diff --git a/browser/app/Makefile.in b/browser/app/Makefile.in
index bdae83a868db..5073ce31afee 100644
--- a/browser/app/Makefile.in
+++ b/browser/app/Makefile.in
@@ -83,7 +83,7 @@ clean clobber repackage::
 MAC_BUNDLE_VERSION = $(shell $(PYTHON) $(srcdir)/macversion.py --version=$(MOZ_APP_VERSION) --buildid=$(DEPTH)/buildid.h)
 
 .PHONY: repackage
-tools repackage:: $(DIST)/bin/$(MOZ_APP_NAME) features
+tools repackage:: $(DIST)/bin/$(MOZ_APP_NAME)
 	rm -rf $(dist_dest)
 	$(MKDIR) -p $(dist_dest)/Contents/MacOS
 	$(MKDIR) -p $(dist_dest)/$(LPROJ)
@@ -104,7 +104,3 @@ ifdef MOZ_UPDATER
 endif
 	printf APPLMOZB > $(dist_dest)/Contents/PkgInfo
 endif
-
-.PHONY: features
-tools features::
-	$(PYTHON) -c 'import os, json; listing = {"system": os.listdir("$(DIST)/bin/browser/features")}; print json.dumps(listing)' > $(DIST)/bin/browser/chrome/browser/content/browser/built_in_addons.json
diff --git a/config/faster/rules.mk b/config/faster/rules.mk
index 56db1c6429af..e55c84841d14 100644
--- a/config/faster/rules.mk
+++ b/config/faster/rules.mk
@@ -110,9 +110,3 @@ endif
 $(TOPOBJDIR)/config/makefiles/xpidl/xpidl: $(addprefix install-,$(filter dist/bin%,$(INSTALL_MANIFESTS)))
 
 $(TOPOBJDIR)/build/application.ini: $(TOPOBJDIR)/buildid.h $(TOPOBJDIR)/source-repo.h
-
-# The manifest of allowed system add-ons should be re-built when using
-# "build faster".
-ifeq ($(MOZ_BUILD_APP),browser/app)
-default: $(TOPOBJDIR)/browser/app/features
-endif
diff --git a/js/xpconnect/idl/xpccomponents.idl b/js/xpconnect/idl/xpccomponents.idl
index a82f11e2d2d8..4cace8db96a0 100644
--- a/js/xpconnect/idl/xpccomponents.idl
+++ b/js/xpconnect/idl/xpccomponents.idl
@@ -15,7 +15,6 @@ interface nsIClassInfo;
 interface nsIComponentManager;
 interface nsICycleCollectorListener;
 interface nsIFile;
-interface nsIURI;
 interface nsIJSCID;
 interface nsIJSIID;
 interface nsIPrincipal;
@@ -702,12 +701,6 @@ interface nsIXPCComponents_Utils : nsISupports
      * startup so its contents will be available the next time they're read.
      */
     ACString readFile(in nsIFile file);
-
-    /*
-     * Reads the given local file URL and returns its contents. This has the
-     * same semantics of readFile.
-     */
-    ACString readURI(in nsIURI url);
 };
 
 /**
diff --git a/js/xpconnect/loader/URLPreloader.cpp b/js/xpconnect/loader/URLPreloader.cpp
index b9876d86938b..e7ddaeae216a 100644
--- a/js/xpconnect/loader/URLPreloader.cpp
+++ b/js/xpconnect/loader/URLPreloader.cpp
@@ -83,20 +83,22 @@ URLPreloader::CollectReports(nsIHandleReportCallback* aHandleReport,
     return NS_OK;
 }
 
+
 URLPreloader&
 URLPreloader::GetSingleton()
 {
-    if (!sSingleton) {
-        sSingleton = new URLPreloader();
-        ClearOnShutdown(&sSingleton);
+    static RefPtr<URLPreloader> singleton;
+
+    if (!singleton) {
+        singleton = new URLPreloader();
+        ClearOnShutdown(&singleton);
     }
 
-    return *sSingleton;
+    return *singleton;
 }
 
-bool URLPreloader::sInitialized = false;
 
-StaticRefPtr<URLPreloader> URLPreloader::sSingleton;
+bool URLPreloader::sInitialized = false;
 
 URLPreloader::URLPreloader()
 {
@@ -154,14 +156,6 @@ URLPreloader::InitInternal()
     return Ok();
 }
 
-URLPreloader&
-URLPreloader::ReInitialize()
-{
-    sSingleton = new URLPreloader();
-
-    return *sSingleton;
-}
-
 Result<nsCOMPtr<nsIFile>, nsresult>
 URLPreloader::GetCacheFile(const nsAString& suffix)
 {
diff --git a/js/xpconnect/loader/URLPreloader.h b/js/xpconnect/loader/URLPreloader.h
index 0fadc8dd2d75..8825a02db980 100644
--- a/js/xpconnect/loader/URLPreloader.h
+++ b/js/xpconnect/loader/URLPreloader.h
@@ -96,18 +96,13 @@ private:
 
     static bool sInitialized;
 
