Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

37 files changed, 5993 insertions, 0 deletions

diff --git a/browser/base/content/test/performance/PerfTestHelpers.sys.mjs b/browser/base/content/test/performance/PerfTestHelpers.sys.mjs
new file mode 100644
index 0000000000..caa832c2e5
--- /dev/null
+++ b/browser/base/content/test/performance/PerfTestHelpers.sys.mjs
@@ -0,0 +1,77 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  NetUtil: "resource://gre/modules/NetUtil.sys.mjs",
+});
+
+export var PerfTestHelpers = {
+  /**
+   * Maps the entries in the given iterable to the given
+   * promise-returning task function, and waits for all returned
+   * promises to have resolved. At most `limit` promises may remain
+   * unresolved at a time. When the limit is reached, the function will
+   * wait for some to resolve before spawning more tasks.
+   */
+  async throttledMapPromises(iterable, task, limit = 64) {
+    let pending = new Set();
+    let promises = [];
+    for (let data of iterable) {
+      while (pending.size >= limit) {
+        await Promise.race(pending);
+      }
+
+      let promise = task(data);
+      promises.push(promise);
+      if (promise) {
+        promise.finally(() => pending.delete(promise));
+        pending.add(promise);
+      }
+    }
+
+    return Promise.all(promises);
+  },
+
+  /**
+   * Returns a promise which resolves to true if the resource at the
+   * given URI exists, false if it doesn't. This should only be used
+   * with local resources, such as from resource:/chrome:/jar:/file:
+   * URIs.
+   */
+  checkURIExists(uri) {
+    return new Promise(resolve => {
+      try {
+        let channel = lazy.NetUtil.newChannel({
+          uri,
+          loadUsingSystemPrincipal: true,
+          // Avoid crashing for non-existant files. If the file not existing
+          // is bad, we can deal with it in the test instead.
+          contentPolicyType: Ci.nsIContentPolicy.TYPE_FETCH,
+        });
+
+        channel.asyncOpen({
+          onStartRequest(request) {
+            resolve(Components.isSuccessCode(request.status));
+            request.cancel(Cr.NS_BINDING_ABORTED);
+          },
+
+          onStopRequest(request, status) {
+            // We should have already resolved from `onStartRequest`, but
+            // we resolve again here just as a failsafe.
+            resolve(Components.isSuccessCode(status));
+          },
+        });
+      } catch (e) {
+        if (
+          e.result != Cr.NS_ERROR_FILE_NOT_FOUND &&
+          e.result != Cr.NS_ERROR_NOT_AVAILABLE
+        ) {
+          throw e;
+        }
+        resolve(false);
+      }
+    });
+  },
+};
diff --git a/browser/base/content/test/performance/StartupContentSubframe.sys.mjs b/browser/base/content/test/performance/StartupContentSubframe.sys.mjs
new file mode 100644
index 0000000000..a78e456afb
--- /dev/null
+++ b/browser/base/content/test/performance/StartupContentSubframe.sys.mjs
@@ -0,0 +1,55 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * test helper JSWindowActors used by the browser_startup_content_subframe.js test.
+ */
+
+export class StartupContentSubframeParent extends JSWindowActorParent {
+  receiveMessage(msg) {
+    // Tell the test about the data we received from the content process.
+    Services.obs.notifyObservers(
+      msg.data,
+      "startup-content-subframe-loaded-scripts"
+    );
+  }
+}
+
+export class StartupContentSubframeChild extends JSWindowActorChild {
+  async handleEvent(event) {
+    // When the remote subframe is loaded, an event will be fired to this actor,
+    // which will cause us to send the `LoadedScripts` message to the parent
+    // process.
+    // Wait a spin of the event loop before doing so to ensure we don't
+    // miss any scripts loaded immediately after the load event.
+    await new Promise(resolve => Services.tm.dispatchToMainThread(resolve));
+
+    const Cm = Components.manager;
+    Cm.QueryInterface(Ci.nsIServiceManager);
+    const { AppConstants } = ChromeUtils.importESModule(
+      "resource://gre/modules/AppConstants.sys.mjs"
+    );
+    let collectStacks = AppConstants.NIGHTLY_BUILD || AppConstants.DEBUG;
+
+    let modules = {};
+    for (let module of Cu.loadedJSModules) {
+      modules[module] = collectStacks ? Cu.getModuleImportStack(module) : "";
+    }
+    for (let module of Cu.loadedESModules) {
+      modules[module] = collectStacks ? Cu.getModuleImportStack(module) : "";
+    }
+
+    let services = {};
+    for (let contractID of Object.keys(Cc)) {
+      try {
+        if (Cm.isServiceInstantiatedByContractID(contractID, Ci.nsISupports)) {
+          services[contractID] = "";
+        }
+      } catch (e) {}
+    }
+    this.sendAsyncMessage("LoadedScripts", {
+      modules,
+      services,
+    });
+  }
+}
diff --git a/browser/base/content/test/performance/browser.toml b/browser/base/content/test/performance/browser.toml
new file mode 100644
index 0000000000..bd7d56e762
--- /dev/null
+++ b/browser/base/content/test/performance/browser.toml
@@ -0,0 +1,115 @@
+[DEFAULT]
+# to avoid overhead when running the browser normally, StartupRecorder.sys.mjs will
+# do almost nothing unless browser.startup.record is true.
+# gfx.canvas.willReadFrequently.enable is just an optimization, but needs to be
+# set during early startup to have an impact as a canvas will be used by
+# StartupRecorder.sys.mjs
+prefs = [
+  # Skip migration work in BG__migrateUI for browser_startup.js since it isn't
+  # representative of common startup.
+  "browser.migration.version=9999999",
+  "browser.startup.record=true",
+  "gfx.canvas.willReadFrequently.enable=true",
+  # The form autofill framescript is only used in certain locales if this
+  # pref is set to 'detect', which is the default value on non-Nightly.
+  "extensions.formautofill.addresses.available='on'",
+  "browser.urlbar.disableExtendForTests=true",
+  # For perfomance tests do not enable the remote control cue, which gets set
+  # when Marionette is enabled, but users normally don't see.
+  "browser.chrome.disableRemoteControlCueForTests=true",
+  # The Screenshots extension is disabled by default in Mochitests. We re-enable
+  # it here, since it's a more realistic configuration.
+  "extensions.screenshots.disabled=false",
+]
+support-files = ["head.js"]
+
+["browser_appmenu.js"]
+skip-if = [
+  "asan",
+  "debug",
+  "os == 'win'", # Bug 1775626
+  "os == 'linux' && socketprocess_networking", # Bug 1382809, bug 1369959
+]
+
+["browser_hidden_browser_vsync.js"]
+
+["browser_panel_vsync.js"]
+support-files = ["!/browser/components/downloads/test/browser/head.js"]
+
+["browser_preferences_usage.js"]
+https_first_disabled = true
+skip-if = [
+  "!debug",
+  "apple_catalina",  # platform migration
+  "socketprocess_networking",
+]
+
+["browser_startup.js"]
+
+["browser_startup_content.js"]
+support-files = ["file_empty.html"]
+
+["browser_startup_content_subframe.js"]
+skip-if = ["!fission"]
+support-files = [
+  "file_empty.html",
+  "StartupContentSubframe.sys.mjs",
+]
+
+["browser_startup_flicker.js"]
+run-if = [
+  "debug",
+  "nightly_build", # Requires StartupRecorder.sys.mjs, which isn't shipped everywhere by default
+]
+
+["browser_startup_hiddenwindow.js"]
+skip-if = ["os == 'mac'"]
+
+["browser_tabclose.js"]
+skip-if = [
+  "os == 'linux' && devedition", # Bug 1737131
+  "os == 'mac'", # Bug 1531417
+  "os == 'win'", # Bug 1488537, Bug 1497713
+]
+
+["browser_tabclose_grow.js"]
+
+["browser_tabdetach.js"]
+
+["browser_tabopen.js"]
+skip-if = [
+  "apple_catalina", # Bug 1594274
+  "os == 'mac' && !debug", # Bug 1705492
+  "os == 'linux' && !debug", # Bug 1705492
+]
+
+["browser_tabopen_squeeze.js"]
+
+["browser_tabstrip_overflow_underflow.js"]
+skip-if = [
+  "os == 'win' && verify && !debug",
+  "win11_2009 && bits == 32",
+]
+
+["browser_tabswitch.js"]
+skip-if = ["os == 'win'"] #Bug 1455054
+
+["browser_toolbariconcolor_restyles.js"]
+
+["browser_urlbar_keyed_search.js"]
+skip-if = ["win11_2009 && bits == 32"] # # Disabled on Win32 because of intermittent OOM failures (bug 1448241)
+
+["browser_urlbar_search.js"]
+skip-if = [
+  "os == 'linux' && (debug || ccov)", # Disabled on Linux debug and ccov due to intermittent timeouts. Bug 1414126.
+  "os == 'win' && (debug || ccov)", # Disabled on Windows debug and ccov due to intermittent timeouts. bug 1426611.
+  "win11_2009 && bits == 32",
+]
+
+["browser_vsync_accessibility.js"]
+
+["browser_window_resize.js"]
+
+["browser_windowclose.js"]
+
+["browser_windowopen.js"]
diff --git a/browser/base/content/test/performance/browser_appmenu.js b/browser/base/content/test/performance/browser_appmenu.js
new file mode 100644
index 0000000000..37e7482c51
--- /dev/null
+++ b/browser/base/content/test/performance/browser_appmenu.js
@@ -0,0 +1,130 @@
+"use strict";
+/* global PanelUI */
+
+const { CustomizableUITestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/CustomizableUITestUtils.sys.mjs"
+);
+let gCUITestUtils = new CustomizableUITestUtils(window);
+
+/**
+ * WHOA THERE: We should never be adding new things to
+ * EXPECTED_APPMENU_OPEN_REFLOWS. This list should slowly go
+ * away as we improve the performance of the front-end. Instead of adding more
+ * reflows to the list, you should be modifying your code to avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_APPMENU_OPEN_REFLOWS = [
+  {
+    stack: [
+      "openPopup/this._openPopupPromise<@resource:///modules/PanelMultiView.sys.mjs",
+    ],
+  },
+
+  {
+    stack: [
+      "_calculateMaxHeight@resource:///modules/PanelMultiView.sys.mjs",
+      "handleEvent@resource:///modules/PanelMultiView.sys.mjs",
+    ],
+
+    maxCount: 7, // This number should only ever go down - never up.
+  },
+];
+
+add_task(async function () {
+  await ensureNoPreloadedBrowser();
+  await ensureAnimationsFinished();
+  await disableFxaBadge();
+
+  let textBoxRect = gURLBar
+    .querySelector("moz-input-box")
+    .getBoundingClientRect();
+  let menuButtonRect = document
+    .getElementById("PanelUI-menu-button")
+    .getBoundingClientRect();
+  let firstTabRect = gBrowser.selectedTab.getBoundingClientRect();
+  let frameExpectations = {
+    filter: rects => {
+      // We expect the menu button to get into the active state.
+      //
+      // XXX For some reason the menu panel isn't in our screenshots, but
+      // that's where we actually expect many changes.
+      return rects.filter(r => !rectInBoundingClientRect(r, menuButtonRect));
+    },
+    exceptions: [
+      {
+        name: "the urlbar placeholder moves up and down by a few pixels",
+        condition: r => rectInBoundingClientRect(r, textBoxRect),
+      },
+      {
+        name: "bug 1547341 - a first tab gets drawn early",
+        condition: r => rectInBoundingClientRect(r, firstTabRect),
+      },
+    ],
+  };
+
+  // First, open the appmenu.
+  await withPerfObserver(() => gCUITestUtils.openMainMenu(), {
+    expectedReflows: EXPECTED_APPMENU_OPEN_REFLOWS,
+    frames: frameExpectations,
+  });
+
+  // Now open a series of subviews, and then close the appmenu. We
+  // should not reflow during any of this.
+  await withPerfObserver(
+    async function () {
+      // This recursive function will take the current main or subview,
+      // find all of the buttons that navigate to subviews inside it,
+      // and click each one individually. Upon entering the new view,
+      // we recurse. When the subviews within a view have been
+      // exhausted, we go back up a level.
+      async function openSubViewsRecursively(currentView) {
+        let navButtons = Array.from(
+          // Ensure that only enabled buttons are tested
+          currentView.querySelectorAll(".subviewbutton-nav:not([disabled])")
+        );
+        if (!navButtons) {
+          return;
+        }
+
+        for (let button of navButtons) {
+          info("Click " + button.id);
+          let promiseViewShown = BrowserTestUtils.waitForEvent(
+            PanelUI.panel,
+            "ViewShown"
+          );
+          button.click();
+          let viewShownEvent = await promiseViewShown;
+
+          // Workaround until bug 1363756 is fixed, then this can be removed.
+          let container = PanelUI.multiView.querySelector(
+            ".panel-viewcontainer"
+          );
+          await TestUtils.waitForCondition(() => {
+            return !container.hasAttribute("width");
+          });
+
+          info("Shown " + viewShownEvent.originalTarget.id);
+          await openSubViewsRecursively(viewShownEvent.originalTarget);
+          promiseViewShown = BrowserTestUtils.waitForEvent(
+            currentView,
+            "ViewShown"
+          );
+          PanelUI.multiView.goBack();
+          await promiseViewShown;
+
+          // Workaround until bug 1363756 is fixed, then this can be removed.
+          await TestUtils.waitForCondition(() => {
+            return !container.hasAttribute("width");
+          });
+        }
+      }
+
+      await openSubViewsRecursively(PanelUI.mainView);
+
+      await gCUITestUtils.hideMainMenu();
+    },
+    { expectedReflows: [], frames: frameExpectations }
+  );
+});
diff --git a/browser/base/content/test/performance/browser_hidden_browser_vsync.js b/browser/base/content/test/performance/browser_hidden_browser_vsync.js
new file mode 100644
index 0000000000..a2e5cc990c
--- /dev/null
+++ b/browser/base/content/test/performance/browser_hidden_browser_vsync.js
@@ -0,0 +1,56 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_hidden_sidebar() {
+  let b = document.createXULElement("browser");
+  for (let [k, v] of Object.entries({
+    type: "content",
+    disablefullscreen: "true",
+    disablehistory: "true",
+    flex: "1",
+    style: "min-width: 300px",
+    message: "true",
+    remote: "true",
+    maychangeremoteness: "true",
+  })) {
+    b.setAttribute(k, v);
+  }
+  let mainBrowser = gBrowser.selectedBrowser;
+  let panel = gBrowser.getPanel(mainBrowser);
+  panel.append(b);
+  let loaded = BrowserTestUtils.browserLoaded(b);
+  BrowserTestUtils.startLoadingURIString(
+    b,
+    `data:text/html,<!doctype html><style>
+  @keyframes fade-in {
+   from {
+     opacity: .25;
+   }
+   to {
+     opacity: 1;
+   }
+  </style>
+  <div style="
+    animation-name: fade-in;
+    animation-direction: alternate;
+    animation-duration: 1s;
+    animation-iteration-count: infinite;
+    animation-timing-function: ease-in-out;
+    background-color: red;
+    height: 500px;
+    width: 100%;
+  "></div>`
+  );
+  await loaded;
+  ok(b, "Browser was created.");
+  await SpecialPowers.spawn(b, [], async () => {
+    await new Promise(r =>
+      content.requestAnimationFrame(() => content.requestAnimationFrame(r))
+    );
+  });
+  b.hidden = true;
+  ok(b.hidden, "Browser should be hidden.");
+  // Now the framework will test to see vsync goes away
+});
diff --git a/browser/base/content/test/performance/browser_panel_vsync.js b/browser/base/content/test/performance/browser_panel_vsync.js
new file mode 100644
index 0000000000..73c56b9095
--- /dev/null
+++ b/browser/base/content/test/performance/browser_panel_vsync.js
@@ -0,0 +1,69 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Services.scriptloader.loadSubScript(
+  "chrome://mochitests/content/browser/browser/components/downloads/test/browser/head.js",
+  this
+);
+
+add_task(
+  async function test_opening_panel_and_closing_should_not_leave_vsync() {
+    await SpecialPowers.pushPrefEnv({
+      set: [["browser.download.autohideButton", false]],
+    });
+    await promiseButtonShown("downloads-button");
+
+    const downloadsButton = document.getElementById("downloads-button");
+    const shownPromise = promisePanelOpened();
+    EventUtils.synthesizeNativeMouseEvent({
+      type: "click",
+      target: downloadsButton,
+      atCenter: true,
+    });
+    await shownPromise;
+
+    is(DownloadsPanel.panel.state, "open", "Check that panel state is 'open'");
+
+    await TestUtils.waitForCondition(
+      () => !ChromeUtils.vsyncEnabled(),
+      "Make sure vsync disabled"
+    );
+    // Should not already be using vsync
+    ok(!ChromeUtils.vsyncEnabled(), "vsync should be off initially");
+
+    if (
+      AppConstants.platform == "linux" &&
+      DownloadsPanel.panel.state != "open"
+    ) {
+      // Panels sometime receive spurious popuphiding events on Linux.
+      // Given the main target of this test is Windows, avoid causing
+      // intermittent failures and just make the test return early.
+      todo(
+        false,
+        "panel should still be 'open', current state: " +
+          DownloadsPanel.panel.state
+      );
+      return;
+    }
+
+    const hiddenPromise = BrowserTestUtils.waitForEvent(
+      DownloadsPanel.panel,
+      "popuphidden"
+    );
+    EventUtils.synthesizeKey("VK_ESCAPE", {}, window);
+    await hiddenPromise;
+    await TestUtils.waitForCondition(
+      () => !ChromeUtils.vsyncEnabled(),
+      "wait for vsync to be disabled again"
+    );
+
+    ok(!ChromeUtils.vsyncEnabled(), "vsync should still be off");
+    is(
+      DownloadsPanel.panel.state,
+      "closed",
+      "Check that panel state is 'closed'"
+    );
+  }
+);
diff --git a/browser/base/content/test/performance/browser_preferences_usage.js b/browser/base/content/test/performance/browser_preferences_usage.js
new file mode 100644
index 0000000000..6bc623a360
--- /dev/null
+++ b/browser/base/content/test/performance/browser_preferences_usage.js
@@ -0,0 +1,272 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+if (SpecialPowers.useRemoteSubframes) {
+  requestLongerTimeout(2);
+}
+
+const DEFAULT_PROCESS_COUNT = Services.prefs
+  .getDefaultBranch(null)
+  .getIntPref("dom.ipc.processCount");
+
+/**
+ * A test that checks whether any preference getter from the given list
+ * of stats was called more often than the max parameter.
+ *
+ * @param {Array}  stats - an array of [prefName, accessCount] tuples
+ * @param {Number} max - the maximum number of times any of the prefs should
+ *                 have been called.
+ * @param {Object} knownProblematicPrefs (optional) - an object that defines
+ *                 prefs that should be exempt from checking the
+ *                 maximum access. It looks like the following:
+ *
+ *                 pref_name: {
+ *                   min: [Number] the minimum amount of times this should have
+ *                                 been called (to avoid keeping around dead items)
+ *                   max: [Number] the maximum amount of times this should have
+ *                                 been called (to avoid this creeping up further)
+ *                 }
+ */
+function checkPrefGetters(stats, max, knownProblematicPrefs = {}) {
+  let getterStats = Object.entries(stats).sort(
+    ([, val1], [, val2]) => val2 - val1
+  );
+
+  // Clone the list to be able to delete entries to check if we
+  // forgot any later on.
+  knownProblematicPrefs = Object.assign({}, knownProblematicPrefs);
+
+  for (let [pref, count] of getterStats) {
+    let prefLimits = knownProblematicPrefs[pref];
+    if (!prefLimits) {
+      Assert.lessOrEqual(
+        count,
+        max,
+        `${pref} should not be accessed more than ${max} times.`
+      );
+    } else {
+      // Still record how much this pref was accessed even if we don't do any real assertions.
+      if (!prefLimits.min && !prefLimits.max) {
+        info(
+          `${pref} should not be accessed more than ${max} times and was accessed ${count} times.`
+        );
+      }
+
+      if (prefLimits.min) {
+        Assert.lessOrEqual(
+          prefLimits.min,
+          count,
+          `${pref} should be accessed at least ${prefLimits.min} times.`
+        );
+      }
+      if (prefLimits.max) {
+        Assert.lessOrEqual(
+          count,
+          prefLimits.max,
+          `${pref} should be accessed at most ${prefLimits.max} times.`
+        );
+      }
+      delete knownProblematicPrefs[pref];
+    }
+  }
+
+  // This pref will be accessed by mozJSComponentLoader when loading modules,
+  // which fails TV runs since they run the test multiple times without restarting.
+  // We just ignore this pref, since it's for testing only anyway.
+  if (knownProblematicPrefs["browser.startup.record"]) {
+    delete knownProblematicPrefs["browser.startup.record"];
+  }
+
+  let unusedPrefs = Object.keys(knownProblematicPrefs);
+  is(
+    unusedPrefs.length,
+    0,
+    `Should have accessed all known problematic prefs. Remaining: ${unusedPrefs}`
+  );
+}
+
+/**
+ * A helper function to read preference access data
+ * using the Services.prefs.readStats() function.
+ */
+function getPreferenceStats() {
+  let stats = {};
+  Services.prefs.readStats((key, value) => (stats[key] = value));
+  return stats;
+}
+
+add_task(async function debug_only() {
+  ok(AppConstants.DEBUG, "You need to run this test on a debug build.");
+});
+
+// Just checks how many prefs were accessed during startup.
+add_task(async function startup() {
+  let max = 40;
+
+  let knownProblematicPrefs = {
+    "browser.startup.record": {
+      // This pref is accessed in Nighly and debug builds only.
+      min: 200,
+      max: 400,
+    },
+    "network.loadinfo.skip_type_assertion": {
+      // This is accessed in debug only.
+    },
+    "chrome.override_package.global": {
+      min: 0,
+      max: 50,
+    },
+  };
+
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  ok(startupRecorder.data.prefStats, "startupRecorder has prefStats");
+
+  checkPrefGetters(startupRecorder.data.prefStats, max, knownProblematicPrefs);
+});
+
+// This opens 10 tabs and checks pref getters.
+add_task(async function open_10_tabs() {
+  // This is somewhat arbitrary. When we had a default of 4 content processes
+  // the value was 15. We need to scale it as we increase the number of
+  // content processes so we approximate with 4 * process_count.
+  const max = 4 * DEFAULT_PROCESS_COUNT;
+
+  let knownProblematicPrefs = {
+    "browser.startup.record": {
+      max: 20,
+    },
+    "browser.tabs.remote.logSwitchTiming": {
+      max: 35,
+    },
+    "network.loadinfo.skip_type_assertion": {
+      // This is accessed in debug only.
+    },
+  };
+
+  Services.prefs.resetStats();
+
+  let tabs = [];
+  while (tabs.length < 10) {
+    tabs.push(
+      await BrowserTestUtils.openNewForegroundTab(
+        gBrowser,
+        // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+        "http://example.com",
+        true,
+        true
+      )
+    );
+  }
+
+  for (let tab of tabs) {
+    await BrowserTestUtils.removeTab(tab);
+  }
+
+  checkPrefGetters(getPreferenceStats(), max, knownProblematicPrefs);
+});
+
+// This navigates to 50 sites and checks pref getters.
+add_task(async function navigate_around() {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      // Disable bfcache so that we can measure more accurately the number of
+      // pref accesses in the child processes.
+      // If bfcache is enabled on Fission
+      // dom.ipc.keepProcessesAlive.webIsolated.perOrigin and
+      // security.sandbox.content.force-namespace are accessed only a couple of
+      // times.
+      ["browser.sessionhistory.max_total_viewers", 0],
+    ],
+  });
+
+  let max = 40;
+
+  let knownProblematicPrefs = {
+    "network.loadinfo.skip_type_assertion": {
+      // This is accessed in debug only.
