--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -7557,34 +7557,35 @@ nsGlobalWindow::CanMoveResizeWindows(Cal
       return false;
     }
 
     if (!CanSetProperty("dom.disable_window_move_resize")) {
       return false;
     }
 
     // Ignore the request if we have more than one tab in the window.
-    uint32_t itemCount = 0;
     if (XRE_IsContentProcess()) {
       nsCOMPtr<nsIDocShell> docShell = GetDocShell();
       if (docShell) {
         nsCOMPtr<nsITabChild> child = docShell->GetTabChild();
-        if (child) {
-          child->SendGetTabCount(&itemCount);
+        bool hasSiblings = true;
+        if (child &&
+            NS_SUCCEEDED(child->GetHasSiblings(&hasSiblings)) &&
+            hasSiblings) {
+          return false;
         }
       }
     } else {
       nsCOMPtr<nsIDocShellTreeOwner> treeOwner = GetTreeOwner();
-      if (!treeOwner || NS_FAILED(treeOwner->GetTabCount(&itemCount))) {
-        itemCount = 0;
+      uint32_t itemCount = 0;
+      if (treeOwner && NS_SUCCEEDED(treeOwner->GetTabCount(&itemCount)) &&
+          itemCount > 1) {
+        return false;
       }
     }
-    if (itemCount > 1) {
-      return false;
-    }
   }
 
   if (mDocShell) {
     bool allow;
     nsresult rv = mDocShell->GetAllowWindowControl(&allow);
     if (NS_SUCCEEDED(rv) && !allow)
       return false;
   }

--- a/dom/interfaces/base/nsITabChild.idl
+++ b/dom/interfaces/base/nsITabChild.idl
@@ -16,26 +16,29 @@ native CommandsArray(nsTArray<nsCString>
 interface nsITabChild : nsISupports
 {
   readonly attribute nsIContentFrameMessageManager messageManager;
 
   attribute nsIWebBrowserChrome3 webBrowserChrome;
 
   [notxpcom] void sendRequestFocus(in boolean canFocus);
 
-  [notxpcom] void sendGetTabCount(out uint32_t tabCount);
-
   [noscript, notxpcom] void enableDisableCommands(in AString action,
                                                   in CommandsArrayRef enabledCommands,
                                                   in CommandsArrayRef disabledCommands);
 
   [noscript] void remoteSizeShellTo(in int32_t width, in int32_t height,
                                     in int32_t shellItemWidth, in int32_t shellItemHeight);
 
   [noscript] void remoteDropLinks(in unsigned long linksCount,
                                   [array, size_is(linksCount)] in nsIDroppedLinkItem links);
 
   readonly attribute uint64_t tabId;
 
+  /*
+   * Indicates whether or not there are other tabs in this tab's window.
+   */
+  attribute boolean hasSiblings;
+
   [noscript, notxpcom] void beforeUnloadAdded();
   [noscript, notxpcom] void beforeUnloadRemoved();
 };
 

--- a/dom/ipc/PBrowser.ipdl
+++ b/dom/ipc/PBrowser.ipdl
@@ -511,24 +511,16 @@ parent:
                                      double aPointerPressure,
                                      uint32_t aPointerOrientation,
                                      uint64_t aObserverId);
     async SynthesizeNativeTouchTap(LayoutDeviceIntPoint aPoint,
                                    bool aLongTap,
                                    uint64_t aObserverId);
     async ClearNativeTouchSequence(uint64_t aObserverId);
 
-    /**
-     * Returns the number of tabs in the window via the out parameter.
-     * If the number of tabs can't be determined, returns 0.
-     *
-     * @param aValue where to store the tab count
-     */
-    sync GetTabCount() returns (uint32_t value);
-
     async AccessKeyNotHandled(WidgetKeyboardEvent event);
 
     async SetHasBeforeUnload(bool aHasBeforeUnload);
 
 child:
     async NativeSynthesisResponse(uint64_t aObserverId, nsCString aResponse);
 
 parent:

--- a/dom/ipc/TabChild.cpp
+++ b/dom/ipc/TabChild.cpp
@@ -420,16 +420,17 @@ TabChild::TabChild(nsIContentChild* aMan
   , mDidFakeShow(false)
   , mNotified(false)
   , mTriedBrowserInit(false)
   , mOrientation(eScreenOrientation_PortraitPrimary)
   , mIgnoreKeyPressEvent(false)
   , mHasValidInnerSize(false)
   , mDestroyed(false)
   , mUniqueId(aTabId)
+  , mHasSiblings(true)
   , mIsTransparent(false)
   , mIPCOpen(false)
   , mParentIsActive(false)
   , mDidSetRealShowInfo(false)
   , mDidLoadURLInit(false)
   , mAwaitingLA(false)
   , mSkipKeyPress(false)
   , mLayerObserverEpoch(0)
@@ -2947,22 +2948,16 @@ TabChild::SetWebBrowserChrome(nsIWebBrow
 
 void
 TabChild::SendRequestFocus(bool aCanFocus)
 {
   PBrowserChild::SendRequestFocus(aCanFocus);
 }
 
 void
-TabChild::SendGetTabCount(uint32_t* tabCount)
-{
-  PBrowserChild::SendGetTabCount(tabCount);
-}
-
-void
 TabChild::EnableDisableCommands(const nsAString& aAction,
                                 nsTArray<nsCString>& aEnabledCommands,
                                 nsTArray<nsCString>& aDisabledCommands)
 {
   PBrowserChild::SendEnableDisableCommands(PromiseFlatString(aAction),
                                            aEnabledCommands, aDisabledCommands);
 }
 
