diff --git a/build/clang-plugin/mozsearch-plugin/MozsearchIndexer.cpp b/build/clang-plugin/mozsearch-plugin/MozsearchIndexer.cpp
index c1fec0df189c..2af05ed04328 100644
--- a/build/clang-plugin/mozsearch-plugin/MozsearchIndexer.cpp
+++ b/build/clang-plugin/mozsearch-plugin/MozsearchIndexer.cpp
@@ -459,7 +459,9 @@ public:
       size_t Length = 0;
       for (std::string &Line : Nodupes) {
         Length += Line.length();
-        fwrite(Line.c_str(), Line.length(), 1, Fp);
+        if (fwrite(Line.c_str(), Line.length(), 1, Fp) != 1) {
+          fprintf(stderr, "Unable to write to output file %s\n", Filename.c_str());
+        }
       }
       fclose(Fp);
 
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 2cd82215ec0a..641ecb33c946 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -1542,28 +1542,6 @@ nsContentUtils::IsJavaScriptLanguage(const nsString& aName)
          aName.LowerCaseEqualsLiteral("javascript1.5");
 }
 
-JSVersion
-nsContentUtils::ParseJavascriptVersion(const nsAString& aVersionStr)
-{
-  if (aVersionStr.Length() != 3 || aVersionStr[0] != '1' ||
-      aVersionStr[1] != '.') {
-    return JSVERSION_UNKNOWN;
-  }
-
-  switch (aVersionStr[2]) {
-  case '0': /* fall through */
-  case '1': /* fall through */
-  case '2': /* fall through */
-  case '3': /* fall through */
-  case '4': /* fall through */
-  case '5': return JSVERSION_DEFAULT;
-  case '6': return JSVERSION_1_6;
-  case '7': return JSVERSION_1_7;
-  case '8': return JSVERSION_1_8;
-  default:  return JSVERSION_UNKNOWN;
-  }
-}
-
 void
 nsContentUtils::SplitMimeType(const nsAString& aValue, nsString& aType,
                               nsString& aParams)
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index abf2e68da00c..b401488267f8 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2633,12 +2633,6 @@ public:
    */
   static bool IsJavaScriptLanguage(const nsString& aName);
 
-  /**
-   * Returns the JSVersion for a string of the form '1.n', n = 0, ..., 8, and
-   * JSVERSION_UNKNOWN for other strings.
-   */
-  static JSVersion ParseJavascriptVersion(const nsAString& aVersionStr);
-
   static bool IsJavascriptMIMEType(const nsAString& aMIMEType);
 
   static void SplitMimeType(const nsAString& aValue, nsString& aType,
diff --git a/dom/script/ModuleLoadRequest.cpp b/dom/script/ModuleLoadRequest.cpp
index c9b02b3f1126..af85028c4db7 100644
--- a/dom/script/ModuleLoadRequest.cpp
+++ b/dom/script/ModuleLoadRequest.cpp
@@ -29,13 +29,13 @@ NS_IMPL_ADDREF_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
 NS_IMPL_RELEASE_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
 
 ModuleLoadRequest::ModuleLoadRequest(nsIScriptElement* aElement,
-                                     uint32_t aVersion,
+                                     ValidJSVersion aValidJSVersion,
                                      CORSMode aCORSMode,
                                      const SRIMetadata& aIntegrity,
                                      ScriptLoader* aLoader)
   : ScriptLoadRequest(ScriptKind::Module,
                       aElement,
-                      aVersion,
+                      aValidJSVersion,
                       aCORSMode,
                       aIntegrity),
     mIsTopLevel(true),
diff --git a/dom/script/ModuleLoadRequest.h b/dom/script/ModuleLoadRequest.h
index fff8a35e030b..01619d391346 100644
--- a/dom/script/ModuleLoadRequest.h
+++ b/dom/script/ModuleLoadRequest.h
@@ -32,7 +32,7 @@ public:
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ModuleLoadRequest, ScriptLoadRequest)
 
