diff --git a/babel.config.js b/babel.config.js
index f3315bbd44..2e9a4a5ddb 100644
--- a/babel.config.js
+++ b/babel.config.js
@@ -4,6 +4,7 @@ module.exports = {
             "@babel/preset-env",
             {
                 corejs: 3,
+                loose: true,  // Loose mode for…of loops are 5× faster in Firefox
                 useBuiltIns: "usage",
             },
         ],
diff --git a/static/js/bundles/common.js b/static/js/bundles/common.js
index ffe04c178a..fca9df10f2 100644
--- a/static/js/bundles/common.js
+++ b/static/js/bundles/common.js
@@ -1,4 +1,5 @@
 import "core-js/features/promise";
+import "core-js/features/symbol";
 import "jquery/dist/jquery.js";
 import "underscore/underscore.js";
 import "../page_params.js";
diff --git a/static/js/unread.js b/static/js/unread.js
index e7a4c90bbe..e8889833e9 100644
--- a/static/js/unread.js
+++ b/static/js/unread.js
@@ -297,7 +297,7 @@ exports.unread_topic_counter = (function () {
 
             return {
                 pretty_name: topic_name,
-                message_id: Math.max(...msgs),
+                message_id: Math.max(...Array.from(msgs)),
             };
         });