--- a/browser/base/content/test/performance/browser.ini
+++ b/browser/base/content/test/performance/browser.ini
@@ -1,6 +1,7 @@
 [DEFAULT]
 support-files =
   head.js
+[browser_tabclose_reflows.js]
 [browser_tabopen_reflows.js]
 [browser_toolbariconcolor_restyles.js]
 [browser_windowopen_reflows.js]

new file mode 100644
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabclose_reflows.js
@@ -0,0 +1,71 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS. This
+ * is a whitelist that should slowly go away as we improve the performance of
+ * the front-end. Instead of adding more reflows to the whitelist, you should
+ * be modifying your code to avoid the reflow.
+ *
+ * See https://developer.mozilla.org/en-US/Firefox/Performance_best_practices_for_Firefox_fe_engineers
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  [
+    "_adjustFocusAfterTabSwitch@chrome://browser/content/tabbrowser.xml",
+  ],
+];
+
+if (gMultiProcessBrowser) {
+  EXPECTED_REFLOWS.push(
+    [
+      "_adjustFocusAfterTabSwitch@chrome://browser/content/tabbrowser.xml",
+    ],
+  );
+}
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when closing new tabs.
+ */
+add_task(async function() {
+  // If we've got a preloaded browser, get rid of it so that it
+  // doesn't interfere with the test if it's loading. We have to
+  // do this before we disable preloading or changing the new tab
+  // URL, otherwise _getPreloadedBrowser will return null, despite
+  // the preloaded browser existing.
+  let preloaded = gBrowser._getPreloadedBrowser();
+  if (preloaded) {
+    preloaded.remove();
+  }
+
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.newtab.preload", false]],
+  });
+
+  let aboutNewTabService = Cc["@mozilla.org/browser/aboutnewtab-service;1"]
+                             .getService(Ci.nsIAboutNewTabService);
+  aboutNewTabService.newTabURL = "about:blank";
+
+  registerCleanupFunction(() => {
+    aboutNewTabService.resetNewTabURL();
+  });
+
+  // Because the tab strip is a scrollable frame, we can't use the
+  // default dirtying function from withReflowObserver and reliably
+  // get reflows for the strip. Instead, we provide a node that's
+  // already in the scrollable frame to dirty - in this case, the
+  // original tab.
+  let origTab = gBrowser.selectedTab;
+
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+  await BrowserTestUtils.waitForCondition(() => tab._fullyOpen);
+
+  // Add a reflow observer and open a new tab.
+  await withReflowObserver(async function() {
+    let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+    gBrowser.removeTab(tab, { animate: true });
+    await BrowserTestUtils.waitForEvent(tab, "transitionend",
+        false, e => e.propertyName === "max-width");
+    await switchDone;
+  }, EXPECTED_REFLOWS, window, origTab);
+});