+    },
+  };
+
+  if (SpecialPowers.useRemoteSubframes) {
+    // We access this when considering starting a new content process.
+    // Because there is no complete list of content process types,
+    // caching this is not trivial. Opening 50 different content
+    // processes and throwing them away immediately is a bit artificial;
+    // we're more likely to keep some around so this shouldn't be quite
+    // this bad in practice. Fixing this is
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1600266
+    knownProblematicPrefs["dom.ipc.processCount.webIsolated"] = {
+      min: 50,
+      max: 51,
+    };
+    // This pref is only accessed in automation to speed up tests.
+    knownProblematicPrefs["dom.ipc.keepProcessesAlive.webIsolated.perOrigin"] =
+      {
+        min: 100,
+        max: 102,
+      };
+    if (AppConstants.platform == "linux") {
+      // The following sandbox pref is covered by
+      // https://bugzilla.mozilla.org/show_bug.cgi?id=1600189
+      knownProblematicPrefs["security.sandbox.content.force-namespace"] = {
+        min: 45,
+        max: 55,
+      };
+    } else if (AppConstants.platform == "win") {
+      // The following 2 graphics prefs are covered by
+      // https://bugzilla.mozilla.org/show_bug.cgi?id=1639497
+      knownProblematicPrefs["gfx.canvas.azure.backends"] = {
+        min: 90,
+        max: 110,
+      };
+      knownProblematicPrefs["gfx.content.azure.backends"] = {
+        min: 90,
+        max: 110,
+      };
+      // The following 2 sandbox prefs are covered by
+      // https://bugzilla.mozilla.org/show_bug.cgi?id=1639494
+      knownProblematicPrefs["security.sandbox.content.read_path_whitelist"] = {
+        min: 47,
+        max: 55,
+      };
+      knownProblematicPrefs["security.sandbox.logging.enabled"] = {
+        min: 47,
+        max: 55,
+      };
+    }
+  }
+
+  Services.prefs.resetStats();
+
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    "http://example.com",
+    true,
+    true
+  );
+
+  let urls = [
+    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    "http://example.com/",
+    "https://example.com/",
+    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    "http://example.org/",
+    "https://example.org/",
+  ];
+
+  for (let i = 0; i < 50; i++) {
+    let url = urls[i % urls.length];
+    info(`Navigating to ${url}...`);
+    BrowserTestUtils.startLoadingURIString(tab.linkedBrowser, url);
+    await BrowserTestUtils.browserLoaded(tab.linkedBrowser, false, url);
+    info(`Loaded ${url}.`);
+  }
+
+  await BrowserTestUtils.removeTab(tab);
+
+  checkPrefGetters(getPreferenceStats(), max, knownProblematicPrefs);
+});
diff --git a/browser/base/content/test/performance/browser_startup.js b/browser/base/content/test/performance/browser_startup.js
new file mode 100644
index 0000000000..9d75c14e0f
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup.js
@@ -0,0 +1,246 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* This test records at which phase of startup the JS modules are first
+ * loaded.
+ * If you made changes that cause this test to fail, it's likely because you
+ * are loading more JS code during startup.
+ * Most code has no reason to run off of the app-startup notification
+ * (this is very early, before we have selected the user profile, so
+ *  preferences aren't accessible yet).
+ * If your code isn't strictly required to show the first browser window,
+ * it shouldn't be loaded before we are done with first paint.
+ * Finally, if your code isn't really needed during startup, it should not be
+ * loaded before we have started handling user events.
+ */
+
+"use strict";
+
+/* Set this to true only for debugging purpose; it makes the output noisy. */
+const kDumpAllStacks = false;
+
+const startupPhases = {
+  // For app-startup, we have an allowlist of acceptable JS files.
+  // Anything loaded during app-startup must have a compelling reason
+  // to run before we have even selected the user profile.
+  // Consider loading your code after first paint instead,
+  // eg. from BrowserGlue.sys.mjs' _onFirstWindowLoaded method).
+  "before profile selection": {
+    allowlist: {
+      modules: new Set([
+        "resource:///modules/BrowserGlue.sys.mjs",
+        "resource:///modules/StartupRecorder.sys.mjs",
+        "resource://gre/modules/AppConstants.sys.mjs",
+        "resource://gre/modules/ActorManagerParent.sys.mjs",
+        "resource://gre/modules/CustomElementsListener.sys.mjs",
+        "resource://gre/modules/MainProcessSingleton.sys.mjs",
+        "resource://gre/modules/XPCOMUtils.sys.mjs",
+      ]),
+    },
+  },
+
+  // For the following phases of startup we have only a list of files that
+  // are **not** allowed to load in this phase, as too many other scripts
+  // load during this time.
+
+  // We are at this phase after creating the first browser window (ie. after final-ui-startup).
+  "before opening first browser window": {
+    denylist: {
+      modules: new Set([]),
+    },
+  },
+
+  // We reach this phase right after showing the first browser window.
+  // This means that anything already loaded at this point has been loaded
+  // before first paint and delayed it.
+  "before first paint": {
+    denylist: {
+      modules: new Set([
+        "resource:///modules/AboutNewTab.sys.mjs",
+        "resource:///modules/BrowserUsageTelemetry.sys.mjs",
+        "resource:///modules/ContentCrashHandlers.sys.mjs",
+        "resource:///modules/ShellService.sys.mjs",
+        "resource://gre/modules/NewTabUtils.sys.mjs",
+        "resource://gre/modules/PageThumbs.sys.mjs",
+        "resource://gre/modules/PlacesUtils.sys.mjs",
+        "resource://gre/modules/Preferences.sys.mjs",
+        "resource://gre/modules/SearchService.sys.mjs",
+        // Sqlite.sys.mjs commented out because of bug 1828735.
+        // "resource://gre/modules/Sqlite.sys.mjs"
+      ]),
+      services: new Set(["@mozilla.org/browser/search-service;1"]),
+    },
+  },
+
+  // We are at this phase once we are ready to handle user events.
+  // Anything loaded at this phase or before gets in the way of the user
+  // interacting with the first browser window.
+  "before handling user events": {
+    denylist: {
+      modules: new Set([
+        "resource://gre/modules/Blocklist.sys.mjs",
+        // Bug 1391495 - BrowserWindowTracker.sys.mjs is intermittently used.
+        // "resource:///modules/BrowserWindowTracker.sys.mjs",
+        "resource://gre/modules/BookmarkHTMLUtils.sys.mjs",
+        "resource://gre/modules/Bookmarks.sys.mjs",
+        "resource://gre/modules/ContextualIdentityService.sys.mjs",
+        "resource://gre/modules/FxAccounts.sys.mjs",
+        "resource://gre/modules/FxAccountsStorage.sys.mjs",
+        "resource://gre/modules/PlacesBackups.sys.mjs",
+        "resource://gre/modules/PlacesExpiration.sys.mjs",
+        "resource://gre/modules/PlacesSyncUtils.sys.mjs",
+        "resource://gre/modules/PushComponents.sys.mjs",
+      ]),
+      services: new Set(["@mozilla.org/browser/nav-bookmarks-service;1"]),
+    },
+  },
+
+  // Things that are expected to be completely out of the startup path
+  // and loaded lazily when used for the first time by the user should
+  // be listed here.
+  "before becoming idle": {
+    denylist: {
+      modules: new Set([
+        "resource://gre/modules/AsyncPrefs.sys.mjs",
+        "resource://gre/modules/LoginManagerContextMenu.sys.mjs",
+        "resource://pdf.js/PdfStreamConverter.sys.mjs",
+      ]),
+    },
+  },
+};
+
+if (
+  Services.prefs.getBoolPref("browser.startup.blankWindow") &&
+  Services.prefs.getCharPref(
+    "extensions.activeThemeID",
+    "default-theme@mozilla.org"
+  ) == "default-theme@mozilla.org"
+) {
+  startupPhases["before profile selection"].allowlist.modules.add(
+    "resource://gre/modules/XULStore.sys.mjs"
+  );
+}
+
+if (AppConstants.MOZ_CRASHREPORTER) {
+  startupPhases["before handling user events"].denylist.modules.add(
+    "resource://gre/modules/CrashSubmit.sys.mjs"
+  );
+}
+
+add_task(async function () {
+  if (
+    !AppConstants.NIGHTLY_BUILD &&
+    !AppConstants.MOZ_DEV_EDITION &&
+    !AppConstants.DEBUG
+  ) {
+    ok(
+      !("@mozilla.org/test/startuprecorder;1" in Cc),
+      "the startup recorder component shouldn't exist in this non-nightly/non-devedition/" +
+        "non-debug build."
+    );
+    return;
+  }
+
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  let data = Cu.cloneInto(startupRecorder.data.code, {});
+  function getStack(scriptType, name) {
+    if (scriptType == "modules") {
+      return Cu.getModuleImportStack(name);
+    }
+    return "";
+  }
+
+  // This block only adds debug output to help find the next bugs to file,
+  // it doesn't contribute to the actual test.
+  SimpleTest.requestCompleteLog();
+  let previous;
+  for (let phase in data) {
+    for (let scriptType in data[phase]) {
+      for (let f of data[phase][scriptType]) {
+        // phases are ordered, so if a script wasn't loaded yet at the immediate
+        // previous phase, it wasn't loaded during any of the previous phases
+        // either, and is new in the current phase.
+        if (!previous || !data[previous][scriptType].includes(f)) {
+          info(`${scriptType} loaded ${phase}: ${f}`);
+          if (kDumpAllStacks) {
+            info(getStack(scriptType, f));
+          }
+        }
+      }
+    }
+    previous = phase;
+  }
+
+  for (let phase in startupPhases) {
+    let loadedList = data[phase];
+    let allowlist = startupPhases[phase].allowlist || null;
+    if (allowlist) {
+      for (let scriptType in allowlist) {
+        loadedList[scriptType] = loadedList[scriptType].filter(c => {
+          if (!allowlist[scriptType].has(c)) {
+            return true;
+          }
+          allowlist[scriptType].delete(c);
+          return false;
+        });
+        is(
+          loadedList[scriptType].length,
+          0,
+          `should have no unexpected ${scriptType} loaded ${phase}`
+        );
+        for (let script of loadedList[scriptType]) {
+          let message = `unexpected ${scriptType}: ${script}`;
+          record(false, message, undefined, getStack(scriptType, script));
+        }
+        is(
+          allowlist[scriptType].size,
+          0,
+          `all ${scriptType} allowlist entries should have been used`
+        );
+        for (let script of allowlist[scriptType]) {
+          ok(false, `unused ${scriptType} allowlist entry: ${script}`);
+        }
+      }
+    }
+    let denylist = startupPhases[phase].denylist || null;
+    if (denylist) {
+      for (let scriptType in denylist) {
+        for (let file of denylist[scriptType]) {
+          let loaded = loadedList[scriptType].includes(file);
+          let message = `${file} is not allowed ${phase}`;
+          if (!loaded) {
+            ok(true, message);
+          } else {
+            record(false, message, undefined, getStack(scriptType, file));
+          }
+        }
+      }
+
+      if (denylist.modules) {
+        let results = await PerfTestHelpers.throttledMapPromises(
+          denylist.modules,
+          async uri => ({
+            uri,
+            exists: await PerfTestHelpers.checkURIExists(uri),
+          })
+        );
+
+        for (let { uri, exists } of results) {
+          ok(exists, `denylist entry ${uri} for phase "${phase}" must exist`);
+        }
+      }
+
+      if (denylist.services) {
+        for (let contract of denylist.services) {
+          ok(
+            contract in Cc,
+            `denylist entry ${contract} for phase "${phase}" must exist`
+          );
+        }
+      }
+    }
+  }
+});
diff --git a/browser/base/content/test/performance/browser_startup_content.js b/browser/base/content/test/performance/browser_startup_content.js
new file mode 100644
index 0000000000..b0f861e47f
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_content.js
@@ -0,0 +1,186 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* This test records which services, frame scripts, process scripts, and
+ * JS modules are loaded when creating a new content process.
+ *
+ * If you made changes that cause this test to fail, it's likely because you
+ * are loading more JS code during content process startup. Please try to
+ * avoid this.
+ *
+ * If your code isn't strictly required to show a page, consider loading it
+ * lazily. If you can't, consider delaying its load until after we have started
+ * handling user events.
+ */
+
+"use strict";
+
+/* Set this to true only for debugging purpose; it makes the output noisy. */
+const kDumpAllStacks = false;
+
+const known_scripts = {
+  modules: new Set([
+    "chrome://mochikit/content/ShutdownLeaksCollector.sys.mjs",
+
+    // General utilities
+    "resource://gre/modules/AppConstants.sys.mjs",
+    "resource://gre/modules/Timer.sys.mjs",
+    "resource://gre/modules/XPCOMUtils.sys.mjs",
+
+    // Logging related
+    // eslint-disable-next-line mozilla/use-console-createInstance
+    "resource://gre/modules/Log.sys.mjs",
+
+    // Browser front-end
+    "resource:///actors/AboutReaderChild.sys.mjs",
+    "resource:///actors/InteractionsChild.sys.mjs",
+    "resource:///actors/LinkHandlerChild.sys.mjs",
+    "resource:///actors/SearchSERPTelemetryChild.sys.mjs",
+    "resource://gre/actors/ContentMetaChild.sys.mjs",
+    "resource://gre/modules/Readerable.sys.mjs",
+
+    // Telemetry
+    "resource://gre/modules/TelemetryControllerBase.sys.mjs", // bug 1470339
+    "resource://gre/modules/TelemetryControllerContent.sys.mjs", // bug 1470339
+
+    // Extensions
+    "resource://gre/modules/ExtensionProcessScript.sys.mjs",
+    "resource://gre/modules/ExtensionUtils.sys.mjs",
+  ]),
+  frameScripts: new Set([
+    // Test related
+    "chrome://mochikit/content/shutdown-leaks-collector.js",
+  ]),
+  processScripts: new Set([
+    "chrome://global/content/process-content.js",
+    "resource://gre/modules/extensionProcessScriptLoader.js",
+  ]),
+};
+
+if (!Services.appinfo.sessionHistoryInParent) {
+  known_scripts.modules.add(
+    "resource:///modules/sessionstore/ContentSessionStore.sys.mjs"
+  );
+}
+
+// Items on this list *might* load when creating the process, as opposed to
+// items in the main list, which we expect will always load.
+const intermittently_loaded_scripts = {
+  modules: new Set([
+    "resource://gre/modules/nsAsyncShutdown.sys.mjs",
+    "resource://gre/modules/sessionstore/Utils.sys.mjs",
+
+    // Translations code which may be preffed on.
+    "resource://gre/actors/TranslationsChild.sys.mjs",
+    "resource://gre/modules/translation/LanguageDetector.sys.mjs",
+    "resource://gre/modules/ConsoleAPIStorage.sys.mjs", // Logging related.
+
+    // Session store.
+    "resource://gre/modules/sessionstore/SessionHistory.sys.mjs",
+
+    // Cookie banner handling.
+    "resource://gre/actors/CookieBannerChild.sys.mjs",
+    "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+
+    // Test related
+    "chrome://remote/content/marionette/actors/MarionetteEventsChild.sys.mjs",
+    "chrome://remote/content/shared/Log.sys.mjs",
+    "resource://testing-common/BrowserTestUtilsChild.sys.mjs",
+    "resource://testing-common/ContentEventListenerChild.sys.mjs",
+    "resource://specialpowers/AppTestDelegateChild.sys.mjs",
+    "resource://testing-common/SpecialPowersChild.sys.mjs",
+    "resource://testing-common/WrapPrivileged.sys.mjs",
+  ]),
+  frameScripts: new Set([]),
+  processScripts: new Set([]),
+};
+
+const forbiddenScripts = {
+  services: new Set([
+    "@mozilla.org/base/telemetry-startup;1",
+    "@mozilla.org/embedcomp/default-tooltiptextprovider;1",
+    "@mozilla.org/push/Service;1",
+  ]),
+};
+
+add_task(async function () {
+  SimpleTest.requestCompleteLog();
+
+  let tab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    url:
+      getRootDirectory(gTestPath).replace(
+        "chrome://mochitests/content",
+        "https://example.com"
+      ) + "file_empty.html",
+    forceNewProcess: true,
+  });
+
+  let mm = gBrowser.selectedBrowser.messageManager;
+  let promise = BrowserTestUtils.waitForMessage(mm, "Test:LoadedScripts");
+
+  // Load a custom frame script to avoid using ContentTask which loads Task.jsm
+  mm.loadFrameScript(
+    "data:text/javascript,(" +
+      function () {
+        /* eslint-env mozilla/frame-script */
+        const Cm = Components.manager;
+        Cm.QueryInterface(Ci.nsIServiceManager);
+        const { AppConstants } = ChromeUtils.importESModule(
+          "resource://gre/modules/AppConstants.sys.mjs"
+        );
+        let collectStacks = AppConstants.NIGHTLY_BUILD || AppConstants.DEBUG;
+        let modules = {};
+        for (let module of Cu.loadedJSModules) {
+          modules[module] = collectStacks
+            ? Cu.getModuleImportStack(module)
+            : "";
+        }
+        for (let module of Cu.loadedESModules) {
+          modules[module] = collectStacks
+            ? Cu.getModuleImportStack(module)
+            : "";
+        }
+        let services = {};
+        for (let contractID of Object.keys(Cc)) {
+          try {
+            if (
+              Cm.isServiceInstantiatedByContractID(contractID, Ci.nsISupports)
+            ) {
+              services[contractID] = "";
+            }
+          } catch (e) {}
+        }
+        sendAsyncMessage("Test:LoadedScripts", {
+          modules,
+          services,
+        });
+      } +
+      ")()",
+    false
+  );
+
+  let loadedInfo = await promise;
+
+  // Gather loaded frame scripts.
+  loadedInfo.frameScripts = {};
+  for (let [uri] of Services.mm.getDelayedFrameScripts()) {
+    loadedInfo.frameScripts[uri] = "";
+  }
+
+  // Gather loaded process scripts.
+  loadedInfo.processScripts = {};
+  for (let [uri] of Services.ppmm.getDelayedProcessScripts()) {
+    loadedInfo.processScripts[uri] = "";
+  }
+
+  await checkLoadedScripts({
+    loadedInfo,
+    known: known_scripts,
+    intermittent: intermittently_loaded_scripts,
+    forbidden: forbiddenScripts,
+    dumpAllStacks: kDumpAllStacks,
+  });
+
+  BrowserTestUtils.removeTab(tab);
+});
diff --git a/browser/base/content/test/performance/browser_startup_content_mainthreadio.js b/browser/base/content/test/performance/browser_startup_content_mainthreadio.js
new file mode 100644
index 0000000000..e60b95bb3c
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_content_mainthreadio.js
@@ -0,0 +1,465 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* This test records I/O syscalls done on the main thread during startup.
+ *
+ * To run this test similar to try server, you need to run:
+ *   ./mach package
+ *   ./mach test --appname=dist <path to test>
+ *
+ * If you made changes that cause this test to fail, it's likely because you
+ * are touching more files or directories during startup.
+ * Most code has no reason to use main thread I/O.
+ * If for some reason accessing the file system on the main thread is currently
+ * unavoidable, consider defering the I/O as long as you can, ideally after
+ * the end of startup.
+ */
+
+"use strict";
+
+/* Set this to true only for debugging purpose; it makes the output noisy. */
+const kDumpAllStacks = false;
+
+// Shortcuts for conditions.
+const LINUX = AppConstants.platform == "linux";
+const WIN = AppConstants.platform == "win";
+const MAC = AppConstants.platform == "macosx";
+const FORK_SERVER = Services.prefs.getBoolPref(
+  "dom.ipc.forkserver.enable",
+  false
+);
+
+/* This is an object mapping string process types to lists of known cases
+ * of IO happening on the main thread. Ideally, IO should not be on the main
+ * thread, and should happen as late as possible (see above).
+ *
+ * Paths in the entries in these lists can:
+ *  - be a full path, eg. "/etc/mime.types"
+ *  - have a prefix which will be resolved using Services.dirsvc
+ *    eg. "GreD:omni.ja"
+ *    It's possible to have only a prefix, in thise case the directory will
+ *    still be resolved, eg. "UAppData:"
+ *  - use * at the begining and/or end as a wildcard
+ * The folder separator is '/' even for Windows paths, where it'll be
+ * automatically converted to '\'.
+ *
+ * Specifying 'ignoreIfUnused: true' will make the test ignore unused entries;
+ * without this the test is strict and will fail if the described IO does not
+ * happen.
+ *
+ * Each entry specifies the maximum number of times an operation is expected to
+ * occur.
+ * The operations currently reported by the I/O interposer are:
+ *   create/open: only supported on Windows currently. The test currently
+ *     ignores these markers to have a shorter initial list of IO operations.
+ *     Adding Unix support is bug 1533779.
+ *   stat: supported on all platforms when checking the last modified date or
+ *     file size. Supported only on Windows when checking if a file exists;
+ *     fixing this inconsistency is bug 1536109.
+ *   read: supported on all platforms, but unix platforms will only report read
+ *     calls going through NSPR.
+ *   write: supported on all platforms, but Linux will only report write calls
+ *     going through NSPR.
+ *   close: supported only on Unix, and only for close calls going through NSPR.
+ *     Adding Windows support is bug 1524574.
+ *   fsync: supported only on Windows.
+ *
+ * If an entry specifies more than one operation, if at least one of them is
+ * encountered, the test won't report a failure for the entry if other
+ * operations are not encountered. This helps when listing cases where the
+ * reported operations aren't the same on all platforms due to the I/O
+ * interposer inconsistencies across platforms documented above.
+ */
+const processes = {
+  "Web Content": [
+    {
+      path: "GreD:omni.ja",
+      // Visible on Windows with an open marker.
+      // The fork server preloads the omnijars.
+      condition: !WIN && !FORK_SERVER,
+      stat: 1,
+    },
+    {
+      // bug 1376994
+      path: "XCurProcD:omni.ja",
+      // Visible on Windows with an open marker.
+      // The fork server preloads the omnijars.
+      condition: !WIN && !FORK_SERVER,
+      stat: 1,
+    },
+    {
+      // Exists call in ScopedXREEmbed::SetAppDir
+      path: "XCurProcD:",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1357205
+      path: "XREAppFeat:formautofill@mozilla.org.xpi",
+      condition: !WIN,
+      ignoreIfUnused: true,
+      stat: 1,
+    },
+    {
+      path: "*ShaderCache*", // Bug 1660480 - seen on hardware
+      condition: WIN,
+      ignoreIfUnused: true,
+      stat: 3,
+    },
+  ],
+  "Privileged Content": [
+    {
+      path: "GreD:omni.ja",
+      // Visible on Windows with an open marker.
+      // The fork server preloads the omnijars.
+      condition: !WIN && !FORK_SERVER,
+      stat: 1,
+    },
+    {
+      // bug 1376994
+      path: "XCurProcD:omni.ja",
+      // Visible on Windows with an open marker.
+      // The fork server preloads the omnijars.
+      condition: !WIN && !FORK_SERVER,
+      stat: 1,
+    },
+    {
+      // Exists call in ScopedXREEmbed::SetAppDir
+      path: "XCurProcD:",
+      condition: WIN,
+      stat: 1,
+    },
+  ],
+  WebExtensions: [
+    {
+      path: "GreD:omni.ja",
+      // Visible on Windows with an open marker.
+      // The fork server preloads the omnijars.
+      condition: !WIN && !FORK_SERVER,
+      stat: 1,
+    },
+    {
+      // bug 1376994
+      path: "XCurProcD:omni.ja",
+      // Visible on Windows with an open marker.
+      // The fork server preloads the omnijars.
+      condition: !WIN && !FORK_SERVER,
+      stat: 1,
+    },
+    {
+      // Exists call in ScopedXREEmbed::SetAppDir
+      path: "XCurProcD:",
+      condition: WIN,
+      stat: 1,
+    },
+  ],
+};
+
+function expandPathWithDirServiceKey(path) {
+  if (path.includes(":")) {
+    let [prefix, suffix] = path.split(":");
+    let [key, property] = prefix.split(".");
+    let dir = Services.dirsvc.get(key, Ci.nsIFile);
+    if (property) {
+      dir = dir[property];
+    }
+
+    // Resolve symLinks.