   ModuleLoadRequest(nsIScriptElement* aElement,
-                    uint32_t aVersion,
+                    ValidJSVersion aValidJSVersion,
                     CORSMode aCORSMode,
                     const SRIMetadata& aIntegrity,
                     ScriptLoader* aLoader);
diff --git a/dom/script/ScriptLoadRequest.cpp b/dom/script/ScriptLoadRequest.cpp
index 4875ba787543..fb5068ce6d66 100644
--- a/dom/script/ScriptLoadRequest.cpp
+++ b/dom/script/ScriptLoadRequest.cpp
@@ -40,7 +40,7 @@ NS_IMPL_CYCLE_COLLECTION_TRACE_END
 
 ScriptLoadRequest::ScriptLoadRequest(ScriptKind aKind,
                                      nsIScriptElement* aElement,
-                                     uint32_t aVersion,
+                                     ValidJSVersion aValidJSVersion,
                                      mozilla::CORSMode aCORSMode,
                                      const mozilla::dom::SRIMetadata& aIntegrity)
   : mKind(aKind)
@@ -63,7 +63,7 @@ ScriptLoadRequest::ScriptLoadRequest(ScriptKind aKind,
   , mScriptText()
   , mScriptBytecode()
   , mBytecodeOffset(0)
-  , mJSVersion(aVersion)
+  , mValidJSVersion(aValidJSVersion)
   , mLineNo(1)
   , mCORSMode(aCORSMode)
   , mIntegrity(aIntegrity)
diff --git a/dom/script/ScriptLoadRequest.h b/dom/script/ScriptLoadRequest.h
index 62ab177d51f9..41245d956e54 100644
--- a/dom/script/ScriptLoadRequest.h
+++ b/dom/script/ScriptLoadRequest.h
@@ -29,6 +29,11 @@ enum class ScriptKind {
   Module
 };
 
+enum class ValidJSVersion : bool {
+  Invalid,
+  Valid
+};
+
 /*
  * A class that handles loading and evaluation of <script> elements.
  */
@@ -48,7 +53,7 @@ protected:
 public:
   ScriptLoadRequest(ScriptKind aKind,
                     nsIScriptElement* aElement,
-                    uint32_t aVersion,
+                    ValidJSVersion aValidJSVersion,
                     mozilla::CORSMode aCORSMode,
                     const mozilla::dom::SRIMetadata &aIntegrity);
 
@@ -178,7 +183,7 @@ public:
   mozilla::Vector<uint8_t> mScriptBytecode;
   uint32_t mBytecodeOffset; // Offset of the bytecode in mScriptBytecode
 
-  uint32_t mJSVersion;
+  ValidJSVersion mValidJSVersion;
   nsCOMPtr<nsIURI> mURI;
   nsCOMPtr<nsIPrincipal> mTriggeringPrincipal;
   nsCOMPtr<nsIPrincipal> mOriginPrincipal;
diff --git a/dom/script/ScriptLoader.cpp b/dom/script/ScriptLoader.cpp
index 10173e2b7d34..32ea73c80222 100644
--- a/dom/script/ScriptLoader.cpp
+++ b/dom/script/ScriptLoader.cpp
@@ -746,7 +746,7 @@ ScriptLoader::StartFetchingModuleAndDependencies(ModuleLoadRequest* aRequest,
   MOZ_ASSERT(aURI);
 
   RefPtr<ModuleLoadRequest> childRequest =
-    new ModuleLoadRequest(aRequest->mElement, aRequest->mJSVersion,
+    new ModuleLoadRequest(aRequest->mElement, aRequest->mValidJSVersion,
                             aRequest->mCORSMode, aRequest->mIntegrity, this);
 
   childRequest->mIsTopLevel = false;
@@ -1039,7 +1039,7 @@ ScriptLoader::StartLoad(ScriptLoadRequest* aRequest)
   aRequest->mCacheInfo = nullptr;
   nsCOMPtr<nsICacheInfoChannel> cic(do_QueryInterface(channel));
   if (cic && nsContentUtils::IsBytecodeCacheEnabled() &&
-      aRequest->mJSVersion == JSVERSION_DEFAULT) {
+      aRequest->mValidJSVersion == ValidJSVersion::Valid) {
     if (!aRequest->IsLoadingSource()) {
       // Inform the HTTP cache that we prefer to have information coming from the
       // bytecode cache instead of the sources, if such entry is already registered.
@@ -1169,12 +1169,29 @@ public:
   }
 };
 
