--- a/widget/gtk/WidgetStyleCache.cpp
+++ b/widget/gtk/WidgetStyleCache.cpp
@@ -703,19 +703,30 @@ CreateStyleForWidget(GtkWidget* aWidget,
 
 GtkStyleContext*
 CreateCSSNode(const char* aName, GtkStyleContext* aParentStyle, GType aType)
 {
   static auto sGtkWidgetPathIterSetObjectName =
     reinterpret_cast<void (*)(GtkWidgetPath *, gint, const char *)>
     (dlsym(RTLD_DEFAULT, "gtk_widget_path_iter_set_object_name"));
 
-  GtkWidgetPath* path = aParentStyle ?
-    gtk_widget_path_copy(gtk_style_context_get_path(aParentStyle)) :
-    gtk_widget_path_new();
+  GtkWidgetPath* path;
+  if (aParentStyle) {
+    path = gtk_widget_path_copy(gtk_style_context_get_path(aParentStyle));
+    // Copy classes from the parent style context to its corresponding node in
+    // the path, because GTK will only match against ancestor classes if they
+    // are on the path.
+    GList* classes = gtk_style_context_list_classes(aParentStyle);
+    for (GList* link = classes; link; link = link->next) {
+      gtk_widget_path_iter_add_class(path, -1, static_cast<gchar*>(link->data));
+    }
+    g_list_free(classes);
+  } else {
+    path = gtk_widget_path_new();
+  }
 
   gtk_widget_path_append_type(path, aType);
 
   if (sGtkWidgetPathIterSetObjectName) {
     (*sGtkWidgetPathIterSetObjectName)(path, -1, aName);
   }
 
   GtkStyleContext *context = gtk_style_context_new();

--- a/widget/gtk/mozgtk/mozgtk.c
+++ b/widget/gtk/mozgtk/mozgtk.c
@@ -575,16 +575,17 @@ STUB(gtk_style_context_set_state)
 STUB(gtk_style_properties_lookup_property)
 STUB(gtk_tree_view_column_get_button)
 STUB(gtk_widget_get_preferred_size)
 STUB(gtk_widget_get_state_flags)
 STUB(gtk_widget_get_style_context)
 STUB(gtk_widget_path_append_type)
 STUB(gtk_widget_path_copy)
 STUB(gtk_widget_path_free)
+STUB(gtk_widget_path_iter_add_class)
 STUB(gtk_widget_path_new)
 STUB(gtk_widget_path_unref)
 STUB(gtk_widget_set_visual)
 STUB(gtk_app_chooser_dialog_new_for_content_type)
 STUB(gtk_app_chooser_get_type)
 STUB(gtk_app_chooser_get_app_info)
 STUB(gtk_app_chooser_dialog_get_type)
 STUB(gtk_app_chooser_dialog_set_heading)