+    let dirPath = dir.path;
+    while (dir && !dir.isSymlink()) {
+      dir = dir.parent;
+    }
+    if (dir) {
+      dirPath = dirPath.replace(dir.path, dir.target);
+    }
+
+    path = dirPath;
+
+    if (suffix) {
+      path += "/" + suffix;
+    }
+  }
+  if (AppConstants.platform == "win") {
+    path = path.replace(/\//g, "\\");
+  }
+  return path;
+}
+
+function getStackFromProfile(profile, stack) {
+  const stackPrefixCol = profile.stackTable.schema.prefix;
+  const stackFrameCol = profile.stackTable.schema.frame;
+  const frameLocationCol = profile.frameTable.schema.location;
+
+  let result = [];
+  while (stack) {
+    let sp = profile.stackTable.data[stack];
+    let frame = profile.frameTable.data[sp[stackFrameCol]];
+    stack = sp[stackPrefixCol];
+    frame = profile.stringTable[frame[frameLocationCol]];
+    if (frame != "js::RunScript" && !frame.startsWith("next (self-hosted:")) {
+      result.push(frame);
+    }
+  }
+  return result;
+}
+
+function getIOMarkersFromProfile(profile) {
+  const nameCol = profile.markers.schema.name;
+  const dataCol = profile.markers.schema.data;
+
+  let markers = [];
+  for (let m of profile.markers.data) {
+    let markerName = profile.stringTable[m[nameCol]];
+
+    if (markerName != "FileIO") {
+      continue;
+    }
+
+    let markerData = m[dataCol];
+    if (markerData.source == "sqlite-mainthread") {
+      continue;
+    }
+
+    let samples = markerData.stack.samples;
+    let stack = samples.data[0][samples.schema.stack];
+    markers.push({
+      operation: markerData.operation,
+      filename: markerData.filename,
+      source: markerData.source,
+      stackId: stack,
+    });
+  }
+
+  return markers;
+}
+
+function pathMatches(path, filename) {
+  path = path.toLowerCase();
+  return (
+    path == filename || // Full match
+    // Wildcard on both sides of the path
+    (path.startsWith("*") &&
+      path.endsWith("*") &&
+      filename.includes(path.slice(1, -1))) ||
+    // Wildcard suffix
+    (path.endsWith("*") && filename.startsWith(path.slice(0, -1))) ||
+    // Wildcard prefix
+    (path.startsWith("*") && filename.endsWith(path.slice(1)))
+  );
+}
+
+add_task(async function () {
+  if (
+    !AppConstants.NIGHTLY_BUILD &&
+    !AppConstants.MOZ_DEV_EDITION &&
+    !AppConstants.DEBUG
+  ) {
+    ok(
+      !("@mozilla.org/test/startuprecorder;1" in Cc),
+      "the startup recorder component shouldn't exist in this non-nightly/non-devedition/" +
+        "non-debug build."
+    );
+    return;
+  }
+
+  TestUtils.assertPackagedBuild();
+
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  for (let process in processes) {
+    processes[process] = processes[process].filter(
+      entry => !("condition" in entry) || entry.condition
+    );
+    processes[process].forEach(entry => {
+      entry.listedPath = entry.path;
+      entry.path = expandPathWithDirServiceKey(entry.path);
+    });
+  }
+
+  let tmpPath = expandPathWithDirServiceKey("TmpD:").toLowerCase();
+  let shouldPass = true;
+  for (let procName in processes) {
+    let knownIOList = processes[procName];
+    info(
+      `known main thread IO paths for ${procName} process:\n` +
+        knownIOList
+          .map(e => {
+            let operations = Object.keys(e)
+              .filter(k => !["path", "condition"].includes(k))
+              .map(k => `${k}: ${e[k]}`);
+            return `  ${e.path} - ${operations.join(", ")}`;
+          })
+          .join("\n")
+    );
+
+    let profile;
+    for (let process of startupRecorder.data.profile.processes) {
+      if (process.threads[0].processName == procName) {
+        profile = process.threads[0];
+        break;
+      }
+    }
+    if (procName == "Privileged Content" && !profile) {
+      // The Privileged Content is started from an idle task that may not have
+      // been executed yet at the time we captured the startup profile in
+      // startupRecorder.
+      todo(false, `profile for ${procName} process not found`);
+    } else {
+      ok(profile, `Found profile for ${procName} process`);
+    }
+    if (!profile) {
+      continue;
+    }
+
+    let markers = getIOMarkersFromProfile(profile);
+    for (let marker of markers) {
+      if (marker.operation == "create/open") {
+        // TODO: handle these I/O markers once they are supported on
+        // non-Windows platforms.
+        continue;
+      }
+
+      if (!marker.filename) {
+        // We are still missing the filename on some mainthreadio markers,
+        // these markers are currently useless for the purpose of this test.
+        continue;
+      }
+
+      // Convert to lower case before comparing because the OS X test machines
+      // have the 'Firefox' folder in 'Library/Application Support' created
+      // as 'firefox' for some reason.
+      let filename = marker.filename.toLowerCase();
+
+      if (!WIN && filename == "/dev/urandom") {
+        continue;
+      }
+
+      // /dev/shm is always tmpfs (a memory filesystem); this isn't
+      // really I/O any more than mmap/munmap are.
+      if (LINUX && filename.startsWith("/dev/shm/")) {
+        continue;
+      }
+
+      // "Files" from memfd_create() are similar to tmpfs but never
+      // exist in the filesystem; however, they have names which are
+      // exposed in procfs, and the I/O interposer observes when
+      // they're close()d.
+      if (LINUX && filename.startsWith("/memfd:")) {
+        continue;
+      }
+
+      // Shared memory uses temporary files on MacOS <= 10.11 to avoid
+      // a kernel security bug that will never be patched (see
+      // https://crbug.com/project-zero/1671 for details).  This can
+      // be removed when we no longer support those OS versions.
+      if (MAC && filename.startsWith(tmpPath + "/org.mozilla.ipc.")) {
+        continue;
+      }
+
+      let expected = false;
+      for (let entry of knownIOList) {
+        if (pathMatches(entry.path, filename)) {
+          entry[marker.operation] = (entry[marker.operation] || 0) - 1;
+          entry._used = true;
+          expected = true;
+          break;
+        }
+      }
+      if (!expected) {
+        record(
+          false,
+          `unexpected ${marker.operation} on ${marker.filename} in ${procName} process`,
+          undefined,
+          "  " + getStackFromProfile(profile, marker.stackId).join("\n  ")
+        );
+        shouldPass = false;
+      }
+      info(`(${marker.source}) ${marker.operation} - ${marker.filename}`);
+      if (kDumpAllStacks) {
+        info(
+          getStackFromProfile(profile, marker.stackId)
+            .map(f => "  " + f)
+            .join("\n")
+        );
+      }
+    }
+
+    if (!knownIOList.length) {
+      continue;
+    }
+    // The I/O interposer is disabled if !RELEASE_OR_BETA, so we expect to have
+    // no I/O marker in that case, but it's good to keep the test running to check
+    // that we are still able to produce startup profiles.
+    is(
+      !!markers.length,
+      !AppConstants.RELEASE_OR_BETA,
+      procName +
+        " startup profiles should have IO markers in builds that are not RELEASE_OR_BETA"
+    );
+    if (!markers.length) {
+      // If a profile unexpectedly contains no I/O marker, it's better to return
+      // early to avoid having a lot of of confusing "no main thread IO when we
+      // expected some" failures.
+      continue;
+    }
+
+    for (let entry of knownIOList) {
+      for (let op in entry) {
+        if (
+          [
+            "listedPath",
+            "path",
+            "condition",
+            "ignoreIfUnused",
+            "_used",
+          ].includes(op)
+        ) {
+          continue;
+        }
+        let message = `${op} on ${entry.path} `;
+        if (entry[op] == 0) {
+          message += "as many times as expected";
+        } else if (entry[op] > 0) {
+          message += `allowed ${entry[op]} more times`;
+        } else {
+          message += `${entry[op] * -1} more times than expected`;
+        }
+        Assert.greaterOrEqual(
+          entry[op],
+          0,
+          `${message} in ${procName} process`
+        );
+      }
+      if (!("_used" in entry) && !entry.ignoreIfUnused) {
+        ok(
+          false,
+          `no main thread IO when we expected some for process ${procName}: ${entry.path} (${entry.listedPath})`
+        );
+        shouldPass = false;
+      }
+    }
+  }
+
+  if (shouldPass) {
+    ok(shouldPass, "No unexpected main thread I/O during startup");
+  } else {
+    const filename = "profile_startup_content_mainthreadio.json";
+    let path = Services.env.get("MOZ_UPLOAD_DIR");
+    let helpString;
+    if (path) {
+      let profilePath = PathUtils.join(path, filename);
+      await IOUtils.writeJSON(profilePath, startupRecorder.data.profile);
+      helpString = `open the ${filename} artifact in the Firefox Profiler to see what happened`;
+    } else {
+      helpString =
+        "set the MOZ_UPLOAD_DIR environment variable to record a profile";
+    }
+    ok(
+      false,
+      "Unexpected main thread I/O behavior during child process startup; " +
+        helpString
+    );
+  }
+});
diff --git a/browser/base/content/test/performance/browser_startup_content_subframe.js b/browser/base/content/test/performance/browser_startup_content_subframe.js
new file mode 100644
index 0000000000..3d1ee6352d
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_content_subframe.js
@@ -0,0 +1,151 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* This test records which services, JS components, frame scripts, process
+ * scripts, and JS modules are loaded when creating a new content process for a
+ * subframe.
+ *
+ * If you made changes that cause this test to fail, it's likely because you
+ * are loading more JS code during content process startup. Please try to
+ * avoid this.
+ *
+ * If your code isn't strictly required to show an iframe, consider loading it
+ * lazily. If you can't, consider delaying its load until after we have started
+ * handling user events.
+ *
+ * This test differs from browser_startup_content.js in that it tests a process
+ * with no toplevel browsers opened, but with a single subframe document
+ * loaded. This leads to a different set of scripts being loaded.
+ */
+
+"use strict";
+
+const actorModuleURI =
+  getRootDirectory(gTestPath) + "StartupContentSubframe.sys.mjs";
+const subframeURI =
+  getRootDirectory(gTestPath).replace(
+    "chrome://mochitests/content",
+    "https://example.com"
+  ) + "file_empty.html";
+
+// Set this to true only for debugging purpose; it makes the output noisy.
+const kDumpAllStacks = false;
+
+const known_scripts = {
+  modules: new Set([
+    // Loaded by this test
+    actorModuleURI,
+
+    // General utilities
+    "resource://gre/modules/AppConstants.sys.mjs",
+    "resource://gre/modules/XPCOMUtils.sys.mjs",
+
+    // Logging related
+    // eslint-disable-next-line mozilla/use-console-createInstance
+    "resource://gre/modules/Log.sys.mjs",
+
+    // Telemetry
+    "resource://gre/modules/TelemetryControllerBase.sys.mjs", // bug 1470339
+    "resource://gre/modules/TelemetryControllerContent.sys.mjs", // bug 1470339
+
+    // Extensions
+    "resource://gre/modules/ExtensionProcessScript.sys.mjs",
+    "resource://gre/modules/ExtensionUtils.sys.mjs",
+  ]),
+  processScripts: new Set([
+    "chrome://global/content/process-content.js",
+    "resource://gre/modules/extensionProcessScriptLoader.js",
+  ]),
+};
+
+// Items on this list *might* load when creating the process, as opposed to
+// items in the main list, which we expect will always load.
+const intermittently_loaded_scripts = {
+  modules: new Set([
+    "resource://gre/modules/nsAsyncShutdown.sys.mjs",
+
+    // Cookie banner handling.
+    "resource://gre/actors/CookieBannerChild.sys.mjs",
+    "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+
+    // Test related
+    "chrome://remote/content/marionette/actors/MarionetteEventsChild.sys.mjs",
+    "chrome://remote/content/shared/Log.sys.mjs",
+    "resource://testing-common/BrowserTestUtilsChild.sys.mjs",
+    "resource://testing-common/ContentEventListenerChild.sys.mjs",
+    "resource://testing-common/SpecialPowersChild.sys.mjs",
+    "resource://specialpowers/AppTestDelegateChild.sys.mjs",
+    "resource://testing-common/WrapPrivileged.sys.mjs",
+  ]),
+  processScripts: new Set([]),
+};
+
+const forbiddenScripts = {
+  services: new Set([
+    "@mozilla.org/base/telemetry-startup;1",
+    "@mozilla.org/embedcomp/default-tooltiptextprovider;1",
+    "@mozilla.org/push/Service;1",
+  ]),
+};
+
+add_task(async function () {
+  SimpleTest.requestCompleteLog();
+
+  // Increase the maximum number of webIsolated content processes to make sure
+  // our newly-created iframe is spawned into a new content process.
+  //
+  // Unfortunately, we don't have something like `forceNewProcess` for subframe
+  // loads.
+  await SpecialPowers.pushPrefEnv({
+    set: [["dom.ipc.processCount.webIsolated", 10]],
+  });
+  Services.ppmm.releaseCachedProcesses();
+
+  // Register a custom window actor which will send us a notification when the
+  // script loading information is available.
+  ChromeUtils.registerWindowActor("StartupContentSubframe", {
+    parent: {
+      esModuleURI: actorModuleURI,
+    },
+    child: {
+      esModuleURI: actorModuleURI,
+      events: {
+        load: { mozSystemGroup: true, capture: true },
+      },
+    },
+    matches: [subframeURI],
+    allFrames: true,
+  });
+
+  // Create a tab, and load a remote subframe with the specific URI in it.
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+
+  SpecialPowers.spawn(tab.linkedBrowser, [subframeURI], uri => {
+    let iframe = content.document.createElement("iframe");
+    iframe.src = uri;
+    content.document.body.appendChild(iframe);
+  });
+
+  // Wait for the reply to come in, remove the XPCOM wrapper, and unregister our actor.
+  let [subject] = await TestUtils.topicObserved(
+    "startup-content-subframe-loaded-scripts"
+  );
+  let loadedInfo = subject.wrappedJSObject;
+
+  ChromeUtils.unregisterWindowActor("StartupContentSubframe");
+  BrowserTestUtils.removeTab(tab);
+
+  // Gather loaded process scripts.
+  loadedInfo.processScripts = {};
+  for (let [uri] of Services.ppmm.getDelayedProcessScripts()) {
+    loadedInfo.processScripts[uri] = "";
+  }
+
+  await checkLoadedScripts({
+    loadedInfo,
+    known: known_scripts,
+    intermittent: intermittently_loaded_scripts,
+    forbidden: forbiddenScripts,
+    dumpAllStacks: kDumpAllStacks,
+  });
+});
diff --git a/browser/base/content/test/performance/browser_startup_flicker.js b/browser/base/content/test/performance/browser_startup_flicker.js
new file mode 100644
index 0000000000..16300e1525
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_flicker.js
@@ -0,0 +1,72 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/*
+ * This test ensures that there is no unexpected flicker
+ * on the first window opened during startup.
+ */
+
+add_task(async function () {
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  // Ensure all the frame data is in the test compartment to avoid traversing
+  // a cross compartment wrapper for each pixel.
+  let frames = Cu.cloneInto(startupRecorder.data.frames, {});
+  ok(!!frames.length, "Should have captured some frames.");
+
+  let unexpectedRects = 0;
+  let alreadyFocused = false;
+  for (let i = 1; i < frames.length; ++i) {
+    let frame = frames[i],
+      previousFrame = frames[i - 1];
+    let rects = compareFrames(frame, previousFrame);
+    if (!alreadyFocused && isLikelyFocusChange(rects, frame)) {
+      todo(
+        false,
+        "bug 1445161 - the window should be focused at first paint, " +
+          rects.toSource()
+      );
+      continue;
+    }
+    alreadyFocused = true;
+
+    rects = rects.filter(rect => {
+      let width = frame.width;
+
+      let exceptions = [
+        /**
+         * Please don't add anything new unless justified!
+         */
+      ];
+
+      let rectText = `${rect.toSource()}, window width: ${width}`;
+      for (let e of exceptions) {
+        if (e.condition(rect)) {
+          todo(false, e.name + ", " + rectText);
+          return false;
+        }
+      }
+
+      ok(false, "unexpected changed rect: " + rectText);
+      return true;
+    });
+    if (!rects.length) {
+      info("ignoring identical frame");
+      continue;
+    }
+
+    // Before dumping a frame with unexpected differences for the first time,
+    // ensure at least one previous frame has been logged so that it's possible
+    // to see the differences when examining the log.
+    if (!unexpectedRects) {
+      dumpFrame(previousFrame);
+    }
+    unexpectedRects += rects.length;
+    dumpFrame(frame);
+  }
+  is(unexpectedRects, 0, "should have 0 unknown flickering areas");
+});
diff --git a/browser/base/content/test/performance/browser_startup_hiddenwindow.js b/browser/base/content/test/performance/browser_startup_hiddenwindow.js
new file mode 100644
index 0000000000..27ff837dc4
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_hiddenwindow.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function () {
+  if (
+    !AppConstants.NIGHTLY_BUILD &&
+    !AppConstants.MOZ_DEV_EDITION &&
+    !AppConstants.DEBUG
+  ) {
+    ok(
+      !("@mozilla.org/test/startuprecorder;1" in Cc),
+      "the startup recorder component shouldn't exist in this non-nightly/non-devedition/" +
+        "non-debug build."
+    );
+    return;
+  }
+
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  let extras = Cu.cloneInto(startupRecorder.data.extras, {});
+
+  let phasesExpectations = {
+    "before profile selection": false,
+    "before opening first browser window": false,
+    "before first paint": AppConstants.platform === "macosx",
+    // Bug 1531854
+    "before handling user events": true,
+    "before becoming idle": true,
+  };
+
+  for (let phase in extras) {
+    if (!(phase in phasesExpectations)) {
+      ok(false, `Startup phase '${phase}' should be specified.`);
+      continue;
+    }
+
+    is(
+      extras[phase].hiddenWindowLoaded,
+      phasesExpectations[phase],
+      `Hidden window loaded at '${phase}': ${phasesExpectations[phase]}`
+    );
+  }
+});
diff --git a/browser/base/content/test/performance/browser_startup_images.js b/browser/base/content/test/performance/browser_startup_images.js
new file mode 100644
index 0000000000..5a27b9a8dd
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_images.js
@@ -0,0 +1,136 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * This test checks that any images we load on startup are actually used,
+ * so we don't waste IO and cycles loading images the user doesn't see.
+ * It has a list of known problematic images that we aim to reduce to
+ * empty.
+ */
+
+/* A list of images that are loaded at startup but not shown.
+ * List items support the following attributes:
+ *  - file: The location of the loaded image file.
+ *  - hidpi: An alternative hidpi file location for retina screens, if one exists.
+ *           May be the magic string <not loaded> in strange cases where
+ *           only the low-resolution image is loaded but not shown.
+ *  - platforms: An array of the platforms where the issue is occurring.
+ *               Possible values are linux, win, macosx.
+ *  - intermittentNotLoaded: an array of platforms where this image is
+ *                           intermittently not loaded, e.g. because it is
+ *                           loaded during the time we stop recording.
+ *  - intermittentShown: An array of platforms where this image is
+ *                       intermittently shown, even though the list implies
+ *                       it might not be shown.
+ *
+ * PLEASE do not add items to this list.
+ *
+ * PLEASE DO remove items from this list.
+ */
+const knownUnshownImages = [
+  {
+    file: "chrome://global/skin/icons/arrow-left.svg",
+    platforms: ["linux", "win", "macosx"],
+  },
+
+  {
+    file: "chrome://browser/skin/toolbar-drag-indicator.svg",
+    platforms: ["linux", "win", "macosx"],
+  },
+
+  {
+    file: "chrome://global/skin/icons/chevron.svg",
+    platforms: ["win", "linux", "macosx"],
+    intermittentShown: ["win", "linux"],
+  },
+
+  {
+    file: "chrome://browser/skin/window-controls/maximize.svg",
+    platforms: ["win"],
+    // This is to prevent perma-fails in case Windows machines
+    // go back to running tests in non-maximized windows.
+    intermittentShown: ["win"],
+    // This file is not loaded on Windows 7/8.
+    intermittentNotLoaded: ["win"],
+  },
+];
+
+add_task(async function () {
+  if (!AppConstants.DEBUG) {
+    ok(false, "You need to run this test on a debug build.");
+  }
+
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  let data = Cu.cloneInto(startupRecorder.data.images, {});
+  let knownImagesForPlatform = knownUnshownImages.filter(el => {
+    return el.platforms.includes(AppConstants.platform);
+  });
+
+  {
+    let results = await PerfTestHelpers.throttledMapPromises(
+      knownImagesForPlatform,
+      async image => ({
+        uri: image.file,
+        exists: await PerfTestHelpers.checkURIExists(image.file),
+      })
+    );
+    for (let { uri, exists } of results) {
+      ok(exists, `Unshown image entry ${uri} must exist`);
+    }
+  }
+
+  let loadedImages = data["image-loading"];
+  let shownImages = data["image-drawing"];
+
+  for (let loaded of loadedImages.values()) {
+    let knownImage = knownImagesForPlatform.find(el => {
+      if (window.devicePixelRatio >= 2 && el.hidpi && el.hidpi == loaded) {
+        return true;
+      }
+      return el.file == loaded;
+    });
+    if (knownImage) {
+      if (
+        !knownImage.intermittentShown ||
+        !knownImage.intermittentShown.includes(AppConstants.platform)
+      ) {
+        todo(
+          shownImages.has(loaded),
+          `Image ${loaded} should not have been shown.`
+        );
+      }
+      continue;
+    }
+    ok(
+      shownImages.has(loaded),
+      `Loaded image ${loaded} should have been shown.`
+    );
+  }
+
+  // Check for known images that are no longer used.
+  for (let item of knownImagesForPlatform) {
+    if (
+      !item.intermittentNotLoaded ||
+      !item.intermittentNotLoaded.includes(AppConstants.platform)
+    ) {
+      if (window.devicePixelRatio >= 2 && item.hidpi) {
+        if (item.hidpi != "<not loaded>") {
+          ok(
+            loadedImages.has(item.hidpi),
+            `Image ${item.hidpi} should have been loaded.`
+          );
+        }
+      } else {
+        ok(
+          loadedImages.has(item.file),
+          `Image ${item.file} should have been loaded.`
+        );
+      }
+    }
+  }
+});
diff --git a/browser/base/content/test/performance/browser_startup_mainthreadio.js b/browser/base/content/test/performance/browser_startup_mainthreadio.js
new file mode 100644
index 0000000000..b65ede26d5
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_mainthreadio.js
@@ -0,0 +1,881 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* This test records I/O syscalls done on the main thread during startup.
+ *
+ * To run this test similar to try server, you need to run:
+ *   ./mach package
+ *   ./mach test --appname=dist <path to test>
+ *
+ * If you made changes that cause this test to fail, it's likely because you
+ * are touching more files or directories during startup.
+ * Most code has no reason to use main thread I/O.
+ * If for some reason accessing the file system on the main thread is currently
+ * unavoidable, consider defering the I/O as long as you can, ideally after
+ * the end of startup.
+ * If your code isn't strictly required to show the first browser window,
+ * it shouldn't be loaded before we are done with first paint.
+ * Finally, if your code isn't really needed during startup, it should not be
+ * loaded before we have started handling user events.
+ */
+
+"use strict";
+
+/* Set this to true only for debugging purpose; it makes the output noisy. */
+const kDumpAllStacks = false;
+
+// Shortcuts for conditions.