-    static mozilla::StaticRefPtr<URLPreloader> sSingleton;
-
 protected:
-    friend class AddonManagerStartup;
     friend class ScriptPreloader;
 
     virtual ~URLPreloader();
 
     Result<Ok, nsresult> WriteCache();
 
-    static URLPreloader& ReInitialize();
-
     // Clear leftover entries after the cache has been written.
     void Cleanup();
 
diff --git a/js/xpconnect/src/XPCComponents.cpp b/js/xpconnect/src/XPCComponents.cpp
index be26fc262315..4f0c10001811 100644
--- a/js/xpconnect/src/XPCComponents.cpp
+++ b/js/xpconnect/src/XPCComponents.cpp
@@ -3397,15 +3397,6 @@ nsXPCComponents_Utils::ReadFile(nsIFile* aFile, nsACString& aResult)
     return NS_OK;
 }
 
-NS_IMETHODIMP
-nsXPCComponents_Utils::ReadURI(nsIURI* aURI, nsACString& aResult)
-{
-    NS_ENSURE_TRUE(aURI, NS_ERROR_INVALID_ARG);
-
-    MOZ_TRY_VAR(aResult, URLPreloader::ReadURI(aURI));
-    return NS_OK;
-}
-
 NS_IMETHODIMP
 nsXPCComponents_Utils::Now(double* aRetval)
 {
diff --git a/services/sync/tests/unit/test_addons_store.js b/services/sync/tests/unit/test_addons_store.js
index f5a823650d44..ab5a53dec88a 100644
--- a/services/sync/tests/unit/test_addons_store.js
+++ b/services/sync/tests/unit/test_addons_store.js
@@ -37,7 +37,6 @@ function loadSystemAddon() {
 
 loadAddonTestFunctions();
 loadSystemAddon();
-awaitPromise(overrideBuiltIns({ "system": [SYSTEM_ADDON_ID] }));
 startupManager();
 
 let engine;
diff --git a/toolkit/components/telemetry/tests/unit/head.js b/toolkit/components/telemetry/tests/unit/head.js
index 161201fc8bef..558df77f8c4a 100644
--- a/toolkit/components/telemetry/tests/unit/head.js
+++ b/toolkit/components/telemetry/tests/unit/head.js
@@ -179,7 +179,7 @@ function wrapWithExceptionHandler(f) {
   return wrapper;
 }
 
-async function loadAddonManager(...args) {
+function loadAddonManager(...args) {
   AddonTestUtils.init(gGlobalScope);
   AddonTestUtils.overrideCertDB();
   createAppInfo(...args);
@@ -188,8 +188,6 @@ async function loadAddonManager(...args) {
   // used by system add-ons.
   const distroDir = FileUtils.getDir("ProfD", ["sysfeatures", "app0"], true);
   AddonTestUtils.registerDirectory("XREAppFeat", distroDir);
-  await AddonTestUtils.overrideBuiltIns(
-          {"system": ["tel-system-xpi@tests.mozilla.org"]});
   return AddonTestUtils.promiseStartupManager();
 }
 
diff --git a/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js b/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
index 55d68f2da392..97de62ac0262 100644
--- a/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryEnvironment.js
@@ -858,7 +858,7 @@ add_task(async function setup() {
   let system_addon = FileUtils.File(distroDir.path);
   system_addon.append("tel-system-xpi@tests.mozilla.org.xpi");
   system_addon.lastModifiedTime = SYSTEM_ADDON_INSTALL_DATE;
-  await loadAddonManager(APP_ID, APP_NAME, APP_VERSION, PLATFORM_VERSION);
+  loadAddonManager(APP_ID, APP_NAME, APP_VERSION, PLATFORM_VERSION);
 
   // Spoof the persona ID.
   LightweightThemeManager.currentTheme =
diff --git a/toolkit/mozapps/extensions/AddonManagerStartup.cpp b/toolkit/mozapps/extensions/AddonManagerStartup.cpp
index 0c18fd0adefc..23ce692dae3b 100644
--- a/toolkit/mozapps/extensions/AddonManagerStartup.cpp
+++ b/toolkit/mozapps/extensions/AddonManagerStartup.cpp
@@ -745,15 +745,6 @@ AddonManagerStartup::Reset()
   return NS_OK;
 }
 
-nsresult
-AddonManagerStartup::InitializeURLPreloader()
-{
-  MOZ_RELEASE_ASSERT(xpc::IsInAutomation());
-
-  URLPreloader::ReInitialize();
-
-  return NS_OK;
-}
 
