diff --git a/dom/serviceworkers/ServiceWorkerRegistrar.cpp b/dom/serviceworkers/ServiceWorkerRegistrar.cpp
index ef6943f62299..0aad701e7d3d 100644
--- a/dom/serviceworkers/ServiceWorkerRegistrar.cpp
+++ b/dom/serviceworkers/ServiceWorkerRegistrar.cpp
@@ -31,6 +31,7 @@
 #include "mozilla/ResultExtensions.h"
 #include "mozilla/Services.h"
 #include "mozilla/StaticPtr.h"
+#include "mozJSComponentLoader.h"
 #include "nsAppDirectoryServiceDefs.h"
 #include "nsComponentManagerUtils.h"
 #include "nsContentUtils.h"
@@ -1212,6 +1213,7 @@ ServiceWorkerRegistrar::GetState(nsIPropertyBag** aBagOut) {
 #define RELEASE_ASSERT_SUCCEEDED(rv, name)                                    \
   do {                                                                        \
     if (NS_FAILED(rv)) {                                                      \
+      mozJSComponentLoader::Get()->AnnotateCrashReport();                     \
       if (rv == NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS) {                 \
         if (auto* context = CycleCollectedJSContext::Get()) {                 \
           if (RefPtr<Exception> exn = context->GetPendingException()) {       \
diff --git a/dom/serviceworkers/moz.build b/dom/serviceworkers/moz.build
index 0b074442d79c..7f7fac7ba724 100644
--- a/dom/serviceworkers/moz.build
+++ b/dom/serviceworkers/moz.build
@@ -112,6 +112,10 @@ include("/ipc/chromium/chromium-config.mozbuild")
 
 FINAL_LIBRARY = "xul"
 
+LOCAL_INCLUDES += [
+    "/js/xpconnect/loader",
+]
+
 MOCHITEST_MANIFESTS += [
     "test/mochitest.ini",
 ]
diff --git a/js/xpconnect/loader/mozJSComponentLoader.cpp b/js/xpconnect/loader/mozJSComponentLoader.cpp
index a73013c8d591..db930d28e355 100644
--- a/js/xpconnect/loader/mozJSComponentLoader.cpp
+++ b/js/xpconnect/loader/mozJSComponentLoader.cpp
@@ -335,6 +335,39 @@ static JSObject* ResolveModuleObjectProperty(JSContext* aCx,
   return aModObj;
 }
 
+static mozilla::Result<nsCString, nsresult> ReadScript(
+    ComponentLoaderInfo& aInfo);
+
+static nsresult AnnotateScriptContents(CrashReporter::Annotation aName,
+                                       const nsACString& aURI) {
+  ComponentLoaderInfo info(aURI);
+
+  nsCString str;
+  MOZ_TRY_VAR(str, ReadScript(info));
+
+  // The crash reporter won't accept any strings with embedded nuls. We
+  // shouldn't have any here, but if we do because of data corruption, we
+  // still want the annotation. So replace any embedded nuls before
+  // annotating.
+  str.ReplaceSubstring("\0"_ns, "\\0"_ns);
+
+  CrashReporter::AnnotateCrashReport(aName, str);
+
+  return NS_OK;
+}
+
+nsresult mozJSComponentLoader::AnnotateCrashReport() {
+  Unused << AnnotateScriptContents(
+      CrashReporter::Annotation::nsAsyncShutdownComponent,
+      "resource://gre/components/nsAsyncShutdown.js"_ns);
+
+  Unused << AnnotateScriptContents(
+      CrashReporter::Annotation::AsyncShutdownModule,
+      "resource://gre/modules/AsyncShutdown.jsm"_ns);
+
+  return NS_OK;
+}
+
 const mozilla::Module* mozJSComponentLoader::LoadModule(FileLocation& aFile) {
   if (!NS_IsMainThread()) {
     MOZ_ASSERT(false, "Don't use JS components off the main thread");
@@ -364,12 +397,34 @@ const mozilla::Module* mozJSComponentLoader::LoadModule(FileLocation& aFile) {
   jsapi.Init();
   JSContext* cx = jsapi.cx();
 
+  bool isCriticalModule = StringEndsWith(spec, "/nsAsyncShutdown.js"_ns);
+
   auto entry = MakeUnique<ModuleEntry>(RootingContext::get(cx));
   RootedValue exn(cx);
   rv = ObjectForLocation(info, file, &entry->obj, &entry->thisObjectKey,
-                         &entry->location, /* aPropagateExceptions */ false,
-                         &exn);
-  NS_ENSURE_SUCCESS(rv, nullptr);
+                         &entry->location, isCriticalModule, &exn);
+  if (NS_FAILED(rv)) {
+    // Temporary debugging assertion for bug 1403348:
+    if (isCriticalModule && !exn.isUndefined()) {
+      AnnotateCrashReport();
+
+      JSAutoRealm ar(cx, xpc::PrivilegedJunkScope());
+      JS_WrapValue(cx, &exn);
+
+      nsAutoCString file;
+      uint32_t line;
+      uint32_t column;
+      nsAutoString msg;
+      nsContentUtils::ExtractErrorValues(cx, exn, file, &line, &column, msg);
+
+      NS_ConvertUTF16toUTF8 cMsg(msg);
+      MOZ_CRASH_UNSAFE_PRINTF(
+          "Failed to load module \"%s\": "
+          "[\"%s\" {file: \"%s\", line: %u}]",
+          spec.get(), cMsg.get(), file.get(), line);
+    }
+    return nullptr;
+  }
 
   nsCOMPtr<nsIComponentManager> cm;
   rv = NS_GetComponentManager(getter_AddRefs(cm));
diff --git a/js/xpconnect/loader/mozJSComponentLoader.h b/js/xpconnect/loader/mozJSComponentLoader.h
index 665a2ae7173b..e1a85a321cda 100644
--- a/js/xpconnect/loader/mozJSComponentLoader.h
+++ b/js/xpconnect/loader/mozJSComponentLoader.h
@@ -74,6 +74,14 @@ class mozJSComponentLoader final : public nsIMemoryReporter {
 
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf);
 
+  /**
+   * Temporary diagnostic function for startup crashes in bug 1403348:
+   *
+   * Annotate the crash report with the contents of the async shutdown
+   * module/component scripts.
+   */
+  nsresult AnnotateCrashReport();
+
  protected:
   mozJSComponentLoader();
   ~mozJSComponentLoader();
diff --git a/toolkit/crashreporter/CrashAnnotations.yaml b/toolkit/crashreporter/CrashAnnotations.yaml
index c8c753fcd0d6..e7186191c3f5 100644
--- a/toolkit/crashreporter/CrashAnnotations.yaml
+++ b/toolkit/crashreporter/CrashAnnotations.yaml
@@ -92,6 +92,19 @@ ApplicationBuildID:
     Product application's build ID.
   type: string
 
+AsyncShutdownModule:
+  description: >
+    Holds the contents of the AsyncShutdown.js script
+  type: string
+
+AsyncShutdownTimeout:
+  description: >
+    This annotation is present if a shutdown blocker was not released in time
+    and the browser was crashed instead of waiting for shutdown to finish. The
+    condition that caused the hang is contained in the annotation.
+  type: string
+  ping: true
+
 AvailablePageFile:
   description: >
     Available commit-space in bytes.