+const LINUX = AppConstants.platform == "linux";
+const WIN = AppConstants.platform == "win";
+const MAC = AppConstants.platform == "macosx";
+
+const kSharedFontList = SpecialPowers.getBoolPref("gfx.e10s.font-list.shared");
+
+/* This is an object mapping string phases of startup to lists of known cases
+ * of IO happening on the main thread. Ideally, IO should not be on the main
+ * thread, and should happen as late as possible (see above).
+ *
+ * Paths in the entries in these lists can:
+ *  - be a full path, eg. "/etc/mime.types"
+ *  - have a prefix which will be resolved using Services.dirsvc
+ *    eg. "GreD:omni.ja"
+ *    It's possible to have only a prefix, in thise case the directory will
+ *    still be resolved, eg. "UAppData:"
+ *  - use * at the begining and/or end as a wildcard
+ * The folder separator is '/' even for Windows paths, where it'll be
+ * automatically converted to '\'.
+ *
+ * Specifying 'ignoreIfUnused: true' will make the test ignore unused entries;
+ * without this the test is strict and will fail if the described IO does not
+ * happen.
+ *
+ * Each entry specifies the maximum number of times an operation is expected to
+ * occur.
+ * The operations currently reported by the I/O interposer are:
+ *   create/open: only supported on Windows currently. The test currently
+ *     ignores these markers to have a shorter initial list of IO operations.
+ *     Adding Unix support is bug 1533779.
+ *   stat: supported on all platforms when checking the last modified date or
+ *     file size. Supported only on Windows when checking if a file exists;
+ *     fixing this inconsistency is bug 1536109.
+ *   read: supported on all platforms, but unix platforms will only report read
+ *     calls going through NSPR.
+ *   write: supported on all platforms, but Linux will only report write calls
+ *     going through NSPR.
+ *   close: supported only on Unix, and only for close calls going through NSPR.
+ *     Adding Windows support is bug 1524574.
+ *   fsync: supported only on Windows.
+ *
+ * If an entry specifies more than one operation, if at least one of them is
+ * encountered, the test won't report a failure for the entry if other
+ * operations are not encountered. This helps when listing cases where the
+ * reported operations aren't the same on all platforms due to the I/O
+ * interposer inconsistencies across platforms documented above.
+ */
+const startupPhases = {
+  // Anything done before or during app-startup must have a compelling reason
+  // to run before we have even selected the user profile.
+  "before profile selection": [
+    {
+      // bug 1541200
+      path: "UAppData:Crash Reports/InstallTime20*",
+      condition: AppConstants.MOZ_CRASHREPORTER,
+      stat: 1, // only caught on Windows.
+      read: 1,
+      write: 2,
+      close: 1,
+    },
+    {
+      // bug 1541200
+      path: "UAppData:Crash Reports/LastCrash",
+      condition: WIN && AppConstants.MOZ_CRASHREPORTER,
+      stat: 1, // only caught on Windows.
+      read: 1,
+    },
+    {
+      // bug 1541200
+      path: "UAppData:Crash Reports/LastCrash",
+      condition: !WIN && AppConstants.MOZ_CRASHREPORTER,
+      ignoreIfUnused: true, // only if we ever crashed on this machine
+      read: 1,
+      close: 1,
+    },
+    {
+      // At least the read seems unavoidable for a regular startup.
+      path: "UAppData:profiles.ini",
+      ignoreIfUnused: true,
+      condition: MAC,
+      stat: 1,
+      read: 1,
+      close: 1,
+    },
+    {
+      // At least the read seems unavoidable for a regular startup.
+      path: "UAppData:profiles.ini",
+      condition: WIN,
+      ignoreIfUnused: true, // only if a real profile exists on the system.
+      read: 1,
+      stat: 1,
+    },
+    {
+      // bug 1541226, bug 1363586, bug 1541593
+      path: "ProfD:",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      path: "ProfLD:.startup-incomplete",
+      condition: !WIN, // Visible on Windows with an open marker
+      close: 1,
+    },
+    {
+      // bug 1541491 to stop using this file, bug 1541494 to write correctly.
+      path: "ProfLD:compatibility.ini",
+      write: 18,
+      close: 1,
+    },
+    {
+      path: "GreD:omni.ja",
+      condition: !WIN, // Visible on Windows with an open marker
+      stat: 1,
+    },
+    {
+      // bug 1376994
+      path: "XCurProcD:omni.ja",
+      condition: !WIN, // Visible on Windows with an open marker
+      stat: 1,
+    },
+    {
+      path: "ProfD:parent.lock",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1541603
+      path: "ProfD:minidumps",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1543746
+      path: "XCurProcD:defaults/preferences",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1544034
+      path: "ProfLDS:startupCache/scriptCache-child-current.bin",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1544034
+      path: "ProfLDS:startupCache/scriptCache-child.bin",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1544034
+      path: "ProfLDS:startupCache/scriptCache-current.bin",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1544034
+      path: "ProfLDS:startupCache/scriptCache.bin",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1541601
+      path: "PrfDef:channel-prefs.js",
+      stat: 1,
+      read: 1,
+      close: 1,
+    },
+    {
+      // At least the read seems unavoidable
+      path: "PrefD:prefs.js",
+      stat: 1,
+      read: 1,
+      close: 1,
+    },
+    {
+      // bug 1543752
+      path: "PrefD:user.js",
+      stat: 1,
+      read: 1,
+      close: 1,
+    },
+  ],
+
+  "before opening first browser window": [
+    {
+      // bug 1541226
+      path: "ProfD:",
+      condition: WIN,
+      ignoreIfUnused: true, // Sometimes happens in the next phase
+      stat: 1,
+    },
+    {
+      // bug 1534745
+      path: "ProfD:cookies.sqlite-journal",
+      condition: !LINUX,
+      ignoreIfUnused: true, // Sometimes happens in the next phase
+      stat: 3,
+      write: 4,
+    },
+    {
+      // bug 1534745
+      path: "ProfD:cookies.sqlite",
+      condition: !LINUX,
+      ignoreIfUnused: true, // Sometimes happens in the next phase
+      stat: 2,
+      read: 3,
+      write: 1,
+    },
+    {
+      // bug 1534745
+      path: "ProfD:cookies.sqlite-wal",
+      ignoreIfUnused: true, // Sometimes happens in the next phase
+      condition: WIN,
+      stat: 2,
+    },
+    {
+      // Seems done by OS X and outside of our control.
+      path: "*.savedState/restorecount.plist",
+      condition: MAC,
+      ignoreIfUnused: true,
+      write: 1,
+    },
+    {
+      // Side-effect of bug 1412090, via sandboxing (but the real
+      // problem there is main-thread CPU use; see bug 1439412)
+      path: "*ld.so.conf*",
+      condition: LINUX && !AppConstants.MOZ_CODE_COVERAGE && !kSharedFontList,
+      read: 22,
+      close: 11,
+    },
+    {
+      // bug 1541246
+      path: "ProfD:extensions",
+      ignoreIfUnused: true, // bug 1649590
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1541246
+      path: "UAppData:",
+      ignoreIfUnused: true, // sometimes before opening first browser window,
+      // sometimes before first paint
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1833104 has context - this is artifact-only so doesn't affect
+      // any real users, will just show up for developer builds and
+      // artifact trypushes so we include it here.
+      path: "GreD:jogfile.json",
+      condition:
+        WIN && Services.prefs.getBoolPref("telemetry.fog.artifact_build"),
+      stat: 1,
+    },
+  ],
+
+  // We reach this phase right after showing the first browser window.
+  // This means that any I/O at this point delayed first paint.
+  "before first paint": [
+    {
+      // bug 1545119
+      path: "OldUpdRootD:",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // bug 1446012
+      path: "UpdRootD:updates/0/update.status",
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      path: "XREAppFeat:formautofill@mozilla.org.xpi",
+      condition: !WIN,
+      stat: 1,
+      close: 1,
+    },
+    {
+      path: "XREAppFeat:webcompat@mozilla.org.xpi",
+      condition: LINUX,
+      ignoreIfUnused: true, // Sometimes happens in the previous phase
+      close: 1,
+    },
+    {
+      // We only hit this for new profiles.
+      path: "XREAppDist:distribution.ini",
+      // check we're not msix to disambiguate from the next entry...
+      condition: WIN && !Services.sysinfo.getProperty("hasWinPackageId"),
+      stat: 1,
+    },
+    {
+      // On MSIX, we actually read this file - bug 1833341.
+      path: "XREAppDist:distribution.ini",
+      condition: WIN && Services.sysinfo.getProperty("hasWinPackageId"),
+      stat: 1,
+      read: 1,
+    },
+    {
+      // bug 1545139
+      path: "*Fonts/StaticCache.dat",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win7
+      read: 1,
+    },
+    {
+      // Bug 1626738
+      path: "SysD:spool/drivers/color/*",
+      condition: WIN,
+      read: 1,
+    },
+    {
+      // Sandbox policy construction
+      path: "*ld.so.conf*",
+      condition: LINUX && !AppConstants.MOZ_CODE_COVERAGE,
+      read: 22,
+      close: 11,
+    },
+    {
+      // bug 1541246
+      path: "UAppData:",
+      ignoreIfUnused: true, // sometimes before opening first browser window,
+      // sometimes before first paint
+      condition: WIN,
+      stat: 1,
+    },
+    {
+      // Not in packaged builds; useful for artifact builds.
+      path: "GreD:ScalarArtifactDefinitions.json",
+      condition: WIN && !AppConstants.MOZILLA_OFFICIAL,
+      stat: 1,
+    },
+    {
+      // Not in packaged builds; useful for artifact builds.
+      path: "GreD:EventArtifactDefinitions.json",
+      condition: WIN && !AppConstants.MOZILLA_OFFICIAL,
+      stat: 1,
+    },
+    {
+      // bug 1541226
+      path: "ProfD:",
+      condition: WIN,
+      ignoreIfUnused: true, // Usually happens in the previous phase
+      stat: 1,
+    },
+    {
+      // bug 1534745
+      path: "ProfD:cookies.sqlite-journal",
+      condition: WIN,
+      ignoreIfUnused: true, // Usually happens in the previous phase
+      stat: 3,
+      write: 4,
+    },
+    {
+      // bug 1534745
+      path: "ProfD:cookies.sqlite",
+      condition: WIN,
+      ignoreIfUnused: true, // Usually happens in the previous phase
+      stat: 2,
+      read: 3,
+      write: 1,
+    },
+    {
+      // bug 1534745
+      path: "ProfD:cookies.sqlite-wal",
+      condition: WIN,
+      ignoreIfUnused: true, // Usually happens in the previous phase
+      stat: 2,
+    },
+  ],
+
+  // We are at this phase once we are ready to handle user events.
+  // Any IO at this phase or before gets in the way of the user
+  // interacting with the first browser window.
+  "before handling user events": [
+    {
+      path: "GreD:update.test",
+      ignoreIfUnused: true,
+      condition: LINUX,
+      close: 1,
+    },
+    {
+      path: "XREAppFeat:webcompat-reporter@mozilla.org.xpi",
+      condition: !WIN,
+      ignoreIfUnused: true,
+      stat: 1,
+      close: 1,
+    },
+    {
+      // Bug 1660582 - access while running on windows10 hardware.
+      path: "ProfD:wmfvpxvideo.guard",
+      condition: WIN,
+      ignoreIfUnused: true,
+      stat: 1,
+      close: 1,
+    },
+    {
+      // Bug 1649590
+      path: "ProfD:extensions",
+      ignoreIfUnused: true,
+      condition: WIN,
+      stat: 1,
+    },
+  ],
+
+  // Things that are expected to be completely out of the startup path
+  // and loaded lazily when used for the first time by the user should
+  // be listed here.
+  "before becoming idle": [
+    {
+      // bug 1370516 - NSS should be initialized off main thread.
+      path: `ProfD:cert9.db`,
+      condition: WIN,
+      read: 5,
+      stat: AppConstants.NIGHTLY_BUILD ? 5 : 4,
+    },
+    {
+      // bug 1370516 - NSS should be initialized off main thread.
+      path: `ProfD:cert9.db-journal`,
+      condition: WIN,
+      stat: 3,
+    },
+    {
+      // bug 1370516 - NSS should be initialized off main thread.
+      path: `ProfD:cert9.db-wal`,
+      condition: WIN,
+      stat: 3,
+    },
+    {
+      // bug 1370516 - NSS should be initialized off main thread.
+      path: "ProfD:pkcs11.txt",
+      condition: WIN,
+      read: 2,
+    },
+    {
+      // bug 1370516 - NSS should be initialized off main thread.
+      path: `ProfD:key4.db`,
+      condition: WIN,
+      read: 10,
+      stat: AppConstants.NIGHTLY_BUILD ? 5 : 4,
+    },
+    {
+      // bug 1370516 - NSS should be initialized off main thread.
+      path: `ProfD:key4.db-journal`,
+      condition: WIN,
+      stat: 7,
+    },
+    {
+      // bug 1370516 - NSS should be initialized off main thread.
+      path: `ProfD:key4.db-wal`,
+      condition: WIN,
+      stat: 7,
+    },
+    {
+      path: "XREAppFeat:screenshots@mozilla.org.xpi",
+      ignoreIfUnused: true,
+      close: 1,
+    },
+    {
+      path: "XREAppFeat:webcompat-reporter@mozilla.org.xpi",
+      ignoreIfUnused: true,
+      stat: 1,
+      close: 1,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:places.sqlite-journal",
+      ignoreIfUnused: true,
+      fsync: 1,
+      stat: 4,
+      read: 1,
+      write: 2,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:places.sqlite-wal",
+      ignoreIfUnused: true,
+      stat: 4,
+      fsync: 3,
+      read: 51,
+      write: 178,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:places.sqlite-shm",
+      condition: WIN,
+      ignoreIfUnused: true,
+      stat: 1,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:places.sqlite",
+      ignoreIfUnused: true,
+      fsync: 2,
+      read: 4,
+      stat: 3,
+      write: 1324,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:favicons.sqlite-journal",
+      ignoreIfUnused: true,
+      fsync: 2,
+      stat: 7,
+      read: 2,
+      write: 7,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:favicons.sqlite-wal",
+      ignoreIfUnused: true,
+      fsync: 2,
+      stat: 7,
+      read: 7,
+      write: 15,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:favicons.sqlite-shm",
+      condition: WIN,
+      ignoreIfUnused: true,
+      stat: 2,
+    },
+    {
+      // bug 1391590
+      path: "ProfD:favicons.sqlite",
+      ignoreIfUnused: true,
+      fsync: 3,
+      read: 8,
+      stat: 4,
+      write: 1300,
+    },
+    {
+      path: "ProfD:",
+      condition: WIN,
+      ignoreIfUnused: true,
+      stat: 3,
+    },
+  ],
+};
+
+for (let name of ["d3d11layers", "glcontext", "wmfvpxvideo"]) {
+  startupPhases["before first paint"].push({
+    path: `ProfD:${name}.guard`,
+    ignoreIfUnused: true,
+    stat: 1,
+  });
+}
+
+function expandPathWithDirServiceKey(path) {
+  if (path.includes(":")) {
+    let [prefix, suffix] = path.split(":");
+    let [key, property] = prefix.split(".");
+    let dir = Services.dirsvc.get(key, Ci.nsIFile);
+    if (property) {
+      dir = dir[property];
+    }
+
+    // Resolve symLinks.
+    let dirPath = dir.path;
+    while (dir && !dir.isSymlink()) {
+      dir = dir.parent;
+    }
+    if (dir) {
+      dirPath = dirPath.replace(dir.path, dir.target);
+    }
+
+    path = dirPath;
+
+    if (suffix) {
+      path += "/" + suffix;
+    }
+  }
+  if (AppConstants.platform == "win") {
+    path = path.replace(/\//g, "\\");
+  }
+  return path;
+}
+
+function getStackFromProfile(profile, stack) {
+  const stackPrefixCol = profile.stackTable.schema.prefix;
+  const stackFrameCol = profile.stackTable.schema.frame;
+  const frameLocationCol = profile.frameTable.schema.location;
+
+  let result = [];
+  while (stack) {
+    let sp = profile.stackTable.data[stack];
+    let frame = profile.frameTable.data[sp[stackFrameCol]];
+    stack = sp[stackPrefixCol];
+    frame = profile.stringTable[frame[frameLocationCol]];
+    if (frame != "js::RunScript" && !frame.startsWith("next (self-hosted:")) {
+      result.push(frame);
+    }
+  }
+  return result;
+}
+
+function pathMatches(path, filename) {
+  path = path.toLowerCase();
+  return (
+    path == filename || // Full match
+    // Wildcard on both sides of the path
+    (path.startsWith("*") &&
+      path.endsWith("*") &&
+      filename.includes(path.slice(1, -1))) ||
+    // Wildcard suffix
+    (path.endsWith("*") && filename.startsWith(path.slice(0, -1))) ||
+    // Wildcard prefix
+    (path.startsWith("*") && filename.endsWith(path.slice(1)))
+  );
+}
+
+add_task(async function () {
+  if (
+    !AppConstants.NIGHTLY_BUILD &&
+    !AppConstants.MOZ_DEV_EDITION &&
+    !AppConstants.DEBUG
+  ) {
+    ok(
+      !("@mozilla.org/test/startuprecorder;1" in Cc),
+      "the startup recorder component shouldn't exist in this non-nightly/non-devedition/" +
+        "non-debug build."
+    );
+    return;
+  }
+
+  TestUtils.assertPackagedBuild();
+
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  // Add system add-ons to the list of known IO dynamically.
+  // They should go in the omni.ja file (bug 1357205).
+  {
+    let addons = await AddonManager.getAddonsByTypes(["extension"]);
+    for (let addon of addons) {
+      if (addon.isSystem) {
+        startupPhases["before opening first browser window"].push({
+          path: `XREAppFeat:${addon.id}.xpi`,
+          stat: 3,
+          close: 2,
+        });
+        startupPhases["before handling user events"].push({
+          path: `XREAppFeat:${addon.id}.xpi`,
+          condition: WIN,
+          stat: 2,
+        });
+      }
+    }
+  }
+
+  // Check for main thread I/O markers in the startup profile.
+  let profile = startupRecorder.data.profile.threads[0];
+
+  let phases = {};
+  {
+    const nameCol = profile.markers.schema.name;
+    const dataCol = profile.markers.schema.data;
+
+    let markersForCurrentPhase = [];
+    let foundIOMarkers = false;
+
+    for (let m of profile.markers.data) {
+      let markerName = profile.stringTable[m[nameCol]];
+      if (markerName.startsWith("startupRecorder:")) {
+        phases[markerName.split("startupRecorder:")[1]] =
+          markersForCurrentPhase;
+        markersForCurrentPhase = [];
+        continue;
+      }
+
+      if (markerName != "FileIO") {
+        continue;
+      }
+
+      let markerData = m[dataCol];
+      if (markerData.source == "sqlite-mainthread") {
+        continue;
+      }
+
+      let samples = markerData.stack.samples;
+      let stack = samples.data[0][samples.schema.stack];
+      markersForCurrentPhase.push({
+        operation: markerData.operation,
+        filename: markerData.filename,
+        source: markerData.source,
+        stackId: stack,
+      });
+      foundIOMarkers = true;
+    }
+
+    // The I/O interposer is disabled if !RELEASE_OR_BETA, so we expect to have
+    // no I/O marker in that case, but it's good to keep the test running to check
+    // that we are still able to produce startup profiles.
+    is(
+      foundIOMarkers,
+      !AppConstants.RELEASE_OR_BETA,
+      "The IO interposer should be enabled in builds that are not RELEASE_OR_BETA"
+    );
+    if (!foundIOMarkers) {
+      // If a profile unexpectedly contains no I/O marker, it's better to return
+      // early to avoid having a lot of of confusing "no main thread IO when we
+      // expected some" failures.
+      return;
+    }
+  }
+
+  for (let phase in startupPhases) {
+    startupPhases[phase] = startupPhases[phase].filter(
+      entry => !("condition" in entry) || entry.condition
+    );
+    startupPhases[phase].forEach(entry => {
+      entry.listedPath = entry.path;
+      entry.path = expandPathWithDirServiceKey(entry.path);
+    });
+  }
+
+  let tmpPath = expandPathWithDirServiceKey("TmpD:").toLowerCase();
+  let shouldPass = true;
+  for (let phase in phases) {
+    let knownIOList = startupPhases[phase];
+    info(
+      `known main thread IO paths during ${phase}:\n` +
+        knownIOList
+          .map(e => {
+            let operations = Object.keys(e)
+              .filter(k => k != "path")
+              .map(k => `${k}: ${e[k]}`);
+            return `  ${e.path} - ${operations.join(", ")}`;
+          })
+          .join("\n")
+    );
+
+    let markers = phases[phase];
+    for (let marker of markers) {
+      if (marker.operation == "create/open") {
+        // TODO: handle these I/O markers once they are supported on
+        // non-Windows platforms.
+        continue;
+      }
+
+      if (!marker.filename) {
+        // We are still missing the filename on some mainthreadio markers,
+        // these markers are currently useless for the purpose of this test.
+        continue;
+      }
+
+      // Convert to lower case before comparing because the OS X test machines
+      // have the 'Firefox' folder in 'Library/Application Support' created
+      // as 'firefox' for some reason.
+      let filename = marker.filename.toLowerCase();
+
+      if (!WIN && filename == "/dev/urandom") {
+        continue;
+      }
+
+      // /dev/shm is always tmpfs (a memory filesystem); this isn't
+      // really I/O any more than mmap/munmap are.
+      if (LINUX && filename.startsWith("/dev/shm/")) {
+        continue;
+      }
+
+      // "Files" from memfd_create() are similar to tmpfs but never
+      // exist in the filesystem; however, they have names which are
+      // exposed in procfs, and the I/O interposer observes when
+      // they're close()d.
+      if (LINUX && filename.startsWith("/memfd:")) {
+        continue;
+      }
+
+      // Shared memory uses temporary files on MacOS <= 10.11 to avoid
+      // a kernel security bug that will never be patched (see
+      // https://crbug.com/project-zero/1671 for details).  This can
+      // be removed when we no longer support those OS versions.
+      if (MAC && filename.startsWith(tmpPath + "/org.mozilla.ipc.")) {
+        continue;
+      }
+
+      let expected = false;
+      for (let entry of knownIOList) {
+        if (pathMatches(entry.path, filename)) {
+          entry[marker.operation] = (entry[marker.operation] || 0) - 1;
+          entry._used = true;
+          expected = true;
+          break;
+        }
+      }
+      if (!expected) {
+        record(
+          false,
+          `unexpected ${marker.operation} on ${marker.filename} ${phase}`,
+          undefined,
+          "  " + getStackFromProfile(profile, marker.stackId).join("\n  ")
+        );
+        shouldPass = false;
+      }
+      info(`(${marker.source}) ${marker.operation} - ${marker.filename}`);
+      if (kDumpAllStacks) {
+        info(
+          getStackFromProfile(profile, marker.stackId)
+            .map(f => "  " + f)
+            .join("\n")
+        );
+      }
+    }
+
+    for (let entry of knownIOList) {
+      for (let op in entry) {
+        if (
+          [
+            "listedPath",
+            "path",
+            "condition",
+            "ignoreIfUnused",
+            "_used",
+          ].includes(op)
+        ) {
+          continue;
+        }
+        let message = `${op} on ${entry.path} `;
+        if (entry[op] == 0) {
+          message += "as many times as expected";
+        } else if (entry[op] > 0) {
+          message += `allowed ${entry[op]} more times`;
+        } else {
+          message += `${entry[op] * -1} more times than expected`;
+        }
+        Assert.greaterOrEqual(entry[op], 0, `${message} ${phase}`);
+      }
+      if (!("_used" in entry) && !entry.ignoreIfUnused) {
+        ok(
+          false,
+          `no main thread IO when we expected some during ${phase}: ${entry.path} (${entry.listedPath})`
+        );
+        shouldPass = false;
+      }
+    }
+  }
+
+  if (shouldPass) {
+    ok(shouldPass, "No unexpected main thread I/O during startup");
+  } else {
+    const filename = "profile_startup_mainthreadio.json";
+    let path = Services.env.get("MOZ_UPLOAD_DIR");
+    let profilePath = PathUtils.join(path, filename);
+    await IOUtils.writeJSON(profilePath, startupRecorder.data.profile);
+    ok(
+      false,
+      "Unexpected main thread I/O behavior during startup; open the " +
+        `${filename} artifact in the Firefox Profiler to see what happened`
+    );
+  }
+});
diff --git a/browser/base/content/test/performance/browser_startup_syncIPC.js b/browser/base/content/test/performance/browser_startup_syncIPC.js
new file mode 100644
index 0000000000..32c9450b0e
--- /dev/null
+++ b/browser/base/content/test/performance/browser_startup_syncIPC.js
@@ -0,0 +1,449 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/* This test sync IPC done on the main thread during startup. */
+
+"use strict";
+
+// Shortcuts for conditions.
