--- a/toolkit/components/extensions/Extension.jsm
+++ b/toolkit/components/extensions/Extension.jsm
@@ -580,42 +580,55 @@ this.ExtensionData = class {
         return this.localeData.addLocale(locale, messages, this);
       } catch (e) {
         this.packagingError(`Loading locale file ${file}: ${e}`);
         return new Map();
       }
     })();
   }
 
+  async _promiseLocaleMap() {
+    let locales = new Map();
+
+    let entries = await this.readDirectory("_locales");
+    for (let file of entries) {
+      if (file.isDir) {
+        let locale = this.normalizeLocaleCode(file.name);
+        locales.set(locale, file.name);
+      }
+    }
+
+    return locales;
+  }
+
+  _setupLocaleData(locales) {
+    if (this.localeData) {
+      return this.localeData.locales;
+    }
+
+    this.localeData = new LocaleData({
+      defaultLocale: this.defaultLocale,
+      locales,
+      builtinMessages: this.builtinMessages,
+    });
+
+    return locales;
+  }
+
   // Reads the list of locales available in the extension, and returns a
   // Promise which resolves to a Map upon completion.
   // Each map key is a Gecko-compatible locale code, and each value is the
   // "_locales" subdirectory containing that locale:
   //
   // Map(gecko-locale-code -> locale-directory-name)
   promiseLocales() {
     if (!this._promiseLocales) {
       this._promiseLocales = (async () => {
-        let locales = new Map();
-
-        let entries = await this.readDirectory("_locales");
-        for (let file of entries) {
-          if (file.isDir) {
-            let locale = this.normalizeLocaleCode(file.name);
-            locales.set(locale, file.name);
-          }
-        }
-
-        this.localeData = new LocaleData({
-          defaultLocale: this.defaultLocale,
-          locales,
-          builtinMessages: this.builtinMessages,
-        });
-
-        return locales;
+        let locales = this._promiseLocaleMap();
+        return this._setupLocaleData(locales);
       })();
     }
 
     return this._promiseLocales;
   }
 
   // Reads the locale messages for all locales, and returns a promise which
   // resolves to a Map of locale messages upon completion. Each key in the map
@@ -798,16 +811,23 @@ this.Extension = class extends Extension
   // Checks that the given URL is a child of our baseURI.
   isExtensionURL(url) {
     let uri = Services.io.newURI(url);
 
     let common = this.baseURI.getCommonBaseSpec(uri);
     return common == this.baseURI.spec;
   }
 
+  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, locale],
                                     () => super.readLocaleFile(locale))
       .then(result => {
         this.localeData.messages.set(locale, result);
       });
   }