositioned event is sent.
   nsresult SetPopupPosition(nsIFrame* aAnchorFrame, bool aIsMove,
                             bool aSizedToPopup, bool aNotify);
 
-  bool HasGeneratedChildren() { return mGeneratedChildren; }
-  void SetGeneratedChildren() { mGeneratedChildren = true; }
+  // Force the children to be generated if they have not already been generated.
+  void GenerateFrames();
 
   // called when the Enter key is pressed while the popup is open. This will
   // just pass the call down to the current menu, if any. If a current menu
   // should be opened as a result, this method should return the frame for
   // that menu, or null if no menu should be opened. Also, calling Enter will
   // reset the current incremental search string, calculated in
   // FindMenuWithShortcut.
   nsMenuFrame* Enter(mozilla::WidgetGUIEvent* aEvent);

--- a/layout/xul/nsXULPopupManager.cpp
+++ b/layout/xul/nsXULPopupManager.cpp
@@ -1274,34 +1274,25 @@ void nsXULPopupManager::FirePopupShowing
                                               bool aIsContextMenu,
                                               bool aSelectFirstItem,
                                               Event* aTriggerEvent) {
   nsCOMPtr<nsIContent> popup = aPopup;  // keep a strong reference to the popup
 
   nsMenuPopupFrame* popupFrame = do_QueryFrame(aPopup->GetPrimaryFrame());
   if (!popupFrame) return;
 
-  nsPresContext* presContext = popupFrame->PresContext();
-  nsCOMPtr<nsIPresShell> presShell = presContext->PresShell();
-  nsPopupType popupType = popupFrame->PopupType();
-
-  // generate the child frames if they have not already been generated
-  const bool generateFrames = popupFrame->IsLeaf();
-  MOZ_ASSERT_IF(generateFrames, !popupFrame->HasGeneratedChildren());
-  popupFrame->SetGeneratedChildren();
-  if (generateFrames) {
-    MOZ_ASSERT(popupFrame->PrincipalChildList().IsEmpty());
-    presShell->FrameConstructor()->GenerateChildFrames(popupFrame);
-  }
+  popupFrame->GenerateFrames();
 
   // get the frame again
   nsIFrame* frame = aPopup->GetPrimaryFrame();
   if (!frame) return;
 
-  presShell->FrameNeedsReflow(frame, nsIPresShell::eTreeChange,
+  nsPresContext* presContext = popupFrame->PresContext();
+  nsCOMPtr<nsIPresShell> presShell = presContext->PresShell();
+  presShell->FrameNeedsReflow(popupFrame, nsIPresShell::eTreeChange,
                               NS_FRAME_HAS_DIRTY_CHILDREN);
 
   // cache the popup so that document.popupNode can retrieve the trigger node
   // during the popupshowing event. It will be cleared below after the event
   // has fired.
   mOpeningPopup = aPopup;
 
   nsEventStatus status = nsEventStatus_eIgnore;
@@ -1334,16 +1325,17 @@ void nsXULPopupManager::FirePopupShowing
   mOpeningPopup = nullptr;
 
   mCachedModifiers = 0;
 
   // if a panel, blur whatever has focus so that the panel can take the focus.
   // This is done after the popupshowing event in case that event is cancelled.
   // Using noautofocus="true" will disable this behaviour, which is needed for
   // the autocomplete widget as it manages focus itself.
+  nsPopupType popupType = popupFrame->PopupType();
   if (popupType == ePopupTypePanel &&
       !popup->AsElement()->AttrValueIs(kNameSpaceID_None,
                                        nsGkAtoms::noautofocus, nsGkAtoms::_true,
                                        eCaseMatters)) {
     nsFocusManager* fm = nsFocusManager::GetFocusManager();
     if (fm) {
       Document* doc = popup->GetUncomposedDoc();
 

--- a/toolkit/content/tests/chrome/test_menulist_keynav.xul
+++ b/toolkit/content/tests/chrome/test_menulist_keynav.xul
@@ -22,16 +22,24 @@
 <menulist id="list2">
   <menupopup id="popup" onpopupshown="checkCursorNavigation();">
     <menuitem id="b1" label="One"/>
     <menuitem id="b2" label="Two" selected="true"/>
     <menuitem id="b3" label="Three"/>
     <menuitem id="b4" label="Four"/>
   </menupopup>
 </menulist>
+<menulist id="list3" sizetopopup="none">
+  <menupopup>
+    <menuitem id="s1" label="One"/>
+    <menuitem id="s2" label="Two"/>
+    <menuitem id="s3" label="Three"/>
+    <menuitem id="s4" label="Four"/>
+  </menupopup>
+</menulist>
 
 <script class="testbody" type="application/javascript">
 <![CDATA[
 
 SimpleTest.waitForExplicitFinish();
 
 var gShowPopup = false;
 var gModifiers = 0;
@@ -270,16 +278,26 @@ function checkCursorNavigation()
 
   synthesizeKey("KEY_ArrowUp", {altKey: true});
   is(list.open, ismac, "alt+up closes popup");
 
   if (ismac) {
     list.open = false;
   }
 
+  // Finally, test a menulist with sizetopopup="none" to ensure keyboard navigation
+  // still works when the popup has not been opened.
+  if (!ismac) {
+    let unsizedMenulist = document.getElementById("list3");
+    unsizedMenulist.focus();
+    synthesizeKey("KEY_ArrowDown");
+    is(unsizedMenulist.selectedIndex, 1, "correct menulist index on keydown");
+    is(unsizedMenulist.label, "Two", "correct menulist label on keydown");
+  }
+
   SimpleTest.finish();
 }
 
 SimpleTest.waitForFocus(runTests);
 
 ]]>
 </script>