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diff --git a/browser/base/content/test/performance/browser_startup_content_subframe.js b/browser/base/content/test/performance/browser_startup_content_subframe.js
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+++ b/browser/base/content/test/performance/browser_startup_content_subframe.js
@@ -0,0 +1,150 @@
+/* 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
+    "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,
+  });
+});