+/**
+ * Returns ValidJSVersion::Valid if aVersionStr is a string of the form
+ * '1.n', n = 0, ..., 8, and ValidJSVersion::Invalid for other strings.
+ */
+static ValidJSVersion
+ParseJavascriptVersion(const nsAString& aVersionStr)
+{
+  if (aVersionStr.Length() != 3 || aVersionStr[0] != '1' ||
+      aVersionStr[1] != '.') {
+    return ValidJSVersion::Invalid;
+  }
+  if ('0' <= aVersionStr[2] && aVersionStr[2] <= '8') {
+    return ValidJSVersion::Valid;
+  }
+  return ValidJSVersion::Invalid;
+}
+
 static inline bool
-ParseTypeAttribute(const nsAString& aType, JSVersion* aVersion)
+ParseTypeAttribute(const nsAString& aType, ValidJSVersion* aVersion)
 {
   MOZ_ASSERT(!aType.IsEmpty());
   MOZ_ASSERT(aVersion);
-  MOZ_ASSERT(*aVersion == JSVERSION_DEFAULT);
+  MOZ_ASSERT(*aVersion == ValidJSVersion::Valid);
 
   nsContentTypeParser parser(aType);
 
@@ -1191,7 +1208,7 @@ ParseTypeAttribute(const nsAString& aType, JSVersion* aVersion)
   rv = parser.GetParameter("version", versionName);
 
   if (NS_SUCCEEDED(rv)) {
-    *aVersion = nsContentUtils::ParseJavascriptVersion(versionName);
+    *aVersion = ParseJavascriptVersion(versionName);
   } else if (rv != NS_ERROR_INVALID_ARG) {
     return false;
   }
@@ -1230,11 +1247,13 @@ CSPAllowsInlineScript(nsIScriptElement* aElement, nsIDocument* aDocument)
 ScriptLoadRequest*
 ScriptLoader::CreateLoadRequest(ScriptKind aKind,
                                 nsIScriptElement* aElement,
-                                uint32_t aVersion, CORSMode aCORSMode,
+                                ValidJSVersion aValidJSVersion,
+                                CORSMode aCORSMode,
                                 const SRIMetadata& aIntegrity)
 {
   if (aKind == ScriptKind::Classic) {
-    ScriptLoadRequest* slr = new ScriptLoadRequest(aKind, aElement, aVersion, aCORSMode,
+    ScriptLoadRequest* slr = new ScriptLoadRequest(aKind, aElement,
+                                 aValidJSVersion, aCORSMode,
                                  aIntegrity);
 
     LOG(("ScriptLoader %p creates ScriptLoadRequest %p", this, slr));
@@ -1242,7 +1261,8 @@ ScriptLoader::CreateLoadRequest(ScriptKind aKind,
   }
 
   MOZ_ASSERT(aKind == ScriptKind::Module);
-  return new ModuleLoadRequest(aElement, aVersion, aCORSMode, aIntegrity, this);
+  return new ModuleLoadRequest(aElement, aValidJSVersion, aCORSMode,
+                               aIntegrity, this);
 }
 
 bool
@@ -1265,7 +1285,7 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
     return false;
   }
 
-  JSVersion version = JSVERSION_DEFAULT;
+  ValidJSVersion validJSVersion = ValidJSVersion::Valid;
 
   // Check the type attribute to determine language and version.
   // If type exists, it trumps the deprecated 'language='
@@ -1277,7 +1297,7 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
     if (ModuleScriptsEnabled() && type.LowerCaseEqualsASCII("module")) {
       scriptKind = ScriptKind::Module;
     } else {
-      NS_ENSURE_TRUE(ParseTypeAttribute(type, &version), false);
+      NS_ENSURE_TRUE(ParseTypeAttribute(type, &validJSVersion), false);
     }
   } else if (!hasType) {
     // no 'type=' element
@@ -1380,7 +1400,7 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
         principal = scriptContent->NodePrincipal();
       }
 
