diff --git a/dom/l10n/DocumentL10n.cpp b/dom/l10n/DocumentL10n.cpp
index 212c5b6189d9..442729b0e14d 100644
--- a/dom/l10n/DocumentL10n.cpp
+++ b/dom/l10n/DocumentL10n.cpp
@@ -277,13 +277,9 @@ void DocumentL10n::InitialTranslationCompleted(bool aL10nCached) {
     mContentSink->InitialTranslationCompleted();
   }
 
-  // If sync was true, we want to change the state of
-  // mozILocalization to async now.
-  if (mIsSync) {
-    mIsSync = false;
-
-    mLocalization->SetIsSync(mIsSync);
-  }
+  // From now on, the state of Localization is unconditionally
+  // async.
+  SetIsSync(false);
 }
 
 void DocumentL10n::ConnectRoot(nsINode& aNode, bool aTranslate,
diff --git a/intl/l10n/Localization.cpp b/intl/l10n/Localization.cpp
index 573eef688023..dde048a323fc 100644
--- a/intl/l10n/Localization.cpp
+++ b/intl/l10n/Localization.cpp
@@ -145,7 +145,7 @@ Localization::Observe(nsISupports* aSubject, const char* aTopic,
 
 void Localization::OnChange() {
   if (mLocalization) {
-    mLocalization->OnChange(mResourceIds);
+    mLocalization->OnChange(mResourceIds, mIsSync);
   }
 }
 
@@ -221,10 +221,7 @@ already_AddRefed<Promise> Localization::FormatValue(
   return MaybeWrapPromise(promise);
 }
 
-void Localization::SetIsSync(const bool aIsSync) {
-  mIsSync = aIsSync;
-  mLocalization->SetIsSync(aIsSync);
-}
+void Localization::SetIsSync(const bool aIsSync) { mIsSync = aIsSync; }
 
 already_AddRefed<Promise> Localization::FormatValues(
     JSContext* aCx, const Sequence<L10nKey>& aKeys, ErrorResult& aRv) {
@@ -273,6 +270,11 @@ already_AddRefed<Promise> Localization::FormatMessages(
 void Localization::FormatValueSync(JSContext* aCx, const nsACString& aId,
                                    const Optional<L10nArgs>& aArgs,
                                    nsACString& aRetVal, ErrorResult& aRv) {
+  if (!mIsSync) {
+    aRv.ThrowInvalidStateError(
+        "Can't use formatValueSync when state is async.");
+    return;
+  }
   JS::Rooted<JS::Value> args(aCx);
 
   if (aArgs.WasPassed()) {
@@ -291,6 +293,11 @@ void Localization::FormatValuesSync(JSContext* aCx,
                                     const Sequence<L10nKey>& aKeys,
                                     nsTArray<nsCString>& aRetVal,
                                     ErrorResult& aRv) {
+  if (!mIsSync) {
+    aRv.ThrowInvalidStateError(
+        "Can't use formatValuesSync when state is async.");
+    return;
+  }
   nsTArray<JS::Value> jsKeys;
   SequenceRooter<JS::Value> rooter(aCx, &jsKeys);
   for (auto& key : aKeys) {
@@ -309,6 +316,11 @@ void Localization::FormatMessagesSync(JSContext* aCx,
                                       const Sequence<L10nKey>& aKeys,
                                       nsTArray<Nullable<L10nMessage>>& aRetVal,
                                       ErrorResult& aRv) {
+  if (!mIsSync) {
+    aRv.ThrowInvalidStateError(
+        "Can't use formatMessagesSync when state is async.");
+    return;
+  }
   nsTArray<JS::Value> jsKeys;
   SequenceRooter<JS::Value> rooter(aCx, &jsKeys);
   for (auto& key : aKeys) {
diff --git a/intl/l10n/Localization.jsm b/intl/l10n/Localization.jsm
index b4b6a08eb6fb..0629a77b195a 100644
--- a/intl/l10n/Localization.jsm
+++ b/intl/l10n/Localization.jsm
@@ -219,7 +219,6 @@ class Localization {
     this.resourceIds = [];
     this.generateBundles = undefined;
     this.generateBundlesSync = undefined;
-    this.isSync = undefined;
     this.bundles = undefined;
   }
 
