diff --git a/dom/locales/en-US/chrome/dom/dom.properties b/dom/locales/en-US/chrome/dom/dom.properties
index f0a6363af0..4ae891f327 100644
--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -224,7 +224,7 @@ ServiceWorkerScopePathMismatch=Failed to register a ServiceWorker: The path of t
 # LOCALIZATION NOTE: Do not translate "ServiceWorker". %1$S is a URL representing the scope of the ServiceWorker, %2$S is a stringified numeric HTTP status code like "404" and %3$S is a URL.
 ServiceWorkerRegisterNetworkError=Failed to register/update a ServiceWorker for scope ‘%1$S’: Load failed with status %2$S for script ‘%3$S’.
 # LOCALIZATION NOTE: Do not translate "ServiceWorker". %1$S is a URL representing the scope of the ServiceWorker, %2$S is a MIME Media Type like "text/plain" and %3$S is a URL.
-ServiceWorkerRegisterMimeTypeError=Failed to register/update a ServiceWorker for scope ‘%1$S’: Bad Content-Type of ‘%2$S’ received for script ‘%3$S’.  Must be ‘text/javascript’, ‘application/x-javascript’, or ‘application/javascript’.
+ServiceWorkerRegisterMimeTypeError2=Failed to register/update a ServiceWorker for scope ‘%1$S’: Bad Content-Type of ‘%2$S’ received for script ‘%3$S’.  Must be ‘text/javascript’, ‘application/x-javascript’, or ‘application/javascript’.
 # LOCALIZATION NOTE: Do not translate "ServiceWorker". %1$S is a URL representing the scope of the ServiceWorker.
 ServiceWorkerGraceTimeoutTermination=Terminating ServiceWorker for scope ‘%1$S’ with pending waitUntil/respondWith promises because of grace timeout.
 ExecCommandCutCopyDeniedNotInputDriven=document.execCommand(‘cut’/‘copy’) was denied because it was not called from inside a short running user-generated event handler.
diff --git a/dom/workers/ScriptLoader.cpp b/dom/workers/ScriptLoader.cpp
index 0a9b7207e7..16f0dd1527 100644
--- a/dom/workers/ScriptLoader.cpp
+++ b/dom/workers/ScriptLoader.cpp
@@ -59,6 +59,7 @@
 #include "mozilla/dom/ScriptLoader.h"
 #include "mozilla/dom/ScriptSettings.h"
 #include "mozilla/dom/SRILogHelper.h"
+#include "mozilla/dom/workers/ServiceWorkerManager.h"
 #include "mozilla/UniquePtr.h"
 #include "Principal.h"
 #include "WorkerHolder.h"
@@ -663,6 +664,34 @@ private:
     ScriptLoadInfo& loadInfo = mLoadInfos[aIndex];
 
     nsCOMPtr<nsIChannel> channel = do_QueryInterface(aRequest);
+
+    // Checking the MIME type is only required for ServiceWorkers'
+    // importScripts, per step 10 of https://w3c.github.io/ServiceWorker/#importscripts
+    //
+    // "Extract a MIME type from the response’s header list. If this MIME type
+    // (ignoring parameters) is not a JavaScript MIME type, return a network error."
+    if (mWorkerPrivate->IsServiceWorker()) {
+      nsAutoCString mimeType;
+      channel->GetContentType(mimeType);
+
+      if (!nsContentUtils::IsJavascriptMIMEType(NS_ConvertUTF8toUTF16(mimeType))) {
+        const nsCString& scope =
+          mWorkerPrivate->ServiceWorkerScope();
+
+        ServiceWorkerManager::LocalizeAndReportToAllClients(
+          scope, "ServiceWorkerRegisterMimeTypeError2",
+          nsTArray<nsString> {
+            NS_ConvertUTF8toUTF16(scope),
+            NS_ConvertUTF8toUTF16(mimeType),
+            loadInfo.mURL
+          }
+        );
+
+        channel->Cancel(NS_ERROR_DOM_NETWORK_ERR);
+        return NS_ERROR_DOM_NETWORK_ERR;
+      }
+    }
+
     MOZ_ASSERT(channel == loadInfo.mChannel);
 
     // We synthesize the result code, but its never exposed to content.
diff --git a/dom/workers/ServiceWorkerScriptCache.cpp b/dom/workers/ServiceWorkerScriptCache.cpp
index 00e0446edf..c93ddd7c0e 100644
--- a/dom/workers/ServiceWorkerScriptCache.cpp
+++ b/dom/workers/ServiceWorkerScriptCache.cpp
@@ -801,12 +801,11 @@ CompareNetwork::OnStreamComplete(nsIStreamLoader* aLoader, nsISupports* aContext
     return rv;
   }
 
-  if (!mimeType.LowerCaseEqualsLiteral("text/javascript") &&
-      !mimeType.LowerCaseEqualsLiteral("application/x-javascript") &&
-      !mimeType.LowerCaseEqualsLiteral("application/javascript")) {
+  if (mimeType.IsEmpty() ||
+      !nsContentUtils::IsJavascriptMIMEType(NS_ConvertUTF8toUTF16(mimeType))) {
     RefPtr<ServiceWorkerRegistrationInfo> registration = mManager->GetRegistration();
     ServiceWorkerManager::LocalizeAndReportToAllClients(
-      registration->mScope, "ServiceWorkerRegisterMimeTypeError",
+      registration->mScope, "ServiceWorkerRegisterMimeTypeError2",
       nsTArray<nsString> { NS_ConvertUTF8toUTF16(registration->mScope),
         NS_ConvertUTF8toUTF16(mimeType), mManager->URL() });
     mManager->NetworkFinished(NS_ERROR_DOM_SECURITY_ERR);