diff -up firefox-95.0.2/widget/gtk/nsWindow.cpp.1744896 firefox-95.0.2/widget/gtk/nsWindow.cpp

--- firefox-95.0.2/widget/gtk/nsWindow.cpp.1744896	2021-12-23 11:54:31.522539340 +0100

+++ firefox-95.0.2/widget/gtk/nsWindow.cpp	2021-12-23 11:55:56.070270174 +0100

@@ -5765,6 +5765,17 @@ nsresult nsWindow::Create(nsIWidget* aPa

       }

     }

 #endif

+#ifdef MOZ_WAYLAND

+  // Initialize the window specific VsyncSource early in order to avoid races

+  // with BrowserParent::UpdateVsyncParentVsyncSource().

+  // Only use for toplevel windows for now, see bug 1619246.

+  if (GdkIsWaylandDisplay() &&

+      StaticPrefs::widget_wayland_vsync_enabled_AtStartup() &&

+      mWindowType == eWindowType_toplevel) {

+    mWaylandVsyncSource = new WaylandVsyncSource();

+    MOZ_RELEASE_ASSERT(mWaylandVsyncSource);

+  }

+#endif

     // We create input contexts for all containers, except for

     // toplevel popup windows

@@ -6077,19 +6088,12 @@ void nsWindow::ResumeCompositorFromCompo

 void nsWindow::WaylandStartVsync() {

 #ifdef MOZ_WAYLAND

-  // only use for toplevel windows for now - see bug 1619246

-  if (!GdkIsWaylandDisplay() ||

-      !StaticPrefs::widget_wayland_vsync_enabled_AtStartup() ||

-      mWindowType != eWindowType_toplevel) {

+  if (!mWaylandVsyncSource) {

     return;

   }

   LOG("nsWindow::WaylandStartVsync() [%p]\n", (void*)this);

-  if (!mWaylandVsyncSource) {

-    mWaylandVsyncSource = new WaylandVsyncSource();

-  }

-

   WaylandVsyncSource::WaylandDisplay& display =

       static_cast<WaylandVsyncSource::WaylandDisplay&>(

           mWaylandVsyncSource->GetGlobalDisplay());