diff -up firefox-48.0/widget/gtk/nsWindow.cpp.1225044 firefox-48.0/widget/gtk/nsWindow.cpp
--- firefox-48.0/widget/gtk/nsWindow.cpp.1225044	2016-08-17 11:49:31.675792661 +0200
+++ firefox-48.0/widget/gtk/nsWindow.cpp	2016-08-17 11:51:49.773545394 +0200
@@ -216,6 +216,9 @@ static gboolean window_state_event_cb
 static void     theme_changed_cb          (GtkSettings *settings,
                                            GParamSpec *pspec,
                                            nsWindow *data);
+static void     check_resize_cb           (GtkContainer* container,
+                                           gpointer user_data);
+
 #if (MOZ_WIDGET_GTK == 3)
 static void     scale_changed_cb          (GtkWidget* widget,
                                            GParamSpec* aPSpec,
@@ -472,6 +475,8 @@ nsWindow::nsWindow()
 #if GTK_CHECK_VERSION(3,4,0)
     mLastScrollEventTime = GDK_CURRENT_TIME;
 #endif
+
+    mPendingConfigures = 0;
 }
 
 nsWindow::~nsWindow()
@@ -2413,6 +2418,8 @@ nsWindow::OnConfigureEvent(GtkWidget *aW
     LOG(("configure event [%p] %d %d %d %d\n", (void *)this,
          aEvent->x, aEvent->y, aEvent->width, aEvent->height));
 
+    mPendingConfigures--;
+
     LayoutDeviceIntRect screenBounds;
     GetScreenBounds(screenBounds);
 
@@ -3403,6 +3410,12 @@ nsWindow::OnDPIChanged()
 }
 
 void
+nsWindow::OnCheckResize()
+{
+    mPendingConfigures++;
+}
+
+void
 nsWindow::DispatchDragEvent(EventMessage aMsg, const LayoutDeviceIntPoint& aRefPoint,
                             guint aTime)
 {
@@ -3858,6 +3871,8 @@ nsWindow::Create(nsIWidget* aParent,
                          G_CALLBACK(delete_event_cb), nullptr);
         g_signal_connect(mShell, "window_state_event",
                          G_CALLBACK(window_state_event_cb), nullptr);
+        g_signal_connect(mShell, "check-resize",
+                         G_CALLBACK(check_resize_cb), nullptr);
 
         GtkSettings* default_settings = gtk_settings_get_default();
         g_signal_connect_after(default_settings,
@@ -4177,7 +4192,31 @@ nsWindow::NativeShow(bool aAction)
     }
     else {
         if (mIsTopLevel) {
-            gtk_widget_hide(GTK_WIDGET(mShell));
+            // Workaround window freezes on GTK versions before 3.21.2 by
+            // ensuring that configure events get dispatched to windows before
+            // they are unmapped. See bug 1225044.
+            if (gtk_check_version(3, 21, 2) != nullptr && mPendingConfigures > 0) {
+                GtkAllocation allocation;
+                gtk_widget_get_allocation(GTK_WIDGET(mShell), &allocation);
+
+                GdkEventConfigure event;
+                PodZero(&event);
+                event.type = GDK_CONFIGURE;
+                event.window = mGdkWindow;
+                event.send_event = TRUE;
+                event.x = allocation.x;
+                event.y = allocation.y;
+                event.width = allocation.width;
+                event.height = allocation.height;
+
+                auto shellClass = GTK_WIDGET_GET_CLASS(mShell);
+                for (int i = 0; i < mPendingConfigures; i++) {
+                    Unused << shellClass->configure_event(mShell, &event);
+                }
+                mPendingConfigures = 0;
+            }
+
+            gtk_widget_hide(mShell);
 
             ClearTransparencyBitmap(); // Release some resources
         }
@@ -6028,6 +6067,16 @@ theme_changed_cb (GtkSettings *settings,
     window->ThemeChanged();
 }
 
+static void
+check_resize_cb (GtkContainer* container, gpointer user_data)
+{
+    RefPtr<nsWindow> window = get_window_for_gtk_widget(GTK_WIDGET(container));
+    if (!window) {
+      return;
+    }
+    window->OnCheckResize();
+}
+
 #if (MOZ_WIDGET_GTK == 3)
 static void
 scale_changed_cb (GtkWidget* widget, GParamSpec* aPSpec, gpointer aPointer)
diff -up firefox-48.0/widget/gtk/nsWindow.h.1225044 firefox-48.0/widget/gtk/nsWindow.h
--- firefox-48.0/widget/gtk/nsWindow.h.1225044	2016-07-25 22:22:07.000000000 +0200
+++ firefox-48.0/widget/gtk/nsWindow.h	2016-08-17 11:50:55.274248334 +0200
@@ -254,6 +254,7 @@ public:
 
     void               ThemeChanged(void);
     void               OnDPIChanged(void);
+    void               OnCheckResize(void);
 
 #ifdef MOZ_X11
     Window             mOldFocusWindow;
@@ -463,6 +464,10 @@ private:
     RefPtr<nsShmImage>  mBackShmImage;
 #endif
 
+    // Upper bound on pending ConfigureNotify events to be dispatched to the
+    // window. See bug 1225044.
+    int mPendingConfigures;
+
 #ifdef ACCESSIBILITY
     RefPtr<mozilla::a11y::Accessible> mRootAccessible;