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());