+const LINUX = AppConstants.platform == "linux";
+const WIN = AppConstants.platform == "win";
+const MAC = AppConstants.platform == "macosx";
+const WEBRENDER = window.windowUtils.layerManagerType.startsWith("WebRender");
+const SKELETONUI = Services.prefs.getBoolPref(
+  "browser.startup.preXulSkeletonUI",
+  false
+);
+
+/*
+ * Specifying 'ignoreIfUnused: true' will make the test ignore unused entries;
+ * without this the test is strict and will fail if a list entry isn't used.
+ */
+const startupPhases = {
+  // Anything done before or during app-startup must have a compelling reason
+  // to run before we have even selected the user profile.
+  "before profile selection": [],
+
+  "before opening first browser window": [],
+
+  // We reach this phase right after showing the first browser window.
+  // This means that any I/O at this point delayed first paint.
+  "before first paint": [
+    {
+      name: "PLayerTransaction::Msg_GetTextureFactoryIdentifier",
+      condition: (MAC || LINUX) && !WEBRENDER,
+      maxCount: 1,
+    },
+    {
+      name: "PLayerTransaction::Msg_GetTextureFactoryIdentifier",
+      condition: WIN && !WEBRENDER,
+      maxCount: 3,
+    },
+    {
+      name: "PWebRenderBridge::Msg_EnsureConnected",
+      condition: WIN && WEBRENDER,
+      maxCount: 3,
+    },
+    {
+      name: "PWebRenderBridge::Msg_EnsureConnected",
+      condition: (MAC || LINUX) && WEBRENDER,
+      maxCount: 1,
+    },
+    {
+      // bug 1373773
+      name: "PCompositorBridge::Msg_NotifyChildCreated",
+      condition: !WIN,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_NotifyChildCreated",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win7 32
+      maxCount: 2,
+    },
+    {
+      name: "PCompositorBridge::Msg_MapAndNotifyChildCreated",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 2,
+    },
+    {
+      name: "PCompositorBridge::Msg_FlushRendering",
+      condition: MAC,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_FlushRendering",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win7 32
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_Initialize",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 3,
+    },
+    {
+      name: "PCompositorWidget::Msg_Initialize",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 3,
+    },
+    {
+      name: "PGPU::Msg_AddLayerTreeIdMapping",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 5,
+    },
+    {
+      name: "PCompositorBridge::Msg_MakeSnapshot",
+      condition: WIN && !WEBRENDER,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 1,
+    },
+    {
+      name: "PWebRenderBridge::Msg_GetSnapshot",
+      condition: WIN && WEBRENDER,
+      ignoreIfUnused: true, // Sometimes in the next phase on Windows10 QR
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_WillClose",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 2,
+    },
+    {
+      name: "PAPZInputBridge::Msg_ProcessUnhandledEvent",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 2,
+    },
+    {
+      name: "PGPU::Msg_GetDeviceStatus",
+      // bug 1553740 might want to drop the WEBRENDER clause here.
+      // Additionally, the skeleton UI causes us to attach "before first paint" to a
+      // later event, which lets this sneak in.
+      condition: WIN && (WEBRENDER || SKELETONUI),
+      // If Init() completes before we call EnsureGPUReady we won't send GetDeviceStatus
+      // so we can safely ignore if unused.
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      // bug 1784869
+      // We use Resume signal to propagate correct XWindow/wl_surface
+      // to EGL compositor.
+      name: "PCompositorBridge::Msg_Resume",
+      condition: LINUX,
+      ignoreIfUnused: true, // intermittently occurs in "before handling user events"
+      maxCount: 1,
+    },
+  ],
+
+  // We are at this phase once we are ready to handle user events.
+  // Any IO at this phase or before gets in the way of the user
+  // interacting with the first browser window.
+  "before handling user events": [
+    {
+      name: "PCompositorBridge::Msg_FlushRendering",
+      condition: MAC,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_FlushRendering",
+      condition: LINUX,
+      ignoreIfUnused: true, // intermittently occurs in "before becoming idle"
+      maxCount: 2,
+    },
+    {
+      name: "PLayerTransaction::Msg_GetTextureFactoryIdentifier",
+      condition: (!MAC && !WEBRENDER) || (WIN && WEBRENDER),
+      ignoreIfUnused: true, // intermittently occurs in "before becoming idle"
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_Initialize",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorWidget::Msg_Initialize",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_WillClose",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win7 32
+      maxCount: 2,
+    },
+    {
+      name: "PCompositorBridge::Msg_MakeSnapshot",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win7 32
+      maxCount: 1,
+    },
+    {
+      name: "PWebRenderBridge::Msg_GetSnapshot",
+      condition: WIN && WEBRENDER,
+      ignoreIfUnused: true, // Sometimes in the next phase on Windows10 QR
+      maxCount: 1,
+    },
+    {
+      name: "PAPZInputBridge::Msg_ProcessUnhandledEvent",
+      condition: WIN,
+      ignoreIfUnused: true, // intermittently occurs in "before becoming idle"
+      maxCount: 1,
+    },
+    {
+      name: "PAPZInputBridge::Msg_ReceiveMouseInputEvent",
+      condition: WIN,
+      ignoreIfUnused: true, // intermittently occurs in "before becoming idle"
+      maxCount: 1,
+    },
+    {
+      name: "PWebRenderBridge::Msg_EnsureConnected",
+      condition: WIN && WEBRENDER,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PContent::Reply_BeginDriverCrashGuard",
+      condition: WIN,
+      ignoreIfUnused: true, // Bug 1660590 - found while running test on windows hardware
+      maxCount: 1,
+    },
+    {
+      name: "PContent::Reply_EndDriverCrashGuard",
+      condition: WIN,
+      ignoreIfUnused: true, // Bug 1660590 - found while running test on windows hardware
+      maxCount: 1,
+    },
+    {
+      // bug 1784869
+      // We use Resume signal to propagate correct XWindow/wl_surface
+      // to EGL compositor.
+      name: "PCompositorBridge::Msg_Resume",
+      condition: LINUX,
+      ignoreIfUnused: true, // intermittently occurs in "before first paint"
+      maxCount: 1,
+    },
+  ],
+
+  // Things that are expected to be completely out of the startup path
+  // and loaded lazily when used for the first time by the user should
+  // be listed here.
+  "before becoming idle": [
+    {
+      // bug 1373773
+      name: "PCompositorBridge::Msg_NotifyChildCreated",
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PAPZInputBridge::Msg_ProcessUnhandledEvent",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 1,
+    },
+    {
+      name: "PAPZInputBridge::Msg_ReceiveMouseInputEvent",
+      condition: WIN,
+      ignoreIfUnused: true, // Only on Win10 64
+      maxCount: 1,
+    },
+    {
+      // bug 1554234
+      name: "PLayerTransaction::Msg_GetTextureFactoryIdentifier",
+      condition: WIN || LINUX,
+      ignoreIfUnused: true, // intermittently occurs in "before handling user events"
+      maxCount: 1,
+    },
+    {
+      name: "PWebRenderBridge::Msg_EnsureConnected",
+      condition: (WIN || LINUX) && WEBRENDER,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_Initialize",
+      condition: WIN,
+      ignoreIfUnused: true, // Intermittently occurs in "before handling user events"
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorWidget::Msg_Initialize",
+      condition: WIN,
+      ignoreIfUnused: true, // Intermittently occurs in "before handling user events"
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_MapAndNotifyChildCreated",
+      condition: WIN,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_FlushRendering",
+      condition: MAC || SKELETONUI,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_FlushRendering",
+      condition: LINUX,
+      ignoreIfUnused: true, // intermittently occurs in "before handling user events"
+      maxCount: 1,
+    },
+    {
+      name: "PWebRenderBridge::Msg_GetSnapshot",
+      condition: WIN && WEBRENDER,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_MakeSnapshot",
+      condition: WIN,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+    {
+      name: "PCompositorBridge::Msg_WillClose",
+      condition: WIN,
+      ignoreIfUnused: true,
+      maxCount: 2,
+    },
+    // Added for the search-detection built-in add-on.
+    {
+      name: "PGPU::Msg_AddLayerTreeIdMapping",
+      condition: WIN,
+      ignoreIfUnused: true,
+      maxCount: 1,
+    },
+  ],
+};
+
+add_task(async function () {
+  if (
+    !AppConstants.NIGHTLY_BUILD &&
+    !AppConstants.MOZ_DEV_EDITION &&
+    !AppConstants.DEBUG
+  ) {
+    ok(
+      !("@mozilla.org/test/startuprecorder;1" in Cc),
+      "the startup recorder component shouldn't exist in this non-nightly/non-devedition/" +
+        "non-debug build."
+    );
+    return;
+  }
+
+  let startupRecorder =
+    Cc["@mozilla.org/test/startuprecorder;1"].getService().wrappedJSObject;
+  await startupRecorder.done;
+
+  // Check for sync IPC markers in the startup profile.
+  let profile = startupRecorder.data.profile.threads[0];
+
+  let phases = {};
+  {
+    const nameCol = profile.markers.schema.name;
+    const dataCol = profile.markers.schema.data;
+    const startTimeCol = profile.markers.schema.startTime;
+
+    let markersForCurrentPhase = [];
+    for (let m of profile.markers.data) {
+      let markerName = profile.stringTable[m[nameCol]];
+      if (markerName.startsWith("startupRecorder:")) {
+        phases[markerName.split("startupRecorder:")[1]] =
+          markersForCurrentPhase;
+        markersForCurrentPhase = [];
+        continue;
+      }
+
+      let markerData = m[dataCol];
+      if (
+        !markerData ||
+        markerData.category != "Sync IPC" ||
+        !m[startTimeCol]
+      ) {
+        continue;
+      }
+
+      markersForCurrentPhase.push(markerName);
+    }
+  }
+
+  for (let phase in startupPhases) {
+    startupPhases[phase] = startupPhases[phase].filter(
+      entry => !("condition" in entry) || entry.condition
+    );
+  }
+
+  let shouldPass = true;
+  for (let phase in phases) {
+    let knownIPCList = startupPhases[phase];
+    if (knownIPCList.length) {
+      info(
+        `known sync IPC ${phase}:\n` +
+          knownIPCList
+            .map(e => `  ${e.name} - at most ${e.maxCount} times`)
+            .join("\n")
+      );
+    }
+
+    let markers = phases[phase];
+    for (let marker of markers) {
+      let expected = false;
+      for (let entry of knownIPCList) {
+        if (marker == entry.name) {
+          entry.useCount = (entry.useCount || 0) + 1;
+          expected = true;
+          break;
+        }
+      }
+      if (!expected) {
+        ok(false, `unexpected ${marker} sync IPC ${phase}`);
+        shouldPass = false;
+      }
+    }
+
+    for (let entry of knownIPCList) {
+      // Make sure useCount has been defined.
+      entry.useCount = entry.useCount || 0;
+      let message = `sync IPC ${entry.name} `;
+      if (entry.useCount == entry.maxCount) {
+        message += "happened as many times as expected";
+      } else if (entry.useCount < entry.maxCount) {
+        message += `allowed ${entry.maxCount} but only happened ${entry.useCount} times`;
+      } else {
+        message += `happened ${entry.useCount} but max is ${entry.maxCount}`;
+        shouldPass = false;
+      }
+      Assert.lessOrEqual(entry.useCount, entry.maxCount, `${message} ${phase}`);
+
+      if (entry.useCount == 0 && !entry.ignoreIfUnused) {
+        ok(false, `unused known IPC entry ${phase}: ${entry.name}`);
+        shouldPass = false;
+      }
+    }
+  }
+
+  if (shouldPass) {
+    ok(shouldPass, "No unexpected sync IPC during startup");
+  } else {
+    const filename = "profile_startup_syncIPC.json";
+    let path = Services.env.get("MOZ_UPLOAD_DIR");
+    let profilePath = PathUtils.join(path, filename);
+    await IOUtils.writeJSON(profilePath, startupRecorder.data.profile);
+    ok(
+      false,
+      `Unexpected sync IPC behavior during startup; open the ${filename} ` +
+        "artifact in the Firefox Profiler to see what happened"
+    );
+  }
+});
diff --git a/browser/base/content/test/performance/browser_tabclose.js b/browser/base/content/test/performance/browser_tabclose.js
new file mode 100644
index 0000000000..961686587f
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabclose.js
@@ -0,0 +1,108 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when closing new tabs.
+ */
+add_task(async function () {
+  // Force-enable tab animations
+  gReduceMotionOverride = false;
+
+  await ensureNoPreloadedBrowser();
+  await disableFxaBadge();
+
+  // The test starts on about:blank and opens an about:blank
+  // tab which triggers opening the toolbar since
+  // ensureNoPreloadedBrowser sets AboutNewTab.newTabURL to about:blank.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "never"]],
+  });
+
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+  await TestUtils.waitForCondition(() => tab._fullyOpen);
+
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+  let newTabButtonRect =
+    gBrowser.tabContainer.newTabButton.getBoundingClientRect();
+  let inRange = (val, min, max) => min <= val && val <= max;
+
+  // Add a reflow observer and open a new tab.
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      gBrowser.removeTab(tab, { animate: true });
+      await BrowserTestUtils.waitForEvent(tab, "TabAnimationEnd");
+      await switchDone;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      frames: {
+        filter: rects =>
+          rects.filter(
+            r =>
+              !(
+                // We expect all changes to be within the tab strip.
+                (
+                  r.y1 >= tabStripRect.top &&
+                  r.y2 <= tabStripRect.bottom &&
+                  r.x1 >= tabStripRect.left &&
+                  r.x2 <= tabStripRect.right &&
+                  // The closed tab should disappear at the same time as the previous
+                  // tab gets selected, causing both tab areas to change color at once:
+                  // this should be a single rect of the width of 2 tabs, and can
+                  // include the '+' button if it starts its animation.
+                  ((r.w > gBrowser.selectedTab.clientWidth &&
+                    r.x2 <= newTabButtonRect.right) ||
+                    // The '+' icon moves with an animation. At the end of the animation
+                    // the former and new positions can touch each other causing the rect
+                    // to have twice the icon's width.
+                    (r.h == 13 && r.w <= 2 * 13 + kMaxEmptyPixels) ||
+                    // We sometimes have a rect for the right most 2px of the '+' button.
+                    (r.h == 2 && r.w == 2))
+                )
+              )
+          ),
+        exceptions: [
+          {
+            name:
+              "bug 1444886 - the next tab should be selected at the same time" +
+              " as the closed one disappears",
+            condition: r =>
+              // In tab strip
+              r.y1 >= tabStripRect.top &&
+              r.y2 <= tabStripRect.bottom &&
+              r.x1 >= tabStripRect.left &&
+              r.x2 <= tabStripRect.right &&
+              // Width of one tab plus tab separator(s)
+              inRange(gBrowser.selectedTab.clientWidth - r.w, 0, 2),
+          },
+          {
+            name: "bug 1446449 - spurious tab switch spinner",
+            condition: r =>
+              AppConstants.DEBUG &&
+              // In the content area
+              r.y1 >=
+                document.getElementById("appcontent").getBoundingClientRect()
+                  .top,
+          },
+        ],
+      },
+    }
+  );
+  is(EXPECTED_REFLOWS.length, 0, "No reflows are expected when closing a tab");
+});
diff --git a/browser/base/content/test/performance/browser_tabclose_grow.js b/browser/base/content/test/performance/browser_tabclose_grow.js
new file mode 100644
index 0000000000..7ad43809cd
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabclose_grow.js
@@ -0,0 +1,91 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/*
+ * 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 () {
+  // Force-enable tab animations
+  gReduceMotionOverride = false;
+
+  await ensureNoPreloadedBrowser();
+  await disableFxaBadge();
+
+  // The test starts on about:blank and opens an about:blank
+  // tab which triggers opening the toolbar since
+  // ensureNoPreloadedBrowser sets AboutNewTab.newTabURL to about:blank.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "never"]],
+  });
+
+  // At the time of writing, there are no reflows on tab closing with
+  // tab growth. Mochitest will fail if we have no assertions, so we
+  // add one here to make sure nobody adds any new ones.
+  Assert.equal(
+    EXPECTED_REFLOWS.length,
+    0,
+    "We shouldn't have added any new expected reflows."
+  );
+
+  // 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);
+
+  let lastTab = gBrowser.tabs[gBrowser.tabs.length - 1];
+  await BrowserTestUtils.switchTab(gBrowser, lastTab);
+
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+
+  function isInTabStrip(r) {
+    return (
+      r.y1 >= tabStripRect.top &&
+      r.y2 <= tabStripRect.bottom &&
+      r.x1 >= tabStripRect.left &&
+      r.x2 <= tabStripRect.right &&
+      // It would make sense for each rect to have a width smaller than
+      // a tab (ie. tabstrip.width / tabcount), but tabs are small enough
+      // that they sometimes get reported in the same rect.
+      // So we accept up to the width of n-1 tabs.
+      r.w <=
+        (gBrowser.tabs.length - 1) *
+          Math.ceil(tabStripRect.width / gBrowser.tabs.length)
+    );
+  }
+
+  await withPerfObserver(
+    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, "TabAnimationEnd");
+      await switchDone;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      frames: {
+        filter: rects => rects.filter(r => !isInTabStrip(r)),
+      },
+    }
+  );
+
+  await removeAllButFirstTab();
+});
diff --git a/browser/base/content/test/performance/browser_tabdetach.js b/browser/base/content/test/performance/browser_tabdetach.js
new file mode 100644
index 0000000000..a860362f1f
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabdetach.js
@@ -0,0 +1,118 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS. This
+ * list should slowly go away as we improve the performance of the front-end.
+ * Instead of adding more reflows to the list, you should be modifying your code
+ * to avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  {
+    stack: [
+      "clientX@chrome://browser/content/tabbrowser-tabs.js",
+      "startTabDrag@chrome://browser/content/tabbrowser-tabs.js",
+      "on_dragstart@chrome://browser/content/tabbrowser-tabs.js",
+      "handleEvent@chrome://browser/content/tabbrowser-tabs.js",
+      "synthesizeMouseAtPoint@chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
+      "synthesizeMouse@chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
+      "synthesizePlainDragAndDrop@chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
+    ],
+    maxCount: 2,
+  },
+
+  {
+    stack: [
+      "startTabDrag@chrome://browser/content/tabbrowser-tabs.js",
+      "on_dragstart@chrome://browser/content/tabbrowser-tabs.js",
+      "handleEvent@chrome://browser/content/tabbrowser-tabs.js",
+      "synthesizeMouseAtPoint@chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
+      "synthesizeMouse@chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
+      "synthesizePlainDragAndDrop@chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
+    ],
+  },
+];
+
+/**
+ * This test ensures that there are no unexpected uninterruptible reflows when
+ * detaching a tab via drag and drop. The first testcase tests a non-overflowed
+ * tab strip, and the second tests an overflowed one.
+ */
+
+add_task(async function test_detach_not_overflowed() {
+  await ensureNoPreloadedBrowser();
+  await createTabs(1);
+
+  // Make sure we didn't overflow, as expected
+  await TestUtils.waitForCondition(() => {
+    return !gBrowser.tabContainer.hasAttribute("overflow");
+  });
+
+  let win;
+  await withPerfObserver(
+    async function () {
+      win = await detachTab(gBrowser.tabs[1]);
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      // we are opening a whole new window, so there's no point in tracking
+      // rects being painted
+      frames: { filter: rects => [] },
+    }
+  );
+
+  await BrowserTestUtils.closeWindow(win);
+  win = null;
+});
+
+add_task(async function test_detach_overflowed() {
+  const TAB_COUNT_FOR_OVERFLOW = computeMaxTabCount();
+  await createTabs(TAB_COUNT_FOR_OVERFLOW + 1);
+
+  // Make sure we overflowed, as expected
+  await TestUtils.waitForCondition(() => {
+    return gBrowser.tabContainer.hasAttribute("overflow");
+  });
+
+  let win;
+  await withPerfObserver(
+    async function () {
+      win = await detachTab(
+        gBrowser.tabs[Math.floor(TAB_COUNT_FOR_OVERFLOW / 2)]
+      );
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      // we are opening a whole new window, so there's no point in tracking
+      // rects being painted
+      frames: { filter: rects => [] },
+    }
+  );
+
+  await BrowserTestUtils.closeWindow(win);
+  win = null;
+
+  await removeAllButFirstTab();
+});
+
+async function detachTab(tab) {
+  let newWindowPromise = BrowserTestUtils.waitForNewWindow();
+
+  await EventUtils.synthesizePlainDragAndDrop({
+    srcElement: tab,
+
+    // destElement is null because tab detaching happens due
+    // to a drag'n'drop on an invalid drop target.
+    destElement: null,
+
+    // don't move horizontally because that could cause a tab move
+    // animation, and there's code to prevent a tab detaching if
+    // the dragged tab is released while the animation is running.
+    stepX: 0,
+    stepY: 100,
+  });
+
+  return newWindowPromise;
+}
diff --git a/browser/base/content/test/performance/browser_tabopen.js b/browser/base/content/test/performance/browser_tabopen.js
new file mode 100644
index 0000000000..2457812cb7
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabopen.js
@@ -0,0 +1,201 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when opening new tabs.
+ */
+add_task(async function () {
+  // Force-enable tab animations
+  gReduceMotionOverride = false;
+
+  // TODO (bug 1702653): Disable tab shadows for tests since the shadow
+  // can extend outside of the boundingClientRect. The tabRect will need
+  // to grow to include the shadow size.
+  gBrowser.tabContainer.setAttribute("noshadowfortests", "true");
+
+  await ensureNoPreloadedBrowser();
+  await disableFxaBadge();
+
+  // The test starts on about:blank and opens an about:blank
+  // tab which triggers opening the toolbar since
+  // ensureNoPreloadedBrowser sets AboutNewTab.newTabURL to about:blank.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "never"]],
+  });
+
+  // Prepare the window to avoid flicker and reflow that's unrelated to our
+  // tab opening operation.
+  gURLBar.focus();
+
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+
+  let firstTabRect = gBrowser.selectedTab.getBoundingClientRect();
+  let tabPaddingStart = parseFloat(
+    getComputedStyle(gBrowser.selectedTab).paddingInlineStart
+  );
+  let minTabWidth = firstTabRect.width - 2 * tabPaddingStart;
+  let maxTabWidth = firstTabRect.width;
+  let firstTabLabelRect =
+    gBrowser.selectedTab.textLabel.getBoundingClientRect();
+  let newTabButtonRect = document
+    .getElementById("tabs-newtab-button")
+    .getBoundingClientRect();
+  let textBoxRect = gURLBar
+    .querySelector("moz-input-box")
+    .getBoundingClientRect();
+
+  let inRange = (val, min, max) => min <= val && val <= max;
+
+  info(`tabStripRect=${JSON.stringify(tabStripRect)}`);
+  info(`firstTabRect=${JSON.stringify(firstTabRect)}`);
+  info(`tabPaddingStart=${JSON.stringify(tabPaddingStart)}`);
+  info(`firstTabLabelRect=${JSON.stringify(firstTabLabelRect)}`);
+  info(`newTabButtonRect=${JSON.stringify(newTabButtonRect)}`);
+  info(`textBoxRect=${JSON.stringify(textBoxRect)}`);
+
+  let inTabStrip = function (r) {
+    return (
+      r.y1 >= tabStripRect.top &&
+      r.y2 <= tabStripRect.bottom &&
+      r.x1 >= tabStripRect.left &&
+      r.x2 <= tabStripRect.right
+    );
+  };
+
+  const kTabCloseIconWidth = 13;
+
+  let isExpectedChange = function (r) {
+    // We expect all changes to be within the tab strip.
