diff --git a/dom/serviceworkers/ServiceWorkerShutdownBlocker.cpp b/dom/serviceworkers/ServiceWorkerShutdownBlocker.cpp
new file mode 100644
index 000000000000..534f83094200
--- /dev/null
+++ b/dom/serviceworkers/ServiceWorkerShutdownBlocker.cpp
@@ -0,0 +1,174 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ServiceWorkerShutdownBlocker.h"
+
+#include <utility>
+
+#include "MainThreadUtils.h"
+#include "nsDebug.h"
+#include "nsError.h"
+#include "nsIWritablePropertyBag2.h"
+#include "nsThreadUtils.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/RefPtr.h"
+
+namespace mozilla {
+namespace dom {
+
+NS_IMPL_ISUPPORTS(ServiceWorkerShutdownBlocker, nsIAsyncShutdownBlocker)
+
+NS_IMETHODIMP ServiceWorkerShutdownBlocker::GetName(nsAString& aNameOut) {
+  aNameOut = NS_LITERAL_STRING(
+      "ServiceWorkerShutdownBlocker: shutting down Service Workers");
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+ServiceWorkerShutdownBlocker::BlockShutdown(nsIAsyncShutdownClient* aClient) {
+  AssertIsOnMainThread();
+  MOZ_ASSERT(!mShutdownClient);
+
+  mShutdownClient = aClient;
+  MaybeUnblockShutdown();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP ServiceWorkerShutdownBlocker::GetState(nsIPropertyBag** aBagOut) {
+  AssertIsOnMainThread();
+  MOZ_ASSERT(aBagOut);
+
+  nsCOMPtr<nsIWritablePropertyBag2> propertyBag =
+      do_CreateInstance("@mozilla.org/hash-property-bag;1");
+
+  if (NS_WARN_IF(!propertyBag)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  nsresult rv = propertyBag->SetPropertyAsBool(
+      NS_LITERAL_STRING("acceptingPromises"), IsAcceptingPromises());
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = propertyBag->SetPropertyAsUint32(NS_LITERAL_STRING("pendingPromises"),
+                                        GetPendingPromises());
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  propertyBag.forget(aBagOut);
+
+  return NS_OK;
+}
+
+/* static */ already_AddRefed<ServiceWorkerShutdownBlocker>
+ServiceWorkerShutdownBlocker::CreateAndRegisterOn(
+    nsIAsyncShutdownClient* aShutdownBarrier) {
+  AssertIsOnMainThread();
+  MOZ_ASSERT(aShutdownBarrier);
+
+  RefPtr<ServiceWorkerShutdownBlocker> blocker =
+      new ServiceWorkerShutdownBlocker();
+
+  nsresult rv = aShutdownBarrier->AddBlocker(
+      blocker.get(), NS_LITERAL_STRING(__FILE__), __LINE__,
+      NS_LITERAL_STRING("Service Workers shutdown"));
+
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return nullptr;
+  }
+
+  return blocker.forget();
+}
+
+void ServiceWorkerShutdownBlocker::WaitOnPromise(
+    GenericNonExclusivePromise* aPromise) {
+  AssertIsOnMainThread();
+  MOZ_DIAGNOSTIC_ASSERT(IsAcceptingPromises());
+  MOZ_ASSERT(aPromise);
+
+  ++mState.as<AcceptingPromises>().mPendingPromises;
+
+  RefPtr<ServiceWorkerShutdownBlocker> self = this;
+
+  aPromise->Then(GetCurrentThreadSerialEventTarget(), __func__,
+                 [self = std::move(self)](
+                     const GenericNonExclusivePromise::ResolveOrRejectValue&) {
+                   if (!self->PromiseSettled()) {
+                     self->MaybeUnblockShutdown();
+                   }
+                 });
+}
+
+void ServiceWorkerShutdownBlocker::StopAcceptingPromises() {
+  AssertIsOnMainThread();
+  MOZ_ASSERT(IsAcceptingPromises());
+
+  mState = AsVariant(NotAcceptingPromises(mState.as<AcceptingPromises>()));
+}
+
+ServiceWorkerShutdownBlocker::ServiceWorkerShutdownBlocker()
+    : mState(VariantType<AcceptingPromises>()) {
+  AssertIsOnMainThread();
+}
+
+ServiceWorkerShutdownBlocker::~ServiceWorkerShutdownBlocker() {
+  MOZ_ASSERT(!IsAcceptingPromises());
+  MOZ_ASSERT(!GetPendingPromises());
+  MOZ_ASSERT(!mShutdownClient);
+}
+
+void ServiceWorkerShutdownBlocker::MaybeUnblockShutdown() {
+  AssertIsOnMainThread();
+
+  if (!mShutdownClient || IsAcceptingPromises() || GetPendingPromises()) {
+    return;
+  }
+
+  mShutdownClient->RemoveBlocker(this);
+  mShutdownClient = nullptr;
+}
+
+uint32_t ServiceWorkerShutdownBlocker::PromiseSettled() {
+  AssertIsOnMainThread();