@@ -228,29 +227,24 @@ class Localization {
    *
    * @param {Array<String>}    resourceIds - List of resource ids used by this
    *                                         localization.
-   * @param {bool}                    sync - Whether the instance should be
+   * @param {bool}                  isSync - Whether the instance should be
    *                                         synchronous.
    * @param {bool}                   eager - Whether the initial bundles should be
    *                                         fetched eagerly.
    * @param {Function}     generateBundles - Custom FluentBundle asynchronous generator.
    * @param {Function} generateBundlesSync - Custom FluentBundle generator.
    */
-  activate(resourceIds, sync, eager, generateBundles = defaultGenerateBundles, generateBundlesSync = defaultGenerateBundlesSync) {
+  activate(resourceIds, isSync, eager, generateBundles = defaultGenerateBundles, generateBundlesSync = defaultGenerateBundlesSync) {
     if (this.bundles) {
       throw new Error("Attempt to initialize an already initialized instance.");
     }
     this.generateBundles = generateBundles;
     this.generateBundlesSync = generateBundlesSync;
-    this.isSync = sync;
-    this.regenerateBundles(resourceIds, eager);
+    this.regenerateBundles(resourceIds, isSync, eager);
   }
 
-  setIsSync(isSync) {
-    this.isSync = isSync;
-  }
-
-  cached(iterable) {
-    if (this.isSync) {
+  cached(iterable, isSync) {
+    if (isSync) {
       return CachedSyncIterable.from(iterable);
     } else {
       return CachedAsyncIterable.from(iterable);
@@ -309,9 +303,6 @@ class Localization {
    * @private
    */
   formatWithFallbackSync(keys, method) {
-    if (!this.isSync) {
-      throw new Error("Can't use sync formatWithFallback when state is async.");
-    }
     if (!this.bundles) {
       throw new Error("Attempt to format on an uninitialized instance.");
     }
@@ -464,9 +455,9 @@ class Localization {
    * @param {Array<String>}    resourceIds - List of resource ids used by this
    *                                         localization.
    */
-  onChange(resourceIds) {
+  onChange(resourceIds, isSync) {
     if (this.bundles) {
-      this.regenerateBundles(resourceIds, false);
+      this.regenerateBundles(resourceIds, isSync, false);
     }
   }
 
@@ -478,11 +469,11 @@ class Localization {
    *                                         localization.
    * @param {bool} eager - whether the I/O for new context should begin eagerly
    */
-  regenerateBundles(resourceIds, eager = false) {
+  regenerateBundles(resourceIds, isSync, eager = false) {
     // Store for error reporting from `formatWithFallback`.
     this.resourceIds = resourceIds;
-    let generateMessages = this.isSync ? this.generateBundlesSync : this.generateBundles;
-    this.bundles = this.cached(generateMessages(this.resourceIds));
+    let generateMessages = isSync ? this.generateBundlesSync : this.generateBundles;
+    this.bundles = this.cached(generateMessages(this.resourceIds), isSync);
     if (eager) {
       // If the first app locale is the same as last fallback
       // it means that we have all resources in this locale, and
diff --git a/intl/l10n/mozILocalization.idl b/intl/l10n/mozILocalization.idl
index a906208008de..3622015fc8a5 100644
--- a/intl/l10n/mozILocalization.idl
+++ b/intl/l10n/mozILocalization.idl
@@ -26,7 +26,7 @@ interface mozILocalization : nsISupports
   Array<AUTF8String> formatValuesSync(in Array<jsval> aKeys);
   Array<jsval> formatMessagesSync(in Array<jsval> aKeys);
 
-  void onChange(in Array<AString> aResourceIds);
+  void onChange(in Array<AString> aResourceIds, in bool aIsSync);
 };
 
 [scriptable, uuid(96632d26-1422-12e9-b1ce-9bb586acd241)]