diff --git a/widget/gtk/mozgtk/mozgtk.c b/widget/gtk/mozgtk/mozgtk.c
index 697de376cec9..00e60834b51b 100644
--- a/widget/gtk/mozgtk/mozgtk.c
+++ b/widget/gtk/mozgtk/mozgtk.c
@@ -61,6 +61,7 @@ STUB(gdk_screen_get_resolution)
 STUB(gdk_screen_get_rgba_visual)
 STUB(gdk_screen_get_root_window)
 STUB(gdk_screen_get_system_visual)
+STUB(gdk_screen_get_width)
 STUB(gdk_screen_height)
 STUB(gdk_screen_is_composited)
 STUB(gdk_screen_width)
@@ -437,11 +438,14 @@ STUB(gtk_widget_hide)
 STUB(gtk_widget_is_focus)
 STUB(gtk_widget_is_toplevel)
 STUB(gtk_widget_map)
+STUB(gtk_widget_modify_bg)
 STUB(gtk_widget_realize)
 STUB(gtk_widget_reparent)
 STUB(gtk_widget_set_allocation)
 STUB(gtk_widget_set_app_paintable)
 STUB(gtk_window_set_auto_startup_notification)
+STUB(gtk_window_set_opacity)
+STUB(gtk_window_set_screen)
 STUB(gtk_widget_set_can_focus)
 STUB(gtk_widget_set_direction)
 STUB(gtk_widget_set_double_buffered)
diff --git a/widget/gtk/nsWindow.cpp b/widget/gtk/nsWindow.cpp
index 32b60f837b0d..15ffdafc0b96 100644
--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -4611,6 +4611,117 @@ nsWindow::ConvertBorderStyles(nsBorderStyle aStyle)
     return w;
 }
 
+class FullscreenTransitionWindow final : public nsISupports
+{
+public:
+    NS_DECL_ISUPPORTS
+
+    explicit FullscreenTransitionWindow(GtkWidget* aWidget);
+
+    GtkWidget* mWindow;
+
+private:
+    ~FullscreenTransitionWindow();
+};
+
+NS_IMPL_ISUPPORTS0(FullscreenTransitionWindow)
+
+FullscreenTransitionWindow::FullscreenTransitionWindow(GtkWidget* aWidget)
+{
+    mWindow = gtk_window_new(GTK_WINDOW_POPUP);
+    GtkWindow* gtkWin = GTK_WINDOW(mWindow);
+
+    gtk_window_set_type_hint(gtkWin, GDK_WINDOW_TYPE_HINT_SPLASHSCREEN);
+    gtk_window_set_transient_for(gtkWin, GTK_WINDOW(aWidget));
+    gtk_window_set_decorated(gtkWin, false);
+
+    GdkScreen* screen = gtk_widget_get_screen(aWidget);
+    gint width = gdk_screen_get_width(screen);
+    gint height = gdk_screen_get_height(screen);
+    gtk_window_set_screen(gtkWin, screen);
+    gtk_window_move(gtkWin, 0, 0);
+    gtk_window_resize(gtkWin, width, height);
+
+    GdkColor bgColor;
+    bgColor.red = bgColor.green = bgColor.blue = 0;
+    gtk_widget_modify_bg(mWindow, GTK_STATE_NORMAL, &bgColor);
+
+    gtk_window_set_opacity(gtkWin, 0.0);
+    gtk_widget_show(mWindow);
+}
+
+FullscreenTransitionWindow::~FullscreenTransitionWindow()
+{
+    gtk_widget_destroy(mWindow);
+}
+
+class FullscreenTransitionData
+{
+public:
+    FullscreenTransitionData(nsIWidget::FullscreenTransitionStage aStage,
+                             uint16_t aDuration, nsIRunnable* aCallback,
+                             FullscreenTransitionWindow* aWindow)
+        : mStage(aStage)
+        , mStep(0)
+        , mTotalSteps(aDuration / sInterval)
+        , mCallback(aCallback)
+        , mWindow(aWindow) { }
+
+    static const guint sInterval = 10; // 10ms
+    static gboolean TimeoutCallback(gpointer aData);
+
+private:
+    nsIWidget::FullscreenTransitionStage mStage;
+    uint16_t mStep, mTotalSteps;
+    nsCOMPtr<nsIRunnable> mCallback;
+    nsRefPtr<FullscreenTransitionWindow> mWindow;
+};
+
+/* static */ gboolean
+FullscreenTransitionData::TimeoutCallback(gpointer aData)
+{
+    auto data = static_cast<FullscreenTransitionData*>(aData);
+    data->mStep++;
+    gdouble opacity = data->mStep / gdouble(data->mTotalSteps);
+    if (data->mStage == nsIWidget::eAfterFullscreenToggle) {
+        opacity = 1.0 - opacity;
+    }
+    gtk_window_set_opacity(GTK_WINDOW(data->mWindow->mWindow), opacity);
+
+    if (data->mStep != data->mTotalSteps) {
+        return TRUE;
+    }
+    NS_DispatchToMainThread(data->mCallback.forget());
+    delete data;
+    return FALSE;
+}
+
+/* virtual */ bool
+nsWindow::PrepareForFullscreenTransition(nsISupports** aData)
+{
+    GdkScreen* screen = gtk_widget_get_screen(mShell);
+    if (!gdk_screen_is_composited(screen)) {
+        return false;
+    }
+    *aData = do_AddRef(new FullscreenTransitionWindow(mShell)).take();
+    return true;
+}
+
+/* virtual */ void
+nsWindow::PerformFullscreenTransition(FullscreenTransitionStage aStage,
+                                      uint16_t aDuration, nsISupports* aData,
+                                      nsIRunnable* aCallback)
+{
+    auto data = static_cast<FullscreenTransitionWindow*>(aData);
+    // This will be released at the end of the last timeout callback for it.
+    auto transitionData = new FullscreenTransitionData(aStage, aDuration,
+                                                       aCallback, data);
+    g_timeout_add_full(G_PRIORITY_HIGH,
+                       FullscreenTransitionData::sInterval,
+                       FullscreenTransitionData::TimeoutCallback,
+                       transitionData, nullptr);
+}
+
 NS_IMETHODIMP
 nsWindow::MakeFullScreen(bool aFullScreen, nsIScreen* aTargetScreen)
 {
diff --git a/widget/gtk/nsWindow.h b/widget/gtk/nsWindow.h
index 05ed0e4113ae..59656e40c647 100644
--- a/widget/gtk/nsWindow.h
+++ b/widget/gtk/nsWindow.h
@@ -147,6 +147,11 @@ public:
                                            bool aIntersectWithExisting) override;
     virtual bool       HasPendingInputEvent() override;
 
+    virtual bool PrepareForFullscreenTransition(nsISupports** aData) override;
+    virtual void PerformFullscreenTransition(FullscreenTransitionStage aStage,
+                                             uint16_t aDuration,
+                                             nsISupports* aData,
+                                             nsIRunnable* aCallback) override;
     NS_IMETHOD         MakeFullScreen(bool aFullScreen,
                                       nsIScreen* aTargetScreen = nullptr) override;
     NS_IMETHOD         HideWindowChrome(bool aShouldHide) override;