@@ -3501,16 +3496,30 @@ TabChildSHistoryListener::SHistoryDidUpd
 }
 
 mozilla::dom::TabGroup*
 TabChild::TabGroup()
 {
   return mTabGroup;
 }
 
+nsresult
+TabChild::GetHasSiblings(bool* aHasSiblings)
+{
+  *aHasSiblings = mHasSiblings;
+  return NS_OK;
+}
+
+nsresult
+TabChild::SetHasSiblings(bool aHasSiblings)
+{
+  mHasSiblings = aHasSiblings;
+  return NS_OK;
+}
+
 /*******************************************************************************
  * nsISHistoryListener
  ******************************************************************************/
 
 NS_IMETHODIMP
 TabChildSHistoryListener::OnHistoryNewEntry(nsIURI *aNewURI, int32_t aOldIndex)
 {
   return NS_ERROR_NOT_IMPLEMENTED;

--- a/dom/ipc/TabChild.h
+++ b/dom/ipc/TabChild.h
@@ -913,16 +913,21 @@ private:
   bool mHasValidInnerSize;
   bool mDestroyed;
   // Position of client area relative to the outer window
   LayoutDeviceIntPoint mClientOffset;
   // Position of tab, relative to parent widget (typically the window)
   LayoutDeviceIntPoint mChromeDisp;
   TabId mUniqueId;
 
+  // Whether or not this tab has siblings (other tabs in the same window).
+  // This is one factor used when choosing to allow or deny a non-system
+  // script's attempt to resize the window.
+  bool mHasSiblings;
+
   // Holds the compositor options for the compositor rendering this tab,
   // once we find out which compositor that is.
   Maybe<mozilla::layers::CompositorOptions> mCompositorOptions;
 
   friend class ContentChild;
 
   bool mIsTransparent;
 

--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -3527,32 +3527,16 @@ TabParent::GetShowInfo()
                     isTransparent, mDPI, mRounding, mDefaultScale.scale);
   }
 
   return ShowInfo(EmptyString(), false, false, false,
                   false, mDPI, mRounding, mDefaultScale.scale);
 }
 
 mozilla::ipc::IPCResult
-TabParent::RecvGetTabCount(uint32_t* aValue)
-{
-  *aValue = 0;
-
-  nsCOMPtr<nsIXULBrowserWindow> xulBrowserWindow = GetXULBrowserWindow();
-  NS_ENSURE_TRUE(xulBrowserWindow, IPC_OK());
-
-  uint32_t tabCount;
-  nsresult rv = xulBrowserWindow->GetTabCount(&tabCount);
-  NS_ENSURE_SUCCESS(rv, IPC_OK());
-
-  *aValue = tabCount;
-  return IPC_OK();
-}
-
-mozilla::ipc::IPCResult
 TabParent::RecvLookUpDictionary(const nsString& aText,
                                 nsTArray<FontRange>&& aFontRangeArray,
                                 const bool& aIsVertical,
                                 const LayoutDeviceIntPoint& aPoint)
 {
   nsCOMPtr<nsIWidget> widget = GetWidget();
   if (!widget) {
     return IPC_OK();

--- a/dom/ipc/TabParent.h
+++ b/dom/ipc/TabParent.h
@@ -634,18 +634,16 @@ protected:
   virtual mozilla::ipc::IPCResult RecvRemoteIsReadyToHandleInputEvents() override;
 
   virtual mozilla::ipc::IPCResult RecvForcePaintNoOp(const uint64_t& aLayerObserverEpoch) override;
 
   virtual mozilla::ipc::IPCResult RecvSetDimensions(const uint32_t& aFlags,
                                                     const int32_t& aX, const int32_t& aY,
                                                     const int32_t& aCx, const int32_t& aCy) override;
 
-  virtual mozilla::ipc::IPCResult RecvGetTabCount(uint32_t* aValue) override;
-
   virtual mozilla::ipc::IPCResult RecvSHistoryUpdate(const uint32_t& aCount,
                                                      const uint32_t& aLocalIndex,
                                                      const bool& aTruncate) override;
 
   virtual mozilla::ipc::IPCResult RecvRequestCrossBrowserNavigation(const uint32_t& aGlobalIndex) override;
 
   ContentCacheInParent mContentCache;
 

--- a/ipc/ipdl/sync-messages.ini
+++ b/ipc/ipdl/sync-messages.ini
@@ -831,18 +831,16 @@ description =
 description =
 [PBrowser::GetInputContext]
 description =
 [PBrowser::IsParentWindowMainWidgetVisible]
 description =
 description =
 [PBrowser::RequestNativeKeyBindings]
 description =
-[PBrowser::GetTabCount]
-description =
 [PBrowser::DispatchWheelEvent]
 description =
 [PBrowser::DispatchMouseEvent]
 description =
 [PBrowser::DispatchKeyboardEvent]
 description =
 [PBrowser::EnsureLayersConnected]
 description =