diff --git a/dom/plugins/base/nsPluginNativeWindowGtk.cpp b/dom/plugins/base/nsPluginNativeWindowGtk.cpp
--- a/dom/plugins/base/nsPluginNativeWindowGtk.cpp
+++ b/dom/plugins/base/nsPluginNativeWindowGtk.cpp
@@ -19,16 +19,17 @@
 
 #if (GTK_MAJOR_VERSION == 3)
 #include <gtk/gtkx.h>
 #else
 #include "gtk2xtbin.h"
 #endif
 #include "mozilla/X11Util.h"
 
+static void plug_added_cb(GtkWidget *widget, gpointer data);
 static gboolean plug_removed_cb   (GtkWidget *widget, gpointer data);
 static void socket_unrealize_cb   (GtkWidget *widget, gpointer data);
 
 nsPluginNativeWindowGtk::nsPluginNativeWindowGtk() : nsPluginNativeWindow()
 {
   // initialize the struct fields
   window = nullptr; 
   x = 0; 
@@ -158,16 +159,19 @@ nsresult nsPluginNativeWindowGtk::Create
 
   //attach the socket to the container widget
   gtk_widget_set_parent_window(mSocketWidget, parent_win);
 
   // enable/disable focus event handlers,
   // see plugin_window_filter_func() for details
   g_object_set_data(G_OBJECT(mSocketWidget), "enable-xt-focus", (void *)aEnableXtFocus);
 
+  g_signal_connect(mSocketWidget, "plug_added",
+                   G_CALLBACK(plug_added_cb), nullptr);
+
   // Make sure to handle the plug_removed signal.  If we don't the
   // socket will automatically be destroyed when the plug is
   // removed, which means we're destroying it more than once.
   // SYNTAX ERROR.
   g_signal_connect(mSocketWidget, "plug_removed",
                    G_CALLBACK(plug_removed_cb), nullptr);
 
   g_signal_connect(mSocketWidget, "unrealize",
@@ -273,16 +277,42 @@ nsresult nsPluginNativeWindowGtk::Create
   // Leave mWsInfo.type = 0 - Who knows what this is meant to be?
 
   XFlush(mWsInfo.display);
 
   return NS_OK;
 }
 #endif
 
+static void
+plug_window_finalize_cb(gpointer socket, GObject* plug_window)
+{
+  g_object_unref(socket);
+}
+
+static void
+plug_added_cb(GtkWidget *socket, gpointer data)
+{
+  // The plug window has been embedded, and gtk_socket_add_window() has added
+  // a filter to the socket's plug_window, passing the socket as data for the
+  // filter, so the socket must live as long as events may be received on the
+  // plug window.
+  //
+  // https://git.gnome.org/browse/gtk+/tree/gtk/gtksocket.c?h=3.18.7#n1124
+  g_object_ref(socket);
+  // When the socket is unrealized, perhaps during gtk_widget_destroy() from
+  // ~nsPluginNativeWindowGtk, the plug is removed.  The plug in the child
+  // process then destroys its widget and window.  When the browser process
+  // receives the DestroyNotify event for the plug window, GDK releases its
+  // reference to plugWindow.  This is typically the last reference and so the
+  // weak ref callback triggers release of the socket.
+  GdkWindow* plugWindow = gtk_socket_get_plug_window(GTK_SOCKET(socket));
+  g_object_weak_ref(G_OBJECT(plugWindow), plug_window_finalize_cb, socket);
+}
+
 /* static */
 gboolean
 plug_removed_cb (GtkWidget *widget, gpointer data)
 {
   // Gee, thanks for the info!
   return TRUE;
 }
 
diff --git a/widget/gtk/mozgtk/mozgtk.c b/widget/gtk/mozgtk/mozgtk.c
--- a/widget/gtk/mozgtk/mozgtk.c
+++ b/widget/gtk/mozgtk/mozgtk.c
@@ -384,16 +384,17 @@ STUB(gtk_selection_data_targets_include_
 STUB(gtk_separator_get_type)
 STUB(gtk_separator_menu_item_new)
 STUB(gtk_separator_tool_item_new)
 STUB(gtk_settings_get_default)
 STUB(gtk_settings_get_for_screen)
 STUB(gtk_socket_add_id)
 STUB(gtk_socket_get_id)
 STUB(gtk_socket_get_type)
+STUB(gtk_socket_get_plug_window)
 STUB(gtk_socket_new)
 STUB(gtk_spin_button_new)
 STUB(gtk_statusbar_new)
 STUB(gtk_style_lookup_icon_set)
 STUB(gtk_table_attach)
 STUB(gtk_table_get_type)
 STUB(gtk_table_new)
 STUB(gtk_target_list_add)