--- a/browser/base/content/test/performance/browser_windowopen_reflows.js
+++ b/browser/base/content/test/performance/browser_windowopen_reflows.js
@@ -90,18 +90,17 @@ if (Services.appinfo.OS == "WINNT" || Se
         "init@chrome://browser/content/browser-tabsintitlebar.js",
         "handleEvent@chrome://browser/content/tabbrowser.xml",
       ],
       times: 4, // This number should only ever go down - never up.
     },
   );
 }
 
-// Windows Vista, 7 or 8
-if (navigator.userAgent.indexOf("Windows NT 6") != -1) {
+if (Services.appinfo.OS == "WINNT" && screen.width <= 1280) {
   EXPECTED_REFLOWS.push(
     {
       stack: [
         "handleEvent@chrome://browser/content/tabbrowser.xml",
       ],
     },
   );
 }