--- a/toolkit/components/extensions/Extension.jsm
+++ b/toolkit/components/extensions/Extension.jsm
@@ -1905,24 +1905,16 @@ class Langpack extends ExtensionData {
 
   async promiseLocales(locale) {
     let locales = await StartupCache.locales
       .get([this.id, "@@all_locales"], () => this._promiseLocaleMap());
 
     return this._setupLocaleData(locales);
   }
 
-  readLocaleFile(locale) {
-    return StartupCache.locales.get([this.id, this.version, locale],
-                                    () => super.readLocaleFile(locale))
-      .then(result => {
-        this.localeData.messages.set(locale, result);
-      });
-  }
-
   parseManifest() {
     return StartupCache.manifests.get(this.manifestCacheKey,
                                       () => super.parseManifest());
   }
 
   async startup(reason) {
     this.chromeRegistryHandle = null;
     if (this.startupData.chromeEntries.length > 0) {