+  MOZ_ASSERT(GetPendingPromises());
+
+  if (IsAcceptingPromises()) {
+    return --mState.as<AcceptingPromises>().mPendingPromises;
+  }
+
+  return --mState.as<NotAcceptingPromises>().mPendingPromises;
+}
+
+bool ServiceWorkerShutdownBlocker::IsAcceptingPromises() const {
+  AssertIsOnMainThread();
+
+  return mState.is<AcceptingPromises>();
+}
+
+uint32_t ServiceWorkerShutdownBlocker::GetPendingPromises() const {
+  AssertIsOnMainThread();
+
+  if (IsAcceptingPromises()) {
+    return mState.as<AcceptingPromises>().mPendingPromises;
+  }
+
+  return mState.as<NotAcceptingPromises>().mPendingPromises;
+}
+
+ServiceWorkerShutdownBlocker::NotAcceptingPromises::NotAcceptingPromises(
+    AcceptingPromises aPreviousState)
+    : mPendingPromises(aPreviousState.mPendingPromises) {
+  AssertIsOnMainThread();
+}
+
+}  // namespace dom
+}  // namespace mozilla
diff --git a/dom/serviceworkers/ServiceWorkerShutdownBlocker.h b/dom/serviceworkers/ServiceWorkerShutdownBlocker.h
new file mode 100644
index 000000000000..87373da762e0
--- /dev/null
+++ b/dom/serviceworkers/ServiceWorkerShutdownBlocker.h
@@ -0,0 +1,92 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_serviceworkershutdownblocker_h__
+#define mozilla_dom_serviceworkershutdownblocker_h__
+
+#include "nsCOMPtr.h"
+#include "nsIAsyncShutdown.h"
+#include "nsISupportsImpl.h"
+
+#include "mozilla/MozPromise.h"
+
+namespace mozilla {
+namespace dom {
+
+/**
+ * Main thread only.
+ */
+class ServiceWorkerShutdownBlocker final : public nsIAsyncShutdownBlocker {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIASYNCSHUTDOWNBLOCKER
+
+  /**
+   * Returns the registered shutdown blocker if registration succeeded and
+   * nullptr otherwise.
+   */
+  static already_AddRefed<ServiceWorkerShutdownBlocker> CreateAndRegisterOn(
+      nsIAsyncShutdownClient* aShutdownBarrier);
+
+  /**
+   * Blocks shutdown until `aPromise` settles.
+   *
+   * Can be called multiple times, and shutdown will be blocked until all the
+   * calls' promises settle, but all of these calls must happen before
+   * `StopAcceptingPromises()` is called (assertions will enforce this).
+   */
+  void WaitOnPromise(GenericNonExclusivePromise* aPromise);
+
+  /**
+   * Once this is called, shutdown will be blocked until all promises
+   * passed to `WaitOnPromise()` settle, and there must be no more calls to
+   * `WaitOnPromise()` (assertions will enforce this).
+   */
+  void StopAcceptingPromises();
+
+ private:
+  ServiceWorkerShutdownBlocker();
+
+  ~ServiceWorkerShutdownBlocker();
+
+  /**
+   * No-op if any of the following are true:
+   * 1) `BlockShutdown()` hasn't been called yet, or
+   * 2) `StopAcceptingPromises()` hasn't been called yet, or
+   * 3) `StopAcceptingPromises()` HAS been called, but there are still pending
+   *    promises.
+   */
+  void MaybeUnblockShutdown();
+
+  /**
+   * Returns the remaining pending promise count (i.e. excluding the promise
+   * that just settled).
+   */
+  uint32_t PromiseSettled();
+
+  bool IsAcceptingPromises() const;
+
+  uint32_t GetPendingPromises() const;
+
+  struct AcceptingPromises {
+    uint32_t mPendingPromises = 0;
+  };
+
+  struct NotAcceptingPromises {
+    explicit NotAcceptingPromises(AcceptingPromises aPreviousState);
+
+    uint32_t mPendingPromises = 0;
+  };
+
+  Variant<AcceptingPromises, NotAcceptingPromises> mState;
+
+  nsCOMPtr<nsIAsyncShutdownClient> mShutdownClient;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_serviceworkershutdownblocker_h__
diff --git a/dom/serviceworkers/moz.build b/dom/serviceworkers/moz.build
index d60784941c4b..4f711f902c4c 100644
--- a/dom/serviceworkers/moz.build
+++ b/dom/serviceworkers/moz.build
@@ -65,6 +65,7 @@ UNIFIED_SOURCES += [
     'ServiceWorkerRegistrationParent.cpp',
     'ServiceWorkerRegistrationProxy.cpp',
     'ServiceWorkerScriptCache.cpp',
+    'ServiceWorkerShutdownBlocker.cpp',
     'ServiceWorkerUnregisterCallback.cpp',
     'ServiceWorkerUnregisterJob.cpp',
     'ServiceWorkerUpdateJob.cpp',