+    if (!inTabStrip(r)) {
+      return false;
+    }
+
+    // The first tab should get deselected at the same time as the next tab
+    // starts appearing, so we should have one rect that includes the first tab
+    // but is wider.
+    if (
+      inRange(r.w, minTabWidth, maxTabWidth * 2) &&
+      inRange(r.x1, firstTabRect.x, firstTabRect.x + tabPaddingStart)
+    ) {
+      return true;
+    }
+
+    // The second tab gets painted several times due to tabopen animation.
+    let isSecondTabRect =
+      inRange(
+        r.x1,
+        // When the animation starts the tab close icon overflows.
+        // -1 for the border on Win7
+        firstTabRect.right - kTabCloseIconWidth - 1,
+        firstTabRect.right + firstTabRect.width
+      ) &&
+      r.x2 <
+        firstTabRect.right +
+          firstTabRect.width +
+          // Sometimes the '+' is in the same rect.
+          newTabButtonRect.width;
+
+    if (isSecondTabRect) {
+      return true;
+    }
+    // The '+' icon moves with an animation. At the end of the animation
+    // the former and new positions can touch each other causing the rect
+    // to have twice the icon's width.
+    if (
+      r.h == kTabCloseIconWidth &&
+      r.w <= 2 * kTabCloseIconWidth + kMaxEmptyPixels
+    ) {
+      return true;
+    }
+
+    // We sometimes have a rect for the right most 2px of the '+' button.
+    if (r.h == 2 && r.w == 2) {
+      return true;
+    }
+
+    // Same for the 'X' icon.
+    if (r.h == 10 && r.w <= 2 * 10) {
+      return true;
+    }
+
+    // Other changes are unexpected.
+    return false;
+  };
+
+  // Add a reflow observer and open a new tab.
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      BrowserOpenTab();
+      await BrowserTestUtils.waitForEvent(
+        gBrowser.selectedTab,
+        "TabAnimationEnd"
+      );
+      await switchDone;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      frames: {
+        filter: rects => rects.filter(r => !isExpectedChange(r)),
+        exceptions: [
+          {
+            name:
+              "bug 1446452 - the new tab should appear at the same time as the" +
+              " previous one gets deselected",
+            condition: r =>
+              // In tab strip
+              r.y1 >= tabStripRect.top &&
+              r.y2 <= tabStripRect.bottom &&
+              // Position and size of the first tab.
+              r.x1 == firstTabRect.left &&
+              inRange(
+                r.w,
+                firstTabRect.width - 1, // -1 as the border doesn't change
+                firstTabRect.width
+              ),
+          },
+          {
+            name: "the urlbar placeolder moves up and down by a few pixels",
+            // This seems to only happen on the second run in --verify
+            condition: r =>
+              r.x1 >= textBoxRect.left &&
+              r.x2 <= textBoxRect.right &&
+              r.y1 >= textBoxRect.top &&
+              r.y2 <= textBoxRect.bottom,
+          },
+          {
+            name: "bug 1477966 - the name of a deselected tab should appear immediately",
+            condition: r =>
+              AppConstants.platform == "macosx" &&
+              r.x1 >= firstTabLabelRect.x &&
+              r.x2 <= firstTabLabelRect.right &&
+              r.y1 >= firstTabLabelRect.y &&
+              r.y2 <= firstTabLabelRect.bottom,
+          },
+        ],
+      },
+    }
+  );
+
+  let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+  BrowserTestUtils.removeTab(gBrowser.selectedTab);
+  await switchDone;
+});
diff --git a/browser/base/content/test/performance/browser_tabopen_squeeze.js b/browser/base/content/test/performance/browser_tabopen_squeeze.js
new file mode 100644
index 0000000000..f92bdc2ea4
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabopen_squeeze.js
@@ -0,0 +1,100 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when opening a new tab that will
+ * cause the existing tabs to squeeze smaller.
+ */
+add_task(async function () {
+  // Force-enable tab animations
+  gReduceMotionOverride = false;
+
+  await ensureNoPreloadedBrowser();
+  await disableFxaBadge();
+
+  // The test starts on about:blank and opens an about:blank
+  // tab which triggers opening the toolbar since
+  // ensureNoPreloadedBrowser sets AboutNewTab.newTabURL to about:blank.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "never"]],
+  });
+
+  // Compute the number of tabs we can put into the strip without
+  // overflowing, and remove one, so that we can create
+  // TAB_COUNT_FOR_SQUEEE tabs, and then one more, which should
+  // cause the tab to squeeze to a smaller size rather than overflow.
+  const TAB_COUNT_FOR_SQUEEZE = computeMaxTabCount() - 1;
+
+  await createTabs(TAB_COUNT_FOR_SQUEEZE);
+
+  gURLBar.focus();
+
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+  let textBoxRect = gURLBar
+    .querySelector("moz-input-box")
+    .getBoundingClientRect();
+
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      BrowserOpenTab();
+      await BrowserTestUtils.waitForEvent(
+        gBrowser.selectedTab,
+        "TabAnimationEnd"
+      );
+      await switchDone;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      frames: {
+        filter: rects =>
+          rects.filter(
+            r =>
+              !(
+                // We expect plenty of changed rects within the tab strip.
+                (
+                  r.y1 >= tabStripRect.top &&
+                  r.y2 <= tabStripRect.bottom &&
+                  r.x1 >= tabStripRect.left &&
+                  r.x2 <= tabStripRect.right &&
+                  // It would make sense for each rect to have a width smaller than
+                  // a tab (ie. tabstrip.width / tabcount), but tabs are small enough
+                  // that they sometimes get reported in the same rect.
+                  // So we accept up to the width of n-1 tabs.
+                  r.w <=
+                    (gBrowser.tabs.length - 1) *
+                      Math.ceil(tabStripRect.width / gBrowser.tabs.length)
+                )
+              )
+          ),
+        exceptions: [
+          {
+            name: "the urlbar placeolder moves up and down by a few pixels",
+            condition: r =>
+              r.x1 >= textBoxRect.left &&
+              r.x2 <= textBoxRect.right &&
+              r.y1 >= textBoxRect.top &&
+              r.y2 <= textBoxRect.bottom,
+          },
+        ],
+      },
+    }
+  );
+
+  await removeAllButFirstTab();
+});
diff --git a/browser/base/content/test/performance/browser_tabstrip_overflow_underflow.js b/browser/base/content/test/performance/browser_tabstrip_overflow_underflow.js
new file mode 100644
index 0000000000..1fd33ed836
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabstrip_overflow_underflow.js
@@ -0,0 +1,200 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_*_REFLOWS.
+ * This is a (now empty) list of known reflows.
+ * Instead of adding more reflows to the lists, you should be modifying your
+ * code to avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_OVERFLOW_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+const EXPECTED_UNDERFLOW_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/**
+ * This test ensures that there are no unexpected uninterruptible reflows when
+ * opening a new tab that will cause the existing tabs to overflow and the tab
+ * strip to become scrollable. It also tests that there are no unexpected
+ * uninterruptible reflows when closing that tab, which causes the tab strip to
+ * underflow.
+ */
+add_task(async function () {
+  // Force-enable tab animations
+  gReduceMotionOverride = false;
+
+  await ensureNoPreloadedBrowser();
+
+  // The test starts on about:blank and opens an about:blank
+  // tab which triggers opening the toolbar since
+  // ensureNoPreloadedBrowser sets AboutNewTab.newTabURL to about:blank.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "never"]],
+  });
+
+  const TAB_COUNT_FOR_OVERFLOW = computeMaxTabCount();
+
+  await createTabs(TAB_COUNT_FOR_OVERFLOW);
+
+  gURLBar.focus();
+  await disableFxaBadge();
+
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+  let textBoxRect = gURLBar
+    .querySelector("moz-input-box")
+    .getBoundingClientRect();
+
+  let ignoreTabstripRects = {
+    filter: rects =>
+      rects.filter(
+        r =>
+          !(
+            // We expect plenty of changed rects within the tab strip.
+            (
+              r.y1 >= tabStripRect.top &&
+              r.y2 <= tabStripRect.bottom &&
+              r.x1 >= tabStripRect.left &&
+              r.x2 <= tabStripRect.right
+            )
+          )
+      ),
+    exceptions: [
+      {
+        name: "the urlbar placeolder moves up and down by a few pixels",
+        condition: r =>
+          r.x1 >= textBoxRect.left &&
+          r.x2 <= textBoxRect.right &&
+          r.y1 >= textBoxRect.top &&
+          r.y2 <= textBoxRect.bottom,
+      },
+      {
+        name: "bug 1446449 - spurious tab switch spinner",
+        condition: r =>
+          // In the content area
+          r.y1 >=
+          document.getElementById("appcontent").getBoundingClientRect().top,
+      },
+    ],
+  };
+
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      BrowserOpenTab();
+      await BrowserTestUtils.waitForEvent(
+        gBrowser.selectedTab,
+        "TabAnimationEnd"
+      );
+      await switchDone;
+      await TestUtils.waitForCondition(() => {
+        return gBrowser.tabContainer.arrowScrollbox.hasAttribute(
+          "scrolledtoend"
+        );
+      });
+    },
+    { expectedReflows: EXPECTED_OVERFLOW_REFLOWS, frames: ignoreTabstripRects }
+  );
+
+  Assert.ok(
+    gBrowser.tabContainer.hasAttribute("overflow"),
+    "Tabs should now be overflowed."
+  );
+
+  // Now test that opening and closing a tab while overflowed doesn't cause
+  // us to reflow.
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      BrowserOpenTab();
+      await switchDone;
+      await TestUtils.waitForCondition(() => {
+        return gBrowser.tabContainer.arrowScrollbox.hasAttribute(
+          "scrolledtoend"
+        );
+      });
+    },
+    { expectedReflows: [], frames: ignoreTabstripRects }
+  );
+
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      BrowserTestUtils.removeTab(gBrowser.selectedTab, { animate: true });
+      await switchDone;
+    },
+    { expectedReflows: [], frames: ignoreTabstripRects }
+  );
+
+  // At this point, we have an overflowed tab strip, and we've got the last tab
+  // selected. This should mean that the first tab is scrolled out of view.
+  // Let's test that we don't reflow when switching to that first tab.
+  let lastTab = gBrowser.selectedTab;
+  let arrowScrollbox = gBrowser.tabContainer.arrowScrollbox;
+
+  // First, we'll check that the first tab is actually scrolled
+  // at least partially out of view.
+  Assert.ok(
+    arrowScrollbox.scrollPosition > 0,
+    "First tab should be partially scrolled out of view."
+  );
+
+  // Now switch to the first tab. We shouldn't flush layout at all.
+  await withPerfObserver(
+    async function () {
+      let firstTab = gBrowser.tabs[0];
+      await BrowserTestUtils.switchTab(gBrowser, firstTab);
+      await TestUtils.waitForCondition(() => {
+        return gBrowser.tabContainer.arrowScrollbox.hasAttribute(
+          "scrolledtostart"
+        );
+      });
+    },
+    { expectedReflows: [], frames: ignoreTabstripRects }
+  );
+
+  // Okay, now close the last tab. The tabstrip should stay overflowed, but removing
+  // one more after that should underflow it.
+  BrowserTestUtils.removeTab(lastTab);
+
+  Assert.ok(
+    gBrowser.tabContainer.hasAttribute("overflow"),
+    "Tabs should still be overflowed."
+  );
+
+  // Depending on the size of the window, it might take one or more tab
+  // removals to put the tab strip out of the overflow state, so we'll just
+  // keep testing removals until that occurs.
+  while (gBrowser.tabContainer.hasAttribute("overflow")) {
+    lastTab = gBrowser.tabs[gBrowser.tabs.length - 1];
+    if (gBrowser.selectedTab !== lastTab) {
+      await BrowserTestUtils.switchTab(gBrowser, lastTab);
+    }
+
+    // ... and make sure we don't flush layout when closing it, and exiting
+    // the overflowed state.
+    await withPerfObserver(
+      async function () {
+        let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+        BrowserTestUtils.removeTab(lastTab, { animate: true });
+        await switchDone;
+        await TestUtils.waitForCondition(() => !lastTab.isConnected);
+      },
+      {
+        expectedReflows: EXPECTED_UNDERFLOW_REFLOWS,
+        frames: ignoreTabstripRects,
+      }
+    );
+  }
+
+  await removeAllButFirstTab();
+});
diff --git a/browser/base/content/test/performance/browser_tabswitch.js b/browser/base/content/test/performance/browser_tabswitch.js
new file mode 100644
index 0000000000..bbbbac3a21
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabswitch.js
@@ -0,0 +1,123 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See  https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when switching between two
+ * tabs that are both fully visible.
+ */
+add_task(async function () {
+  // TODO (bug 1702653): Disable tab shadows for tests since the shadow
+  // can extend outside of the boundingClientRect. The tabRect will need
+  // to grow to include the shadow size.
+  gBrowser.tabContainer.setAttribute("noshadowfortests", "true");
+
+  await ensureNoPreloadedBrowser();
+  await disableFxaBadge();
+
+  // The test starts on about:blank and opens an about:blank
+  // tab which triggers opening the toolbar since
+  // ensureNoPreloadedBrowser sets AboutNewTab.newTabURL to about:blank.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "never"]],
+  });
+
+  // At the time of writing, there are no reflows on simple tab switching.
+  // Mochitest will fail if we have no assertions, so we add one here
+  // to make sure nobody adds any new ones.
+  Assert.equal(
+    EXPECTED_REFLOWS.length,
+    0,
+    "We shouldn't have added any new expected reflows."
+  );
+
+  let origTab = gBrowser.selectedTab;
+  let firstSwitchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+  let otherTab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+  await firstSwitchDone;
+
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+  let firstTabRect = origTab.getBoundingClientRect();
+  let tabPaddingStart = parseFloat(
+    getComputedStyle(gBrowser.selectedTab).paddingInlineStart
+  );
+  let minTabWidth = firstTabRect.width - 2 * tabPaddingStart;
+  let maxTabWidth = firstTabRect.width;
+  let inRange = (val, min, max) => min <= val && val <= max;
+
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      gBrowser.selectedTab = origTab;
+      await switchDone;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      frames: {
+        filter: rects =>
+          rects.filter(
+            r =>
+              !(
+                // We expect all changes to be within the tab strip.
+                (
+                  r.y1 >= tabStripRect.top &&
+                  r.y2 <= tabStripRect.bottom &&
+                  r.x1 >= tabStripRect.left &&
+                  r.x2 <= tabStripRect.right &&
+                  // The tab selection changes between 2 adjacent tabs, so we expect
+                  // both to change color at once: this should be a single rect of the
+                  // width of 2 tabs.
+                  inRange(
+                    r.w,
+                    minTabWidth - 1, // -1 for the border on Win7
+                    maxTabWidth * 2
+                  )
+                )
+              )
+          ),
+        exceptions: [
+          {
+            name:
+              "bug 1446454 - the border between tabs should be painted at" +
+              " the same time as the tab switch",
+            condition: r =>
+              // In tab strip
+              r.y1 >= tabStripRect.top &&
+              r.y2 <= tabStripRect.bottom &&
+              // 1px border, 1px before the end of the first tab.
+              r.w == 1 &&
+              r.x1 == firstTabRect.right - 1,
+          },
+          {
+            name: "bug 1446449 - spurious tab switch spinner",
+            condition: r =>
+              AppConstants.DEBUG &&
+              // In the content area
+              r.y1 >=
+                document.getElementById("appcontent").getBoundingClientRect()
+                  .top,
+          },
+        ],
+      },
+    }
+  );
+
+  BrowserTestUtils.removeTab(otherTab);
+});
diff --git a/browser/base/content/test/performance/browser_toolbariconcolor_restyles.js b/browser/base/content/test/performance/browser_toolbariconcolor_restyles.js
new file mode 100644
index 0000000000..890c8f3c80
--- /dev/null
+++ b/browser/base/content/test/performance/browser_toolbariconcolor_restyles.js
@@ -0,0 +1,65 @@
+"use strict";
+
+/**
+ * Ensure redundant style flushes are not triggered when switching between windows
+ */
+add_task(async function test_toolbar_element_restyles_on_activation() {
+  let restyles = {
+    win1: {},
+    win2: {},
+  };
+
+  // create a window and snapshot the elementsStyled
+  let win1 = await BrowserTestUtils.openNewBrowserWindow();
+  await new Promise(resolve => waitForFocus(resolve, win1));
+
+  // create a 2nd window and snapshot the elementsStyled
+  let win2 = await BrowserTestUtils.openNewBrowserWindow();
+  await new Promise(resolve => waitForFocus(resolve, win2));
+
+  // (De)-activate both windows once before we take a measurement. The first
+  // (de-)activation may flush styles, after that the style data should be
+  // cached.
+  win1.focus();
+  win2.focus();
+
+  // Flush any pending styles before we take a measurement.
+  win1.getComputedStyle(win1.document.firstElementChild);
+  win2.getComputedStyle(win2.document.firstElementChild);
+
+  // Clear the focused element from each window so that when
+  // we raise them, the focus of the element doesn't cause an
+  // unrelated style flush.
+  Services.focus.clearFocus(win1);
+  Services.focus.clearFocus(win2);
+
+  let utils1 = SpecialPowers.getDOMWindowUtils(win1);
+  restyles.win1.initial = utils1.restyleGeneration;
+
+  let utils2 = SpecialPowers.getDOMWindowUtils(win2);
+  restyles.win2.initial = utils2.restyleGeneration;
+
+  // switch back to 1st window, and snapshot elementsStyled
+  win1.focus();
+  restyles.win1.activate = utils1.restyleGeneration;
+  restyles.win2.deactivate = utils2.restyleGeneration;
+
+  // switch back to 2nd window, and snapshot elementsStyled
+  win2.focus();
+  restyles.win2.activate = utils2.restyleGeneration;
+  restyles.win1.deactivate = utils1.restyleGeneration;
+
+  is(
+    restyles.win1.activate - restyles.win1.deactivate,
+    0,
+    "No elements restyled when re-activating/deactivating a window"
+  );
+  is(
+    restyles.win2.activate - restyles.win2.deactivate,
+    0,
+    "No elements restyled when re-activating/deactivating a window"
+  );
+
+  await BrowserTestUtils.closeWindow(win1);
+  await BrowserTestUtils.closeWindow(win2);
+});
diff --git a/browser/base/content/test/performance/browser_urlbar_keyed_search.js b/browser/base/content/test/performance/browser_urlbar_keyed_search.js
new file mode 100644
index 0000000000..a44e5d4822
--- /dev/null
+++ b/browser/base/content/test/performance/browser_urlbar_keyed_search.js
@@ -0,0 +1,27 @@
+"use strict";
+
+// This tests searching in the urlbar (a.k.a. the quantumbar).
+
+/**
+ * WHOA THERE: We should never be adding new things to
+ * EXPECTED_REFLOWS_FIRST_OPEN or EXPECTED_REFLOWS_SECOND_OPEN.
+ * Instead of adding reflows to these lists, you should be modifying your code
+ * to avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+
+/* These reflows happen only the first time the panel opens. */
+const EXPECTED_REFLOWS_FIRST_OPEN = [];
+
+/* These reflows happen every time the panel opens. */
+const EXPECTED_REFLOWS_SECOND_OPEN = [];
+
+add_task(async function quantumbar() {
+  await runUrlbarTest(
+    true,
+    EXPECTED_REFLOWS_FIRST_OPEN,
+    EXPECTED_REFLOWS_SECOND_OPEN
+  );
+});
diff --git a/browser/base/content/test/performance/browser_urlbar_search.js b/browser/base/content/test/performance/browser_urlbar_search.js
new file mode 100644
index 0000000000..35961c641f
--- /dev/null
+++ b/browser/base/content/test/performance/browser_urlbar_search.js
@@ -0,0 +1,27 @@
+"use strict";
+
+// This tests searching in the urlbar (a.k.a. the quantumbar).
+
+/**
+ * WHOA THERE: We should never be adding new things to
+ * EXPECTED_REFLOWS_FIRST_OPEN or EXPECTED_REFLOWS_SECOND_OPEN.
+ * Instead of adding reflows to these lists, you should be modifying your code
+ * to avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+
+/* These reflows happen only the first time the panel opens. */
+const EXPECTED_REFLOWS_FIRST_OPEN = [];
+
+/* These reflows happen every time the panel opens. */
+const EXPECTED_REFLOWS_SECOND_OPEN = [];
+
+add_task(async function quantumbar() {
+  await runUrlbarTest(
+    false,
+    EXPECTED_REFLOWS_FIRST_OPEN,
+    EXPECTED_REFLOWS_SECOND_OPEN
+  );
+});
diff --git a/browser/base/content/test/performance/browser_vsync_accessibility.js b/browser/base/content/test/performance/browser_vsync_accessibility.js
new file mode 100644
index 0000000000..64e3dc0b85
--- /dev/null
+++ b/browser/base/content/test/performance/browser_vsync_accessibility.js
@@ -0,0 +1,20 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function () {
+  await TestUtils.waitForCondition(
+    () => !ChromeUtils.vsyncEnabled(),
+    "wait for vsync to be disabled at the start of the test"
+  );
+  Assert.ok(!ChromeUtils.vsyncEnabled(), "vsync should be disabled");
+  Cc["@mozilla.org/accessibilityService;1"].getService(
+    Ci.nsIAccessibilityService
+  );
+  await TestUtils.waitForCondition(
+    () => !ChromeUtils.vsyncEnabled(),
+    "wait for vsync to be disabled after initializing the accessibility service"
+  );
+  Assert.ok(!ChromeUtils.vsyncEnabled(), "vsync should still be disabled");
+});
diff --git a/browser/base/content/test/performance/browser_window_resize.js b/browser/base/content/test/performance/browser_window_resize.js
new file mode 100644
index 0000000000..b529eec040
--- /dev/null
+++ b/browser/base/content/test/performance/browser_window_resize.js
@@ -0,0 +1,132 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+const gToolbar = document.getElementById("PersonalToolbar");
+
+/**
+ * Sets the visibility state on the Bookmarks Toolbar, and
+ * waits for it to transition to fully visible.
+ *
+ * @param visible (bool)
+ *        Whether or not the bookmarks toolbar should be made visible.
+ * @returns Promise
+ */
+async function toggleBookmarksToolbar(visible) {
+  let transitionPromise = BrowserTestUtils.waitForEvent(
+    gToolbar,
+    "transitionend",
+    e => e.propertyName == "max-height"
+  );
+
+  setToolbarVisibility(gToolbar, visible);
+  await transitionPromise;
+}
+
+/**
+ * Resizes a browser window to a particular width and height, and
+ * waits for it to reach a "steady state" with respect to its overflowing
+ * toolbars.
+ * @param win (browser window)
+ *        The window to resize.
+ * @param width (int)
+ *        The width to resize the window to.
+ * @param height (int)
+ *        The height to resize the window to.
+ * @returns Promise
+ */
+async function resizeWindow(win, width, height) {
+  let toolbarEvent = BrowserTestUtils.waitForEvent(
+    win,
+    "BookmarksToolbarVisibilityUpdated"
+  );
+  let resizeEvent = BrowserTestUtils.waitForEvent(win, "resize");
+  win.windowUtils.ensureDirtyRootFrame();
+  win.resizeTo(width, height);
+  await resizeEvent;
+  await toolbarEvent;
+}
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when resizing windows.
