diff --git a/layout/tools/reftest/manifest.sys.mjs b/layout/tools/reftest/manifest.sys.mjs
index 319f27f6ae9a..0820b450ef52 100644
--- a/layout/tools/reftest/manifest.sys.mjs
+++ b/layout/tools/reftest/manifest.sys.mjs
@@ -654,7 +654,7 @@ function BuildConditionSandbox(aURL) {
   sandbox.wayland = mozinfo.display == "wayland";
 
   // data not using mozinfo
-  sandbox.xulRuntime = {};
+  sandbox.xulRuntime = Cu.cloneInto({}, sandbox);
 
   // Do we *not* have a dedicated gpu process.
   sandbox.nogpu =