diff --git a/browser/base/content/test/performance/browser_windowopen_reflows.js b/browser/base/content/test/performance/browser_windowopen_reflows.js
index 9da03f1dfea4..2a8a0e64ca7b 100644
--- a/browser/base/content/test/performance/browser_windowopen_reflows.js
+++ b/browser/base/content/test/performance/browser_windowopen_reflows.js
@@ -20,21 +20,26 @@ const EXPECTED_REFLOWS = [
   ],
 ];
 
-if (Services.appinfo.OS == "Darwin") {
-  // TabsInTitlebar._update causes a reflow on OS X trying to do calculations
-  // since layout info is already dirty. This doesn't seem to happen before
-  // MozAfterPaint on Linux.
-  EXPECTED_REFLOWS.push(
-    [
-      "rect@chrome://browser/content/browser-tabsintitlebar.js",
-      "_update@chrome://browser/content/browser-tabsintitlebar.js",
-      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
-      "handleEvent@chrome://browser/content/tabbrowser.xml",
-    ],
-  );
+if (Services.appinfo.OS == "Linux") {
+  if (gMultiProcessBrowser) {
+    EXPECTED_REFLOWS.push(
+      [
+        "handleEvent@chrome://browser/content/tabbrowser.xml",
+        "EventListener.handleEvent*tabbrowser-tabs_XBL_Constructor@chrome://browser/content/tabbrowser.xml",
+      ],
+    );
+  } else {
+    EXPECTED_REFLOWS.push(
+      [
+        "handleEvent@chrome://browser/content/tabbrowser.xml",
+        "inferFromText@chrome://browser/content/browser.js",
+        "handleEvent@chrome://browser/content/browser.js",
+      ],
+    );
+  }
 }
 
-if (Services.appinfo.OS == "WINNT" || Services.appinfo.OS == "Darwin") {
+if (Services.appinfo.OS == "Darwin") {
   EXPECTED_REFLOWS.push(
     [
       "handleEvent@chrome://browser/content/tabbrowser.xml",
@@ -44,11 +49,93 @@ if (Services.appinfo.OS == "WINNT" || Services.appinfo.OS == "Darwin") {
   );
 }
 
+if (Services.appinfo.OS == "WINNT") {
+  EXPECTED_REFLOWS.push(
+    [
+      "verticalMargins@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+
+    [
+      "verticalMargins@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+
+    [
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+      "inferFromText@chrome://browser/content/browser.js",
+      "handleEvent@chrome://browser/content/browser.js",
+    ],
+
+    [
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+      "EventListener.handleEvent*tabbrowser-tabs_XBL_Constructor@chrome://browser/content/tabbrowser.xml",
+    ],
+  );
+}
+
+if (Services.appinfo.OS == "WINNT" || Services.appinfo.OS == "Darwin") {
+  EXPECTED_REFLOWS.push(
+    [
+      "rect@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+
+    [
+      "rect@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+
+    [
+      "rect@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+
+    [
+      "rect@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+
+    [
+      "verticalMargins@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+
+    [
+      "verticalMargins@chrome://browser/content/browser-tabsintitlebar.js",
+      "_update@chrome://browser/content/browser-tabsintitlebar.js",
+      "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
+      "handleEvent@chrome://browser/content/tabbrowser.xml",
+    ],
+  );
+}
+
 /*
  * This test ensures that there are no unexpected
  * uninterruptible reflows when opening new windows.
  */
 add_task(async function() {
+  // Flushing all caches helps to ensure that we get consistent
+  // behaviour when opening a new window, even if windows have been
+  // opened in previous tests.
+  Services.obs.notifyObservers(null, "startupcache-invalidate");
+  Services.obs.notifyObservers(null, "chrome-flush-skin-caches");
+  Services.obs.notifyObservers(null, "chrome-flush-caches");
+
   let win = OpenBrowserWindow();
 
   await withReflowObserver(async function() {