+ */
+add_task(async function () {
+  const BOOKMARKS_COUNT = 150;
+  const STARTING_WIDTH = 600;
+  const STARTING_HEIGHT = 400;
+  const SMALL_WIDTH = 150;
+  const SMALL_HEIGHT = 150;
+
+  await PlacesUtils.bookmarks.eraseEverything();
+
+  // Add a bunch of bookmarks to display in the Bookmarks toolbar
+  await PlacesUtils.bookmarks.insertTree({
+    guid: PlacesUtils.bookmarks.toolbarGuid,
+    children: Array(BOOKMARKS_COUNT)
+      .fill("")
+      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      .map((_, i) => ({ url: `http://test.places.${i}.x/` })),
+  });
+
+  let wasCollapsed = gToolbar.collapsed;
+  Assert.ok(wasCollapsed, "The toolbar is collapsed by default");
+  if (wasCollapsed) {
+    let promiseReady = BrowserTestUtils.waitForEvent(
+      gToolbar,
+      "BookmarksToolbarVisibilityUpdated"
+    );
+    await toggleBookmarksToolbar(true);
+    await promiseReady;
+  }
+
+  registerCleanupFunction(async () => {
+    if (wasCollapsed) {
+      await toggleBookmarksToolbar(false);
+    }
+    await PlacesUtils.bookmarks.eraseEverything();
+    await PlacesUtils.history.clear();
+  });
+
+  let win = await prepareSettledWindow();
+
+  if (
+    win.screen.availWidth < STARTING_WIDTH ||
+    win.screen.availHeight < STARTING_HEIGHT
+  ) {
+    Assert.ok(
+      false,
+      "This test is running on too small a display - " +
+        `(${STARTING_WIDTH}x${STARTING_HEIGHT} min)`
+    );
+    return;
+  }
+
+  await resizeWindow(win, STARTING_WIDTH, STARTING_HEIGHT);
+
+  await withPerfObserver(
+    async function () {
+      await resizeWindow(win, SMALL_WIDTH, SMALL_HEIGHT);
+      await resizeWindow(win, STARTING_WIDTH, STARTING_HEIGHT);
+    },
+    { expectedReflows: EXPECTED_REFLOWS, frames: { filter: () => [] } },
+    win
+  );
+
+  await BrowserTestUtils.closeWindow(win);
+});
diff --git a/browser/base/content/test/performance/browser_windowclose.js b/browser/base/content/test/performance/browser_windowclose.js
new file mode 100644
index 0000000000..7d11779acc
--- /dev/null
+++ b/browser/base/content/test/performance/browser_windowclose.js
@@ -0,0 +1,67 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/**
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when closing windows. When the
+ * window is closed, the test waits until the original window
+ * has activated.
+ */
+add_task(async function () {
+  // Ensure that this browser window starts focused. This seems to be
+  // necessary to avoid intermittent failures when running this test
+  // on repeat.
+  await new Promise(resolve => {
+    waitForFocus(resolve, window);
+  });
+
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  await new Promise(resolve => {
+    waitForFocus(resolve, win);
+  });
+
+  // At the time of writing, there are no reflows on window closing.
+  // Mochitest will fail if we have no assertions, so we add one here
+  // to make sure nobody adds any new ones.
+  Assert.equal(
+    EXPECTED_REFLOWS.length,
+    0,
+    "We shouldn't have added any new expected reflows for window close."
+  );
+
+  await withPerfObserver(
+    async function () {
+      let promiseOrigBrowserFocused = TestUtils.waitForCondition(() => {
+        return Services.focus.activeWindow == window;
+      });
+      await BrowserTestUtils.closeWindow(win);
+      await promiseOrigBrowserFocused;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      frames: {
+        filter(rects, frame, previousFrame) {
+          // Ignore the focus-out animation.
+          if (isLikelyFocusChange(rects, frame)) {
+            return [];
+          }
+          return rects;
+        },
+      },
+    },
+    win
+  );
+});
diff --git a/browser/base/content/test/performance/browser_windowopen.js b/browser/base/content/test/performance/browser_windowopen.js
new file mode 100644
index 0000000000..02c6172948
--- /dev/null
+++ b/browser/base/content/test/performance/browser_windowopen.js
@@ -0,0 +1,164 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * for tips on how to do that.
+ */
+const EXPECTED_REFLOWS = [
+  /**
+   * Nothing here! Please don't add anything new!
+   */
+];
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows or flickering areas 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-caches");
+
+  let bookmarksToolbarRect = await getBookmarksToolbarRect();
+
+  let win = window.openDialog(
+    AppConstants.BROWSER_CHROME_URL,
+    "_blank",
+    "chrome,all,dialog=no,remote,suppressanimation",
+    "about:home"
+  );
+
+  await disableFxaBadge();
+
+  let alreadyFocused = false;
+  let inRange = (val, min, max) => min <= val && val <= max;
+  let expectations = {
+    expectedReflows: EXPECTED_REFLOWS,
+    frames: {
+      filter(rects, frame, previousFrame) {
+        // The first screenshot we get in OSX / Windows shows an unfocused browser
+        // window for some reason. See bug 1445161.
+        if (!alreadyFocused && isLikelyFocusChange(rects, frame)) {
+          todo(
+            false,
+            "bug 1445161 - the window should be focused at first paint, " +
+              rects.toSource()
+          );
+          return [];
+        }
+        alreadyFocused = true;
+        return rects;
+      },
+      exceptions: [
+        {
+          name: "bug 1421463 - reload toolbar icon shouldn't flicker",
+          condition: r =>
+            inRange(r.h, 13, 14) &&
+            inRange(r.w, 14, 16) && // icon size
+            inRange(r.y1, 40, 80) && // in the toolbar
+            inRange(r.x1, 65, 100), // near the left side of the screen
+        },
+        {
+          name: "bug 1555842 - the urlbar shouldn't flicker",
+          condition: r => {
+            let inputFieldRect = win.gURLBar.inputField.getBoundingClientRect();
+
+            return (
+              (!AppConstants.DEBUG ||
+                (AppConstants.platform == "linux" && AppConstants.ASAN)) &&
+              r.x1 >= inputFieldRect.left &&
+              r.x2 <= inputFieldRect.right &&
+              r.y1 >= inputFieldRect.top &&
+              r.y2 <= inputFieldRect.bottom
+            );
+          },
+        },
+        {
+          name: "Initial bookmark icon appearing after startup",
+          condition: r =>
+            r.w == 16 &&
+            r.h == 16 && // icon size
+            inRange(
+              r.y1,
+              bookmarksToolbarRect.top,
+              bookmarksToolbarRect.top + bookmarksToolbarRect.height / 2
+            ) && // in the toolbar
+            inRange(r.x1, 11, 13), // very close to the left of the screen
+        },
+        {
+          // Note that the length and x values here are a bit weird because on
+          // some fonts, we appear to detect the two words separately.
+          name: "Initial bookmark text ('Getting Started' or 'Get Involved') appearing after startup",
+          condition: r =>
+            inRange(r.w, 25, 120) && // length of text
+            inRange(r.h, 9, 15) && // height of text
+            inRange(
+              r.y1,
+              bookmarksToolbarRect.top,
+              bookmarksToolbarRect.top + bookmarksToolbarRect.height / 2
+            ) && // in the toolbar
+            inRange(r.x1, 30, 90), // close to the left of the screen
+        },
+      ],
+    },
+  };
+
+  await withPerfObserver(
+    async function () {
+      // Avoid showing the remotecontrol UI.
+      await new Promise(resolve => {
+        win.addEventListener(
+          "DOMContentLoaded",
+          () => {
+            delete win.Marionette;
+            win.Marionette = { running: false };
+            resolve();
+          },
+          { once: true }
+        );
+      });
+
+      await TestUtils.topicObserved(
+        "browser-delayed-startup-finished",
+        subject => subject == win
+      );
+
+      let promises = [
+        BrowserTestUtils.firstBrowserLoaded(win, false),
+        BrowserTestUtils.browserStopped(
+          win.gBrowser.selectedBrowser,
+          "about:home"
+        ),
+      ];
+
+      await Promise.all(promises);
+
+      await new Promise(resolve => {
+        // 10 is an arbitrary value here, it needs to be at least 2 to avoid
+        // races with code initializing itself using idle callbacks.
+        (function waitForIdle(count = 10) {
+          if (!count) {
+            resolve();
+            return;
+          }
+          Services.tm.idleDispatchToMainThread(() => {
+            waitForIdle(count - 1);
+          });
+        })();
+      });
+    },
+    expectations,
+    win
+  );
+
+  await BrowserTestUtils.closeWindow(win);
+});
diff --git a/browser/base/content/test/performance/file_empty.html b/browser/base/content/test/performance/file_empty.html
new file mode 100644
index 0000000000..865879c583
--- /dev/null
+++ b/browser/base/content/test/performance/file_empty.html
@@ -0,0 +1 @@
+<!-- this file intentionally left blank -->
diff --git a/browser/base/content/test/performance/head.js b/browser/base/content/test/performance/head.js
new file mode 100644
index 0000000000..29722e6bbe
--- /dev/null
+++ b/browser/base/content/test/performance/head.js
@@ -0,0 +1,1001 @@
+"use strict";
+
+ChromeUtils.defineESModuleGetters(this, {
+  AboutNewTab: "resource:///modules/AboutNewTab.sys.mjs",
+  PerfTestHelpers: "resource://testing-common/PerfTestHelpers.sys.mjs",
+  PlacesTestUtils: "resource://testing-common/PlacesTestUtils.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+  UrlbarTestUtils: "resource://testing-common/UrlbarTestUtils.sys.mjs",
+});
+
+/**
+ * This function can be called if the test needs to trigger frame dirtying
+ * outside of the normal mechanism.
+ *
+ * @param win (dom window)
+ *        The window in which the frame tree needs to be marked as dirty.
+ */
+function dirtyFrame(win) {
+  let dwu = win.windowUtils;
+  try {
+    dwu.ensureDirtyRootFrame();
+  } catch (e) {
+    // If this fails, we should probably make note of it, but it's not fatal.
+    info("Note: ensureDirtyRootFrame threw an exception:" + e);
+  }
+}
+
+/**
+ * Async utility function to collect the stacks of uninterruptible reflows
+ * occuring during some period of time in a window.
+ *
+ * @param testPromise (Promise)
+ *        A promise that is resolved when the data collection should stop.
+ *
+ * @param win (browser window, optional)
+ *        The browser window to monitor. Defaults to the current window.
+ *
+ * @return An array of reflow stacks
+ */
+async function recordReflows(testPromise, win = window) {
+  // Collect all reflow stacks, we'll process them later.
+  let reflows = [];
+
+  let observer = {
+    reflow(start, end) {
+      // Gather information about the current code path.
+      reflows.push(new Error().stack);
+
+      // Just in case, dirty the frame now that we've reflowed.
+      dirtyFrame(win);
+    },
+
+    reflowInterruptible(start, end) {
+      // Interruptible reflows are the reflows caused by the refresh
+      // driver ticking. These are fine.
+    },
+
+    QueryInterface: ChromeUtils.generateQI([
+      "nsIReflowObserver",
+      "nsISupportsWeakReference",
+    ]),
+  };
+
+  let docShell = win.docShell;
+  docShell.addWeakReflowObserver(observer);
+
+  let dirtyFrameFn = event => {
+    if (event.type != "MozAfterPaint") {
+      dirtyFrame(win);
+    }
+  };
+  Services.els.addListenerForAllEvents(win, dirtyFrameFn, true);
+
+  try {
+    dirtyFrame(win);
+    await testPromise;
+  } finally {
+    Services.els.removeListenerForAllEvents(win, dirtyFrameFn, true);
+    docShell.removeWeakReflowObserver(observer);
+  }
+
+  return reflows;
+}
+
+/**
+ * Utility function to report unexpected reflows.
+ *
+ * @param reflows (Array)
+ *        An array of reflow stacks returned by recordReflows.
+ *
+ * @param expectedReflows (Array, optional)
+ *        An Array of Objects representing reflows.
+ *
+ *        Example:
+ *
+ *        [
+ *          {
+ *            // This reflow is caused by lorem ipsum.
+ *            // Sometimes, due to unpredictable timings, the reflow may be hit
+ *            // less times.
+ *            stack: [
+ *              "select@chrome://global/content/bindings/textbox.xml",
+ *              "focusAndSelectUrlBar@chrome://browser/content/browser.js",
+ *              "openLinkIn@chrome://browser/content/utilityOverlay.js",
+ *              "openUILinkIn@chrome://browser/content/utilityOverlay.js",
+ *              "BrowserOpenTab@chrome://browser/content/browser.js",
+ *            ],
+ *            // We expect this particular reflow to happen up to 2 times.
+ *            maxCount: 2,
+ *          },
+ *
+ *          {
+ *            // This reflow is caused by lorem ipsum. We expect this reflow
+ *            // to only happen once, so we can omit the "maxCount" property.
+ *            stack: [
+ *              "get_scrollPosition@chrome://global/content/bindings/scrollbox.xml",
+ *              "_fillTrailingGap@chrome://browser/content/tabbrowser.xml",
+ *              "_handleNewTab@chrome://browser/content/tabbrowser.xml",
+ *              "onxbltransitionend@chrome://browser/content/tabbrowser.xml",
+ *            ],
+ *          }
+ *        ]
+ *
+ *        Note that line numbers are not included in the stacks.
+ *
+ *        Order of the reflows doesn't matter. Expected reflows that aren't seen
+ *        will cause an assertion failure. When this argument is not passed,
+ *        it defaults to the empty Array, meaning no reflows are expected.
+ */
+function reportUnexpectedReflows(reflows, expectedReflows = []) {
+  let knownReflows = expectedReflows.map(r => {
+    return {
+      stack: r.stack,
+      path: r.stack.join("|"),
+      count: 0,
+      maxCount: r.maxCount || 1,
+      actualStacks: new Map(),
+    };
+  });
+  let unexpectedReflows = new Map();
+
+  if (knownReflows.some(r => r.path.includes("*"))) {
+    Assert.ok(
+      false,
+      "Do not include async frames in the stack, as " +
+        "that feature is not available on all trees."
+    );
+  }
+
+  for (let stack of reflows) {
+    let path = stack
+      .split("\n")
+      .slice(1) // the first frame which is our test code.
+      .map(line => line.replace(/:\d+:\d+$/, "")) // strip line numbers.
+      .join("|");
+
+    // Stack trace is empty. Reflow was triggered by native code, which
+    // we ignore.
+    if (path === "") {
+      continue;
+    }
+
+    // Functions from EventUtils.js calculate coordinates and
+    // dimensions, causing us to reflow. That's the test
+    // harness and we don't care about that, so we'll filter that out.
+    if (
+      /^(synthesize|send|createDragEventObject).*?@chrome:\/\/mochikit.*?EventUtils\.js/.test(
+        path
+      )
+    ) {
+      continue;
+    }
+
+    let index = knownReflows.findIndex(reflow => path.startsWith(reflow.path));
+    if (index != -1) {
+      let reflow = knownReflows[index];
+      ++reflow.count;
+      reflow.actualStacks.set(stack, (reflow.actualStacks.get(stack) || 0) + 1);
+    } else {
+      unexpectedReflows.set(stack, (unexpectedReflows.get(stack) || 0) + 1);
+    }
+  }
+
+  let formatStack = stack =>
+    stack
+      .split("\n")
+      .slice(1)
+      .map(frame => "  " + frame)
+      .join("\n");
+  for (let reflow of knownReflows) {
+    let firstFrame = reflow.stack[0];
+    if (!reflow.count) {
+      Assert.ok(
+        false,
+        `Unused expected reflow at ${firstFrame}:\nStack:\n` +
+          reflow.stack.map(frame => "  " + frame).join("\n") +
+          "\n" +
+          "This is probably a good thing - just remove it from the list of reflows."
+      );
+    } else {
+      if (reflow.count > reflow.maxCount) {
+        Assert.ok(
+          false,
+          `reflow at ${firstFrame} was encountered ${reflow.count} times,\n` +
+            `it was expected to happen up to ${reflow.maxCount} times.`
+        );
+      } else {
+        todo(
+          false,
+          `known reflow at ${firstFrame} was encountered ${reflow.count} times`
+        );
+      }
+      for (let [stack, count] of reflow.actualStacks) {
+        info(
+          "Full stack" +
+            (count > 1 ? ` (hit ${count} times)` : "") +
+            ":\n" +
+            formatStack(stack)
+        );
+      }
+    }
+  }
+
+  for (let [stack, count] of unexpectedReflows) {
+    let location = stack.split("\n")[1].replace(/:\d+:\d+$/, "");
+    Assert.ok(
+      false,
+      `unexpected reflow at ${location} hit ${count} times\n` +
+        "Stack:\n" +
+        formatStack(stack)
+    );
+  }
+  Assert.ok(
+    !unexpectedReflows.size,
+    unexpectedReflows.size + " unexpected reflows"
+  );
+}
+
+async function ensureNoPreloadedBrowser(win = window) {
+  // 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.
+  NewTabPagePreloading.removePreloadedBrowser(win);
+
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.newtab.preload", false]],
+  });
+
+  AboutNewTab.newTabURL = "about:blank";
+
+  registerCleanupFunction(() => {
+    AboutNewTab.resetNewTabURL();
+  });
+}
+
+// Onboarding puts a badge on the fxa toolbar button a while after startup
+// which confuses tests that look at repaints in the toolbar.  Use this
+// function to cancel the badge update.
+function disableFxaBadge() {
+  let { ToolbarBadgeHub } = ChromeUtils.importESModule(
+    "resource:///modules/asrouter/ToolbarBadgeHub.sys.mjs"
+  );
+  ToolbarBadgeHub.removeAllNotifications();
+
+  // Also prevent a new timer from being set
+  return SpecialPowers.pushPrefEnv({
+    set: [["identity.fxaccounts.toolbar.accessed", true]],
+  });
+}
+
+function rectInBoundingClientRect(r, bcr) {
+  return (
+    bcr.x <= r.x1 &&
+    bcr.y <= r.y1 &&
+    bcr.x + bcr.width >= r.x2 &&
+    bcr.y + bcr.height >= r.y2
+  );
+}
+
+async function getBookmarksToolbarRect() {
+  // Temporarily open the bookmarks toolbar to measure its rect
+  let bookmarksToolbar = gNavToolbox.querySelector("#PersonalToolbar");
+  let wasVisible = !bookmarksToolbar.collapsed;
+  if (!wasVisible) {
+    setToolbarVisibility(bookmarksToolbar, true, false, false);
+    await TestUtils.waitForCondition(
+      () => bookmarksToolbar.getBoundingClientRect().height > 0,
+      "wait for non-zero bookmarks toolbar height"
+    );
+  }
+  let bookmarksToolbarRect = bookmarksToolbar.getBoundingClientRect();
+  if (!wasVisible) {
+    setToolbarVisibility(bookmarksToolbar, false, false, false);
+    await TestUtils.waitForCondition(
+      () => bookmarksToolbar.getBoundingClientRect().height == 0,
+      "wait for zero bookmarks toolbar height"
+    );
+  }
+  return bookmarksToolbarRect;
+}
+
+async function ensureAnimationsFinished(win = window) {
+  let animations = win.document.getAnimations();
+  info(`Waiting for ${animations.length} animations`);
+  await Promise.allSettled(animations.map(a => a.finished));
+}
+
+async function prepareSettledWindow() {
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  await ensureNoPreloadedBrowser(win);
+  await ensureAnimationsFinished(win);
+  return win;
+}
+
+/**
+ * Calculate and return how many additional tabs can be fit into the
+ * tabstrip without causing it to overflow.
+ *
+ * @return int
+ *         The maximum additional tabs that can be fit into the
+ *         tabstrip without causing it to overflow.
+ */
+function computeMaxTabCount() {
+  let currentTabCount = gBrowser.tabs.length;
+  let newTabButton = gBrowser.tabContainer.newTabButton;
+  let newTabRect = newTabButton.getBoundingClientRect();
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+  let availableTabStripWidth = tabStripRect.width - newTabRect.width;
+
+  let tabMinWidth = parseInt(
+    getComputedStyle(gBrowser.selectedTab, null).minWidth,
+    10
+  );
+
+  let maxTabCount =
+    Math.floor(availableTabStripWidth / tabMinWidth) - currentTabCount;
+  Assert.ok(
+    maxTabCount > 0,
+    "Tabstrip needs to be wide enough to accomodate at least 1 more tab " +
+      "without overflowing."
+  );
+  return maxTabCount;
+}
+
+/**
+ * Helper function that opens up some number of about:blank tabs, and wait
+ * until they're all fully open.
+ *
+ * @param howMany (int)
+ *        How many about:blank tabs to open.
+ */
+async function createTabs(howMany) {
+  let uris = [];
+  while (howMany--) {
+    uris.push("about:blank");
+  }
+
+  gBrowser.loadTabs(uris, {
+    inBackground: true,
+    triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+  });
+
+  await TestUtils.waitForCondition(() => {
+    return Array.from(gBrowser.tabs).every(tab => tab._fullyOpen);
+  });
+}
+
+/**
+ * Removes all of the tabs except the originally selected
+ * tab, and waits until all of the DOM nodes have been
+ * completely removed from the tab strip.
+ */
+async function removeAllButFirstTab() {
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.tabs.warnOnCloseOtherTabs", false]],
+  });
+  gBrowser.removeAllTabsBut(gBrowser.tabs[0]);
+  await TestUtils.waitForCondition(() => gBrowser.tabs.length == 1);
+  await SpecialPowers.popPrefEnv();
+}
+
+/**
+ * Adds some entries to the Places database so that we can
+ * do semi-realistic look-ups in the URL bar.
+ *
+ * @param searchStr (string)
+ *        Optional text to add to the search history items.
+ */
+async function addDummyHistoryEntries(searchStr = "") {
+  await PlacesUtils.history.clear();
+  const NUM_VISITS = 10;
+  let visits = [];
+
+  for (let i = 0; i < NUM_VISITS; ++i) {
+    visits.push({
+      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      uri: `http://example.com/urlbar-reflows-${i}`,
+      title: `Reflow test for URL bar entry #${i} - ${searchStr}`,
+    });
+  }
+
+  await PlacesTestUtils.addVisits(visits);
+
+  registerCleanupFunction(async function () {
+    await PlacesUtils.history.clear();
+  });
+}
+
+/**
+ * Async utility function to capture a screenshot of each painted frame.
+ *
+ * @param testPromise (Promise)
+ *        A promise that is resolved when the data collection should stop.
+ *
+ * @param win (browser window, optional)
+ *        The browser window to monitor. Defaults to the current window.
+ *
+ * @return An array of screenshots
+ */
+async function recordFrames(testPromise, win = window) {
+  let canvas = win.document.createElementNS(
+    "http://www.w3.org/1999/xhtml",
+    "canvas"
+  );
+  canvas.mozOpaque = true;
+  let ctx = canvas.getContext("2d", { alpha: false, willReadFrequently: true });
+
+  let frames = [];
+
+  let afterPaintListener = event => {
+    let width, height;
+    canvas.width = width = win.innerWidth;
+    canvas.height = height = win.innerHeight;
+    ctx.drawWindow(
+      win,
+      0,
+      0,
+      width,
+      height,
+      "white",
+      ctx.DRAWWINDOW_DO_NOT_FLUSH |
+        ctx.DRAWWINDOW_DRAW_VIEW |
+        ctx.DRAWWINDOW_ASYNC_DECODE_IMAGES |
+        ctx.DRAWWINDOW_USE_WIDGET_LAYERS
+    );
+    let data = Cu.cloneInto(ctx.getImageData(0, 0, width, height).data, {});
+    if (frames.length) {
+      // Compare this frame with the previous one to avoid storing duplicate
+      // frames and running out of memory.
+      let previous = frames[frames.length - 1];
+      if (previous.width == width && previous.height == height) {
+        let equals = true;
+        for (let i = 0; i < data.length; ++i) {
+          if (data[i] != previous.data[i]) {
+            equals = false;
+            break;
+          }
+        }
+        if (equals) {
+          return;
+        }
+      }
+    }
+    frames.push({ data, width, height });
+  };
+  win.addEventListener("MozAfterPaint", afterPaintListener);
+
+  // If the test is using an existing window, capture a frame immediately.
