--- a/browser/base/content/test/static/browser_all_files_referenced.js
+++ b/browser/base/content/test/static/browser_all_files_referenced.js
@@ -92,19 +92,16 @@ var whitelist = [
 
   // toolkit/components/places/ColorAnalyzer_worker.js
   {file: "resource://gre/modules/ClusterLib.js"},
   {file: "resource://gre/modules/ColorConversion.js"},
 
   // Needed by HiddenFrame.jsm, but can't be packaged test-only
   {file: "chrome://global/content/win.xul"},
 
-  // List of built-in locales. See bug 1362617 for details.
-  {file: "resource://gre/res/multilocale.json"},
-
   // The l10n build system can't package string files only for some platforms.
   {file: "resource://gre/chrome/en-US/locale/en-US/global-platform/mac/accessible.properties",
    platforms: ["linux", "win"]},
   {file: "resource://gre/chrome/en-US/locale/en-US/global-platform/mac/intl.properties",
    platforms: ["linux", "win"]},
   {file: "resource://gre/chrome/en-US/locale/en-US/global-platform/mac/platformKeys.properties",
    platforms: ["linux", "win"]},
   {file: "resource://gre/chrome/en-US/locale/en-US/global-platform/unix/accessible.properties",

--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -664,21 +664,25 @@ BrowserGlue.prototype = {
         textcolor: "white",
         accentcolor: "black",
         author: vendorShortName,
       });
     }
 
 
     // Initialize the default l10n resource sources for L10nRegistry.
-    const locales = [AppConstants.INSTALL_LOCALE];
-    const toolkitSource = new FileSource("toolkit", locales, "resource://gre/localization/{locale}/");
-    L10nRegistry.registerSource(toolkitSource);
-    const appSource = new FileSource("app", locales, "resource://app/localization/{locale}/");
-    L10nRegistry.registerSource(appSource);
+    const multilocalePath = "resource://gre/res/multilocale.json";
+    L10nRegistry.bootstrap = fetch(multilocalePath).then(d => d.json()).then(({ locales }) => {
+      const toolkitSource = new FileSource("toolkit", locales, "resource://gre/localization/{locale}/");
+      L10nRegistry.registerSource(toolkitSource);
+      const appSource = new FileSource("app", locales, "resource://app/localization/{locale}/");
+      L10nRegistry.registerSource(appSource);
+    }).catch(e => {
+      Services.console.logStringMessage(`Could not load multilocale.json. Error: ${e}`);
+    });
 
     Services.obs.notifyObservers(null, "browser-ui-startup-complete");
   },
 
   _checkForOldBuildUpdates() {
     // check for update if our build is old
     if (AppConstants.MOZ_UPDATER &&
         Services.prefs.getBoolPref("app.update.enabled") &&

--- a/intl/l10n/L10nRegistry.jsm
+++ b/intl/l10n/L10nRegistry.jsm
@@ -73,26 +73,30 @@ Components.utils.importGlobalProperties(
  * If during the life-cycle of the app a new source is added, the generator can be called again
  * and will produce a new set of permutations placing the language pack provided resources
  * at the top.
  */
 
 const L10nRegistry = {
   sources: new Map(),
   ctxCache: new Map(),
+  bootstrap: null,
 
   /**
    * Based on the list of requested languages and resource Ids,
    * this function returns an lazy iterator over message context permutations.
    *
    * @param {Array} requestedLangs
    * @param {Array} resourceIds
    * @returns {AsyncIterator<MessageContext>}
    */
   async * generateContexts(requestedLangs, resourceIds) {
+    if (this.bootstrap !== null) {
+      await this.bootstrap;
+    }
     const sourcesOrder = Array.from(this.sources.keys()).reverse();
     for (const locale of requestedLangs) {
       yield * generateContextsForLocale(locale, sourcesOrder, resourceIds);
     }
   },
 
   /**
    * Adds a new resource source to the L10nRegistry.