--- a/browser/base/content/test/performance/browser.ini
+++ b/browser/base/content/test/performance/browser.ini
@@ -1,10 +1,11 @@
 [DEFAULT]
 support-files =
   head.js
 [browser_startup.js]
+[browser_tabclose_grow_reflows.js]
 [browser_tabclose_reflows.js]
 [browser_tabopen_reflows.js]
 [browser_tabopen_squeeze_reflows.js]
 [browser_toolbariconcolor_restyles.js]
 [browser_windowclose_reflows.js]
 [browser_windowopen_reflows.js]

new file mode 100644
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabclose_grow_reflows.js
@@ -0,0 +1,60 @@
+"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 a tab that will
+ * cause the existing tabs to grow bigger.
+ */
+add_task(async function() {
+  await ensureNoPreloadedBrowser();
+
+  // Compute the number of tabs we can put into the strip without
+  // overflowing. If we remove one of the tabs, we know that the
+  // remaining tabs will grow to fill the remaining space in the
+  // tabstrip.
+  const TAB_COUNT_FOR_GROWTH = computeMaxTabCount();
+  await createTabs(TAB_COUNT_FOR_GROWTH);
+
+  // 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 lastTab = gBrowser.tabs[gBrowser.tabs.length - 1];
+  await BrowserTestUtils.switchTab(gBrowser, lastTab);
+
+  await withReflowObserver(async function() {
+    let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+    let tab = gBrowser.tabs[gBrowser.tabs.length - 1];
+    gBrowser.removeTab(tab, { animate: true });
+    await BrowserTestUtils.waitForEvent(tab, "transitionend",
+      false, e => e.propertyName === "max-width");
+    await switchDone;
+  }, EXPECTED_REFLOWS, window, origTab);
+
+  await removeAllButFirstTab();
+});