+  if (win.document.readyState == "complete") {
+    afterPaintListener();
+  }
+
+  try {
+    await testPromise;
+  } finally {
+    win.removeEventListener("MozAfterPaint", afterPaintListener);
+  }
+
+  return frames;
+}
+
+// How many identical pixels to accept between 2 rects when deciding to merge
+// them.
+const kMaxEmptyPixels = 3;
+function compareFrames(frame, previousFrame) {
+  // Accessing the Math global is expensive as the test executes in a
+  // non-syntactic scope. Accessing it as a lexical variable is enough
+  // to make the code JIT well.
+  const M = Math;
+
+  function expandRect(x, y, rect) {
+    if (rect.x2 < x) {
+      rect.x2 = x;
+    } else if (rect.x1 > x) {
+      rect.x1 = x;
+    }
+    if (rect.y2 < y) {
+      rect.y2 = y;
+    }
+  }
+
+  function isInRect(x, y, rect) {
+    return (
+      (rect.y2 == y || rect.y2 == y - 1) && rect.x1 - 1 <= x && x <= rect.x2 + 1
+    );
+  }
+
+  if (
+    frame.height != previousFrame.height ||
+    frame.width != previousFrame.width
+  ) {
+    // If the frames have different sizes, assume the whole window has
+    // been repainted when the window was resized.
+    return [{ x1: 0, x2: frame.width, y1: 0, y2: frame.height }];
+  }
+
+  let l = frame.data.length;
+  let different = [];
+  let rects = [];
+  for (let i = 0; i < l; i += 4) {
+    let x = (i / 4) % frame.width;
+    let y = M.floor(i / 4 / frame.width);
+    for (let j = 0; j < 4; ++j) {
+      let index = i + j;
+
+      if (frame.data[index] != previousFrame.data[index]) {
+        let found = false;
+        for (let rect of rects) {
+          if (isInRect(x, y, rect)) {
+            expandRect(x, y, rect);
+            found = true;
+            break;
+          }
+        }
+        if (!found) {
+          rects.unshift({ x1: x, x2: x, y1: y, y2: y });
+        }
+
+        different.push(i);
+        break;
+      }
+    }
+  }
+  rects.reverse();
+
+  // The following code block merges rects that are close to each other
+  // (less than kMaxEmptyPixels away).
+  // This is needed to avoid having a rect for each letter when a label moves.
+  let areRectsContiguous = function (r1, r2) {
+    return (
+      r1.y2 >= r2.y1 - 1 - kMaxEmptyPixels &&
+      r2.x1 - 1 - kMaxEmptyPixels <= r1.x2 &&
+      r2.x2 >= r1.x1 - 1 - kMaxEmptyPixels
+    );
+  };
+  let hasMergedRects;
+  do {
+    hasMergedRects = false;
+    for (let r = rects.length - 1; r > 0; --r) {
+      let rr = rects[r];
+      for (let s = r - 1; s >= 0; --s) {
+        let rs = rects[s];
+        if (areRectsContiguous(rs, rr)) {
+          rs.x1 = Math.min(rs.x1, rr.x1);
+          rs.y1 = Math.min(rs.y1, rr.y1);
+          rs.x2 = Math.max(rs.x2, rr.x2);
+          rs.y2 = Math.max(rs.y2, rr.y2);
+          rects.splice(r, 1);
+          hasMergedRects = true;
+          break;
+        }
+      }
+    }
+  } while (hasMergedRects);
+
+  // For convenience, pre-compute the width and height of each rect.
+  rects.forEach(r => {
+    r.w = r.x2 - r.x1 + 1;
+    r.h = r.y2 - r.y1 + 1;
+  });
+
+  return rects;
+}
+
+function dumpFrame({ data, width, height }) {
+  let canvas = document.createElementNS(
+    "http://www.w3.org/1999/xhtml",
+    "canvas"
+  );
+  canvas.mozOpaque = true;
+  canvas.width = width;
+  canvas.height = height;
+
+  canvas
+    .getContext("2d", { alpha: false, willReadFrequently: true })
+    .putImageData(new ImageData(data, width, height), 0, 0);
+
+  info(canvas.toDataURL());
+}
+
+/**
+ * Utility function to report unexpected changed areas on screen.
+ *
+ * @param frames (Array)
+ *        An array of frames captured by recordFrames.
+ *
+ * @param expectations (Object)
+ *        An Object indicating which changes on screen are expected.
+ *        If can contain the following optional fields:
+ *         - filter: a function used to exclude changed rects that are expected.
+ *            It takes the following parameters:
+ *             - rects: an array of changed rects
+ *             - frame: the current frame
+ *             - previousFrame: the previous frame
+ *            It returns an array of rects. This array is typically a copy of
+ *            the rects parameter, from which identified expected changes have
+ *            been excluded.
+ *         - exceptions: an array of objects describing known flicker bugs.
+ *           Example:
+ *             exceptions: [
+ *               {name: "bug 1nnnnnn - the foo icon shouldn't flicker",
+ *                condition: r => r.w == 14 && r.y1 == 0 && ... }
+ *               },
+ *               {name: "bug ...
+ *             ]
+ */
+function reportUnexpectedFlicker(frames, expectations) {
+  info("comparing " + frames.length + " frames");
+
+  let unexpectedRects = 0;
+  for (let i = 1; i < frames.length; ++i) {
+    let frame = frames[i],
+      previousFrame = frames[i - 1];
+    let rects = compareFrames(frame, previousFrame);
+
+    let rectText = r => `${r.toSource()}, window width: ${frame.width}`;
+
+    rects = rects.filter(rect => {
+      for (let e of expectations.exceptions || []) {
+        if (e.condition(rect)) {
+          todo(false, e.name + ", " + rectText(rect));
+          return false;
+        }
+      }
+      return true;
+    });
+
+    if (expectations.filter) {
+      rects = expectations.filter(rects, frame, previousFrame);
+    }
+
+    if (!rects.length) {
+      continue;
+    }
+
+    ok(
+      false,
+      `unexpected ${rects.length} changed rects: ${rects
+        .map(rectText)
+        .join(", ")}`
+    );
+
+    // Before dumping a frame with unexpected differences for the first time,
+    // ensure at least one previous frame has been logged so that it's possible
+    // to see the differences when examining the log.
+    if (!unexpectedRects) {
+      dumpFrame(previousFrame);
+    }
+    unexpectedRects += rects.length;
+    dumpFrame(frame);
+  }
+  is(unexpectedRects, 0, "should have 0 unknown flickering areas");
+}
+
+/**
+ * This is the main function that performance tests in this folder will call.
+ *
+ * The general idea is that individual tests provide a test function (testFn)
+ * that will perform some user interactions we care about (eg. open a tab), and
+ * this withPerfObserver function takes care of setting up and removing the
+ * observers and listener we need to detect common performance issues.
+ *
+ * Once testFn is done, withPerfObserver will analyse the collected data and
+ * report anything unexpected.
+ *
+ * @param testFn (async function)
+ *        An async function that exercises some part of the browser UI.
+ *
+ * @param exceptions (object, optional)
+ *        An Array of Objects representing expectations and known issues.
+ *        It can contain the following fields:
+ *         - expectedReflows: an array of expected reflow stacks.
+ *           (see the comment above reportUnexpectedReflows for an example)
+ *         - frames: an object setting expectations for what will change
+ *           on screen during the test, and the known flicker bugs.
+ *           (see the comment above reportUnexpectedFlicker for an example)
+ */
+async function withPerfObserver(testFn, exceptions = {}, win = window) {
+  let resolveFn, rejectFn;
+  let promiseTestDone = new Promise((resolve, reject) => {
+    resolveFn = resolve;
+    rejectFn = reject;
+  });
+
+  let promiseReflows = recordReflows(promiseTestDone, win);
+  let promiseFrames = recordFrames(promiseTestDone, win);
+
+  testFn().then(resolveFn, rejectFn);
+  await promiseTestDone;
+
+  let reflows = await promiseReflows;
+  reportUnexpectedReflows(reflows, exceptions.expectedReflows);
+
+  let frames = await promiseFrames;
+  reportUnexpectedFlicker(frames, exceptions.frames);
+}
+
+/**
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when typing into the URL bar
+ * with the default values in Places.
+ *
+ * @param {bool} keyed
+ *        Pass true to synthesize typing the search string one key at a time.
+ * @param {array} expectedReflowsFirstOpen
+ *        The array of expected reflow stacks when the panel is first opened.
+ * @param {array} [expectedReflowsSecondOpen]
+ *        The array of expected reflow stacks when the panel is subsequently
+ *        opened, if you're testing opening the panel twice.
+ */
+async function runUrlbarTest(
+  keyed,
+  expectedReflowsFirstOpen,
+  expectedReflowsSecondOpen = null
+) {
+  const SEARCH_TERM = keyed ? "" : "urlbar-reflows-" + Date.now();
+  await addDummyHistoryEntries(SEARCH_TERM);
+
+  let win = await prepareSettledWindow();
+
+  let URLBar = win.gURLBar;
+
+  URLBar.focus();
+  URLBar.value = SEARCH_TERM;
+  let testFn = async function () {
+    let popup = URLBar.view;
+    let oldOnQueryResults = popup.onQueryResults.bind(popup);
+    let oldOnQueryFinished = popup.onQueryFinished.bind(popup);
+
+    // We need to invalidate the frame tree outside of the normal
+    // mechanism since invalidations and result additions to the
+    // URL bar occur without firing JS events (which is how we
+    // normally know to dirty the frame tree).
+    popup.onQueryResults = context => {
+      dirtyFrame(win);
+      oldOnQueryResults(context);
+    };
+
+    popup.onQueryFinished = context => {
+      dirtyFrame(win);
+      oldOnQueryFinished(context);
+    };
+
+    let waitExtra = async () => {
+      // There are several setTimeout(fn, 0); calls inside autocomplete.xml
+      // that we need to wait for. Since those have higher priority than
+      // idle callbacks, we can be sure they will have run once this
+      // idle callback is called. The timeout seems to be required in
+      // automation - presumably because the machines can be pretty busy
+      // especially if it's GC'ing from previous tests.
+      await new Promise(resolve =>
+        win.requestIdleCallback(resolve, { timeout: 1000 })
+      );
+    };
+
+    if (keyed) {
+      // Only keying in 6 characters because the number of reflows triggered
+      // is so high that we risk timing out the test if we key in any more.
+      let searchTerm = "ows-10";
+      for (let i = 0; i < searchTerm.length; ++i) {
+        let char = searchTerm[i];
+        EventUtils.synthesizeKey(char, {}, win);
+        await UrlbarTestUtils.promiseSearchComplete(win);
+        await waitExtra();
+      }
+    } else {
+      await UrlbarTestUtils.promiseAutocompleteResultPopup({
+        window: win,
+        waitForFocus: SimpleTest.waitForFocus,
+        value: URLBar.value,
+      });
+      await waitExtra();
+    }
+
+    await UrlbarTestUtils.promisePopupClose(win);
+  };
+
+  let urlbarRect = URLBar.textbox.getBoundingClientRect();
+  const SHADOW_SIZE = 17;
+  let expectedRects = {
+    filter: rects => {
+      // We put text into the urlbar so expect its textbox to change.
+      // We expect many changes in the results view.
+      // So we just allow changes anywhere in the urlbar. We don't check the
+      // bottom of the rect because the result view height varies depending on
+      // the results.
+      // We use floor/ceil because the Urlbar dimensions aren't always
+      // integers.
+      return rects.filter(
+        r =>
+          !(
+            r.x1 >= Math.floor(urlbarRect.left) - SHADOW_SIZE &&
+            r.x2 <= Math.ceil(urlbarRect.right) + SHADOW_SIZE &&
+            r.y1 >= Math.floor(urlbarRect.top) - SHADOW_SIZE
+          )
+      );
+    },
+  };
+
+  info("First opening");
+  await withPerfObserver(
+    testFn,
+    { expectedReflows: expectedReflowsFirstOpen, frames: expectedRects },
+    win
+  );
+
+  if (expectedReflowsSecondOpen) {
+    info("Second opening");
+    await withPerfObserver(
+      testFn,
+      { expectedReflows: expectedReflowsSecondOpen, frames: expectedRects },
+      win
+    );
+  }
+
+  await BrowserTestUtils.closeWindow(win);
+}
+
+/**
+ * Helper method for checking which scripts are loaded on content process
+ * startup, used by `browser_startup_content.js` and
+ * `browser_startup_content_subframe.js`.
+ *
+ * Parameters to this function are passed in an object literal to avoid
+ * confusion about parameter order.
+ *
+ * @param loadedInfo (Object)
+ *        Mapping from script type to a set of scripts which have been loaded
+ *        of that type.
+ *
+ * @param known (Object)
+ *        Mapping from script type to a set of scripts which must have been
+ *        loaded of that type.
+ *
+ * @param intermittent (Object)
+ *        Mapping from script type to a set of scripts which may have been
+ *        loaded of that type. There must be a script type map for every type
+ *        in `known`.
+ *
+ * @param forbidden (Object)
+ *        Mapping from script type to a set of scripts which must not have been
+ *        loaded of that type.
+ *
+ * @param dumpAllStacks (bool)
+ *        If true, dump the stacks for all loaded modules. Makes the output
+ *        noisy.
+ */
+async function checkLoadedScripts({
+  loadedInfo,
+  known,
+  intermittent,
+  forbidden,
+  dumpAllStacks,
+}) {
+  let loadedList = {};
+
+  async function checkAllExist(scriptType, list, listType) {
+    if (scriptType == "services") {
+      for (let contract of list) {
+        ok(
+          contract in Cc,
+          `${listType} entry ${contract} for content process startup must exist`
+        );
+      }
+    } else {
+      let results = await PerfTestHelpers.throttledMapPromises(
+        list,
+        async uri => ({
+          uri,
+          exists: await PerfTestHelpers.checkURIExists(uri),
+        })
+      );
+
+      for (let { uri, exists } of results) {
+        ok(
+          exists,
+          `${listType} entry ${uri} for content process startup must exist`
+        );
+      }
+    }
+  }
+
+  for (let scriptType in known) {
+    loadedList[scriptType] = Object.keys(loadedInfo[scriptType]).filter(c => {
+      if (!known[scriptType].has(c)) {
+        return true;
+      }
+      known[scriptType].delete(c);
+      return false;
+    });
+
+    loadedList[scriptType] = loadedList[scriptType].filter(c => {
+      return !intermittent[scriptType].has(c);
+    });
+
+    if (loadedList[scriptType].length) {
+      console.log("Unexpected scripts:", loadedList[scriptType]);
+    }
+    is(
+      loadedList[scriptType].length,
+      0,
+      `should have no unexpected ${scriptType} loaded on content process startup`
+    );
+
+    for (let script of loadedList[scriptType]) {
+      record(
+        false,
+        `Unexpected ${scriptType} loaded during content process startup: ${script}`,
+        undefined,
+        loadedInfo[scriptType][script]
+      );
+    }
+
+    await checkAllExist(scriptType, intermittent[scriptType], "intermittent");
+
+    is(
+      known[scriptType].size,
+      0,
+      `all known ${scriptType} scripts should have been loaded`
+    );
+
+    for (let script of known[scriptType]) {
+      ok(
+        false,
+        `${scriptType} is expected to load for content process startup but wasn't: ${script}`
+      );
+    }
+
+    if (dumpAllStacks) {
+      info(`Stacks for all loaded ${scriptType}:`);
+      for (let file in loadedInfo[scriptType]) {
+        if (loadedInfo[scriptType][file]) {
+          info(
+            `${file}\n------------------------------------\n` +
+              loadedInfo[scriptType][file] +
+              "\n"
+          );
+        }
+      }
+    }
+  }
+
+  for (let scriptType in forbidden) {
+    for (let script of forbidden[scriptType]) {
+      let loaded = script in loadedInfo[scriptType];
+      if (loaded) {
+        record(
+          false,
+          `Forbidden ${scriptType} loaded during content process startup: ${script}`,
+          undefined,
+          loadedInfo[scriptType][script]
+        );
+      }
+    }
+
+    await checkAllExist(scriptType, forbidden[scriptType], "forbidden");
+  }
+}
+
+// The first screenshot we get in OSX / Windows shows an unfocused browser
+// window for some reason. See bug 1445161. This function allows to deal with
+// that in a central place.
+function isLikelyFocusChange(rects, frame) {
+  if (rects.length > 3 && rects.every(r => r.y2 < 100)) {
+    // There are at least 4 areas that changed near the top of the screen.
+    // Note that we need a bit more leeway than the titlebar height, because on
+    // OSX other toolbarbuttons in the navigation toolbar also get disabled
+    // state.
+    return true;
+  }
+  if (
+    rects.every(r => r.y1 == 0 && r.x1 == 0 && r.w == frame.width && r.y2 < 100)
+  ) {
+    // Full-width rect in the top of the titlebar.
+    return true;
+  }
+  return false;
+}
diff --git a/browser/base/content/test/performance/hidpi/browser.toml b/browser/base/content/test/performance/hidpi/browser.toml
new file mode 100644
index 0000000000..afcc961963
--- /dev/null
+++ b/browser/base/content/test/performance/hidpi/browser.toml
@@ -0,0 +1,8 @@
+[DEFAULT]
+prefs = [
+  "browser.startup.recordImages=true",
+  "layout.css.devPixelsPerPx='2'",
+]
+
+["../browser_startup_images.js"]
+skip-if = ["!debug"]
diff --git a/browser/base/content/test/performance/io/browser.toml b/browser/base/content/test/performance/io/browser.toml
new file mode 100644
index 0000000000..e581849028
--- /dev/null
+++ b/browser/base/content/test/performance/io/browser.toml
@@ -0,0 +1,38 @@
+[DEFAULT]
+# Currently disabled on debug due to debug-only failures, see bug 1549723.
+# Disabled on Linux asan due to bug 1549729.
+# Disabled on Windows asan due to intermittent startup hangs, bug 1629824.
+skip-if = [
+  "debug",
+  "tsan",
+  "asan",
+]
+# to avoid overhead when running the browser normally, StartupRecorder.sys.mjs will
+# do almost nothing unless browser.startup.record is true.
+# gfx.canvas.willReadFrequently.enable is just an optimization, but needs to be
+# set during early startup to have an impact as a canvas will be used by
+# StartupRecorder.sys.mjs
+prefs = [
+  "browser.startup.record=true",
+  "gfx.canvas.willReadFrequently.enable=true",
+  "extensions.screenshots.disabled=false", # The Screenshots extension is disabled by default in Mochitests. We re-enable it here, since it's a more realistic configuration.
+]
+environment = [
+  "GNOME_ACCESSIBILITY=0",
+  "MOZ_PROFILER_STARTUP=1",
+  "MOZ_PROFILER_STARTUP_PERFORMANCE_TEST=1",
+  "MOZ_PROFILER_STARTUP_FEATURES=js,mainthreadio",
+  "MOZ_PROFILER_STARTUP_ENTRIES=10000000",
+]
+
+["../browser_startup_content_mainthreadio.js"]
+
+["../browser_startup_mainthreadio.js"]
+skip-if = [
+  "apple_silicon", # bug 1707724
+  "socketprocess_networking",
+  "win11_2009 && bits == 32",
+  "os == 'win' && msix", # Bug 1833639
+]
+
+["../browser_startup_syncIPC.js"]
diff --git a/browser/base/content/test/performance/lowdpi/browser.toml b/browser/base/content/test/performance/lowdpi/browser.toml
new file mode 100644
index 0000000000..391bff58af
--- /dev/null
+++ b/browser/base/content/test/performance/lowdpi/browser.toml
@@ -0,0 +1,8 @@
+[DEFAULT]
+prefs = [
+  "browser.startup.recordImages=true",
+  "layout.css.devPixelsPerPx='1'",
+]
+
+["../browser_startup_images.js"]
+skip-if = ["!debug"]
diff --git a/browser/base/content/test/performance/moz.build b/browser/base/content/test/performance/moz.build
new file mode 100644
index 0000000000..dce33b938d
--- /dev/null
+++ b/browser/base/content/test/performance/moz.build
@@ -0,0 +1,17 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+
+BROWSER_CHROME_MANIFESTS += [
+    "browser.toml",
+    "hidpi/browser.toml",
+    "io/browser.toml",
+    "lowdpi/browser.toml",
+]
+
+TESTING_JS_MODULES += [
+    "PerfTestHelpers.sys.mjs",
+]
diff --git a/browser/base/content/test/performance/triage.json b/browser/base/content/test/performance/triage.json
new file mode 100644
index 0000000000..e5a8051bd2
--- /dev/null
+++ b/browser/base/content/test/performance/triage.json
@@ -0,0 +1,70 @@
+{
+  "triagers": {
+    "Gijs": {
+      "bzmail": "gijskruitbosch+bugs@gmail.com"
+    },
+    "Mike Conley": {
+      "bzmail": "mconley@mozilla.com"
+    },
+    "Florian Quèze": {
+      "bzmail": "florian@mozilla.com"
+    },
+    "Alex Thayer": {
+      "bzmail": "dothayer@mozilla.com"
+    }
+  },
+  "duty-start-dates": {
+    "2023-12-07": "Mike Conley",
+    "2023-12-14": "Florian Quèze",
+    "2023-12-21": "Alex Thayer",
+    "2023-12-28": "Gijs Kruitbosch",
+    "2024-01-04": "Mike Conley",
+    "2024-01-11": "Florian Quèze",
+    "2024-01-18": "Alex Thayer",
+    "2024-01-25": "Gijs Kruitbosch",
+    "2024-02-01": "Mike Conley",
+    "2024-02-08": "Florian Quèze",
+    "2024-02-15": "Alex Thayer",
+    "2024-02-22": "Gijs Kruitbosch",
+    "2024-02-29": "Mike Conley",
+    "2024-03-07": "Florian Quèze",
+    "2024-03-14": "Alex Thayer",
+    "2024-03-21": "Gijs Kruitbosch",
+    "2024-03-28": "Mike Conley",
+    "2024-04-04": "Florian Quèze",
+    "2024-04-11": "Alex Thayer",
+    "2024-04-18": "Gijs Kruitbosch",
+    "2024-04-25": "Mike Conley",
+    "2024-05-02": "Florian Quèze",
+    "2024-05-09": "Alex Thayer",
+    "2024-05-16": "Gijs Kruitbosch",
+    "2024-05-23": "Mike Conley",
+    "2024-05-30": "Florian Quèze",
+    "2024-06-06": "Alex Thayer",
+    "2024-06-13": "Gijs Kruitbosch",
+    "2024-06-20": "Mike Conley",
+    "2024-06-27": "Florian Quèze",
+    "2024-07-04": "Alex Thayer",
+    "2024-07-11": "Gijs Kruitbosch",
+    "2024-07-18": "Mike Conley",
+    "2024-07-25": "Florian Quèze",
+    "2024-08-01": "Alex Thayer",
+    "2024-08-08": "Gijs Kruitbosch",
+    "2024-08-15": "Mike Conley",
+    "2024-08-22": "Florian Quèze",
+    "2024-08-29": "Alex Thayer",
+    "2024-09-05": "Gijs Kruitbosch",
+    "2024-09-12": "Mike Conley",
+    "2024-09-19": "Florian Quèze",
+    "2024-09-26": "Alex Thayer",
+    "2024-10-03": "Gijs Kruitbosch",
+    "2024-10-10": "Mike Conley",
+    "2024-10-17": "Florian Quèze",
+    "2024-10-24": "Alex Thayer",
+    "2024-10-31": "Gijs Kruitbosch",
+    "2024-11-07": "Mike Conley",
+    "2024-11-14": "Florian Quèze",
+    "2024-11-21": "Gijs Kruitbosch",
+    "2024-11-28": "Alex Thayer"
+  }
+}