-      request = CreateLoadRequest(scriptKind, aElement, version, ourCORSMode,
+      request = CreateLoadRequest(scriptKind, aElement, validJSVersion, ourCORSMode,
                                   sriMetadata);
       request->mURI = scriptURI;
       request->mTriggeringPrincipal = Move(principal);
@@ -1422,7 +1442,7 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
     NS_ASSERTION(!request->InCompilingStage(),
                  "Request should not yet be in compiling stage.");
 
-    request->mJSVersion = version;
+    request->mValidJSVersion = validJSVersion;
 
     if (aElement->GetScriptAsync()) {
       request->mIsAsync = true;
@@ -1523,9 +1543,9 @@ ScriptLoader::ProcessScriptElement(nsIScriptElement* aElement)
   }
 
   // Inline scripts ignore ther CORS mode and are always CORS_NONE
-  request = CreateLoadRequest(scriptKind, aElement, version, CORS_NONE,
+  request = CreateLoadRequest(scriptKind, aElement, validJSVersion, CORS_NONE,
                               SRIMetadata()); // SRI doesn't apply
-  request->mJSVersion = version;
+  request->mValidJSVersion = validJSVersion;
   request->mIsInline = true;
   request->mURI = mDocument->GetDocumentURI();
   request->mTriggeringPrincipal = mDocument->NodePrincipal();
@@ -2145,8 +2165,7 @@ ScriptLoader::EvaluateScript(ScriptLoadRequest* aRequest)
     return NS_ERROR_FAILURE;
   }
 
-  JSVersion version = JSVersion(aRequest->mJSVersion);
-  if (version == JSVERSION_UNKNOWN) {
+  if (aRequest->mValidJSVersion == ValidJSVersion::Invalid) {
     return NS_OK;
   }
 
@@ -3110,7 +3129,7 @@ ScriptLoader::PreloadURI(nsIURI* aURI, const nsAString& aCharset,
   }
 
   RefPtr<ScriptLoadRequest> request =
-    CreateLoadRequest(ScriptKind::Classic, nullptr, 0,
+    CreateLoadRequest(ScriptKind::Classic, nullptr, ValidJSVersion::Valid,
                       Element::StringToCORSMode(aCrossOrigin), sriMetadata);
   request->mURI = aURI;
   request->mTriggeringPrincipal = mDocument->NodePrincipal();
diff --git a/dom/script/ScriptLoader.h b/dom/script/ScriptLoader.h
index a9097377cf8d..133b472604d5 100644
--- a/dom/script/ScriptLoader.h
+++ b/dom/script/ScriptLoader.h
@@ -341,7 +341,7 @@ private:
 
   ScriptLoadRequest* CreateLoadRequest(ScriptKind aKind,
                                        nsIScriptElement* aElement,
-                                       uint32_t aVersion,
+                                       ValidJSVersion aValidJSVersion,
                                        mozilla::CORSMode aCORSMode,
                                        const mozilla::dom::SRIMetadata& aIntegrity);
 
diff --git a/dom/xul/nsXULContentSink.cpp b/dom/xul/nsXULContentSink.cpp
index 1c5670708802..d2786802f133 100644
--- a/dom/xul/nsXULContentSink.cpp
+++ b/dom/xul/nsXULContentSink.cpp
@@ -835,7 +835,6 @@ XULContentSinkImpl::OpenScript(const char16_t** aAttributes,
                                const uint32_t aLineNumber)
 {
   bool isJavaScript = true;
-  uint32_t version = JSVERSION_DEFAULT;
   nsresult rv;
 
   // Look for SRC attribute and look for a LANGUAGE attribute
@@ -861,7 +860,6 @@ XULContentSinkImpl::OpenScript(const char16_t** aAttributes,
 
           if (nsContentUtils::IsJavascriptMIMEType(mimeType)) {
               isJavaScript = true;
-              version = JSVERSION_DEFAULT;
 
               // Get the version string, and ensure that JavaScript supports it.
               nsAutoString versionName;
@@ -888,7 +886,6 @@ XULContentSinkImpl::OpenScript(const char16_t** aAttributes,
           nsAutoString lang(aAttributes[1]);
           if (nsContentUtils::IsJavaScriptLanguage(lang)) {
               isJavaScript = true;
-              version = JSVERSION_DEFAULT;
           }
       }
       aAttributes += 2;
@@ -904,7 +901,7 @@ XULContentSinkImpl::OpenScript(const char16_t** aAttributes,
   if (doc)
       globalObject = do_QueryInterface(doc->GetWindow());
   RefPtr<nsXULPrototypeScript> script =
-      new nsXULPrototypeScript(aLineNumber, version);
+      new nsXULPrototypeScript(aLineNumber);
 
