--- mozilla/dom/base/nsGlobalWindow.cpp.orig	2021-10-26 19:49:54.000000000 +0300
+++ mozilla/dom/base/nsGlobalWindow.cpp	2021-12-15 00:29:59.801185237 +0300
@@ -9254,6 +9254,22 @@ nsGlobalWindow::PostMessageMozOuter(JSCo
     return;
   }
 
+  //
+  // Do not generate new event here when something is pending.
+  // It avoids cpu hogs (esp. when run without e10s), preventing
+  // permanent event generation loop by badly written scripts.
+  //
+  // See also https://html.spec.whatwg.org/multipage/web-messaging.html
+  // p. 9.4.2.2 about rate-limiting etc.
+  //
+  if (!aTargetOrigin.EqualsASCII("/")) {
+    nsIThread* thread = NS_GetCurrentThread();
+
+    while (NS_HasPendingEvents(thread)) {
+      (void)NS_ProcessNextEvent(thread, false);
+    }
+  }
+
   aError = Dispatch(TaskCategory::Other, event.forget());
 }