diff --git a/toolkit/moz.configure b/toolkit/moz.configure
--- a/toolkit/moz.configure
+++ b/toolkit/moz.configure
@@ -28,7 +28,7 @@ project_flag(
 
 project_flag(
     env="MOZ_APP_UA_NAME",
-    default="",
+    default="Firefox",
     nargs=1,
     help="Application name in the User Agent string",
 )