   // If there is a SRC attribute...
   if (! src.IsEmpty()) {
diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
index b442f4acfc61..a8a5bc6f9063 100644
--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -2251,7 +2251,7 @@ nsXULPrototypeElement::Deserialize(nsIObjectInputStream* aStream,
                 break;
             case eType_Script: {
                 // language version/options obtained during deserialization.
-                RefPtr<nsXULPrototypeScript> script = new nsXULPrototypeScript(0, 0);
+                RefPtr<nsXULPrototypeScript> script = new nsXULPrototypeScript(0);
 
                 rv = aStream->ReadBoolean(&script->mOutOfLine);
                 if (NS_WARN_IF(NS_FAILED(rv))) return rv;
@@ -2392,13 +2392,12 @@ nsXULPrototypeElement::TraceAllScripts(JSTracer* aTrc)
 // nsXULPrototypeScript
 //
 
-nsXULPrototypeScript::nsXULPrototypeScript(uint32_t aLineNo, uint32_t aVersion)
+nsXULPrototypeScript::nsXULPrototypeScript(uint32_t aLineNo)
     : nsXULPrototypeNode(eType_Script),
       mLineNo(aLineNo),
       mSrcLoading(false),
       mOutOfLine(true),
       mSrcLoadWaiters(nullptr),
-      mLangVersion(aVersion),
       mScriptObject(nullptr)
 {
 }
@@ -2431,7 +2430,7 @@ nsXULPrototypeScript::Serialize(nsIObjectOutputStream* aStream,
     nsresult rv;
     rv = aStream->Write32(mLineNo);
     if (NS_FAILED(rv)) return rv;
-    rv = aStream->Write32(mLangVersion);
+    rv = aStream->Write32(0); // See bug 1418294.
     if (NS_FAILED(rv)) return rv;
 
     JSContext* cx = jsapi.cx();
@@ -2501,7 +2500,8 @@ nsXULPrototypeScript::Deserialize(nsIObjectInputStream* aStream,
     // Read basic prototype data
     rv = aStream->Read32(&mLineNo);
     if (NS_FAILED(rv)) return rv;
-    rv = aStream->Read32(&mLangVersion);
+    uint32_t dummy;
+    rv = aStream->Read32(&dummy); // See bug 1418294.
     if (NS_FAILED(rv)) return rv;
 
     AutoJSAPI jsapi;
@@ -2696,7 +2696,6 @@ nsXULPrototypeScript::Compile(JS::SourceBufferHolder& aSrcBuf,
     }
 
     // Ok, compile it to create a prototype script object!
-    NS_ENSURE_TRUE(JSVersion(mLangVersion) != JSVERSION_UNKNOWN, NS_OK);
     JS::CompileOptions options(cx);
     options.setIntroductionType("scriptElement")
            .setFileAndLine(urlspec.get(), aLineNo);
diff --git a/dom/xul/nsXULElement.h b/dom/xul/nsXULElement.h
index 98386657da4a..6f7c82f25a09 100644
--- a/dom/xul/nsXULElement.h
+++ b/dom/xul/nsXULElement.h
@@ -212,7 +212,7 @@ class XULDocument;
 class nsXULPrototypeScript : public nsXULPrototypeNode
 {
 public:
-    nsXULPrototypeScript(uint32_t aLineNo, uint32_t version);
+    explicit nsXULPrototypeScript(uint32_t aLineNo);
     virtual ~nsXULPrototypeScript();
 
     virtual nsresult Serialize(nsIObjectOutputStream* aStream,
@@ -269,7 +269,6 @@ public:
     bool                     mSrcLoading;
     bool                     mOutOfLine;
     mozilla::dom::XULDocument* mSrcLoadWaiters;   // [OWNER] but not COMPtr
-    uint32_t                 mLangVersion;
 private:
     JS::Heap<JSScript*>      mScriptObject;
 };