 /******************************************************************************
  * RegisterChrome
diff --git a/toolkit/mozapps/extensions/amIAddonManagerStartup.idl b/toolkit/mozapps/extensions/amIAddonManagerStartup.idl
index 82d63779986d..dfbec7b75821 100644
--- a/toolkit/mozapps/extensions/amIAddonManagerStartup.idl
+++ b/toolkit/mozapps/extensions/amIAddonManagerStartup.idl
@@ -75,13 +75,5 @@ interface amIAddonManagerStartup : nsISupports
    * NOT FOR USE OUTSIDE OF UNIT TESTS.
    */
   void reset();
-
-  /**
-   * Initializes the URL Preloader.
-   *
-   * NOT FOR USE OUTSIDE OF UNIT TESTS.
-   */
-  void initializeURLPreloader();
-
 };
 
diff --git a/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm b/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
index 903ca4cf4230..e9834973d8a9 100644
--- a/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
+++ b/toolkit/mozapps/extensions/internal/AddonTestUtils.jsm
@@ -220,7 +220,6 @@ var AddonTestUtils = {
   testUnpacked: false,
   useRealCertChecks: false,
   usePrivilegedSignatures: true,
-  aomStartup: null,
 
   init(testScope) {
     this.testScope = testScope;
@@ -1274,28 +1273,6 @@ var AddonTestUtils = {
       });
     });
   },
-
-  /**
-   * Override chrome URL for specifying allowed built-in add-ons.
-   *
-   * @param {object} data - An object specifying which add-on IDs are permitted
-   *                        to load, for instance: { "system": ["id1", "..."] }
-   */
-  async overrideBuiltIns(data) {
-    // We need to set this in order load the URL preloader service, which
-    // is only possible when running in automation.
-    Services.prefs.setBoolPref(PREF_DISABLE_SECURITY, true);
-    aomStartup.initializeURLPreloader();
-
-    let file = FileUtils.getFile("TmpD", "override.txt");
-    let manifest = Services.io.newFileURI(file);
-    await OS.File.writeAtomic(file.path,
-      new TextEncoder().encode(JSON.stringify(data)));
-    this.aomStartup = aomStartup.registerChrome(manifest, [
-      ["override", "chrome://browser/content/built_in_addons.json",
-       Services.io.newFileURI(file).spec],
-    ]);
-  }
 };
 
 for (let [key, val] of Object.entries(AddonTestUtils)) {
diff --git a/toolkit/mozapps/extensions/internal/XPIProvider.jsm b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
index d9b7fd6306a6..fa5ed01d08c7 100644
--- a/toolkit/mozapps/extensions/internal/XPIProvider.jsm
+++ b/toolkit/mozapps/extensions/internal/XPIProvider.jsm
@@ -102,9 +102,6 @@ const PREF_EM_MIN_COMPAT_PLATFORM_VERSION = "extensions.minCompatiblePlatformVer
 const PREF_EM_HOTFIX_ID               = "extensions.hotfix.id";
 const PREF_EM_LAST_APP_BUILD_ID       = "extensions.lastAppBuildId";
 
-// Specify a list of valid built-in add-ons to load.
-const BUILT_IN_ADDONS_URI             = "chrome://browser/content/built_in_addons.json";
-
 const OBSOLETE_PREFERENCES = [
   "extensions.bootstrappedAddons",
   "extensions.enabledAddons",
@@ -2042,18 +2039,6 @@ this.XPIProvider = {
       XPIProvider.installLocationsByName[location.name] = location;
     }
 
-    function addBuiltInInstallLocation(name, key, paths, scope) {
-      try {
-        let dir = FileUtils.getDir(key, paths);
-        let location = new BuiltInInstallLocation(name, dir, scope);
-
-        XPIProvider.installLocations.push(location);
-        XPIProvider.installLocationsByName[location.name] = location;
-      } catch (e) {
-        logger.warn(`Failed to add built-in install location ${name}`, e);
-      }
-    }
-
     function addSystemAddonInstallLocation(aName, aKey, aPaths, aScope) {
       try {
         var dir = FileUtils.getDir(aKey, aPaths);
@@ -2122,8 +2107,8 @@ this.XPIProvider = {
                                     [DIR_SYSTEM_ADDONS],
                                     AddonManager.SCOPE_PROFILE);
 
-      addBuiltInInstallLocation(KEY_APP_SYSTEM_DEFAULTS, KEY_APP_FEATURES,
-                                [], AddonManager.SCOPE_PROFILE);
+      addDirectoryInstallLocation(KEY_APP_SYSTEM_DEFAULTS, KEY_APP_FEATURES,
+                                  [], AddonManager.SCOPE_PROFILE, true);
 
       if (enabledScopes & AddonManager.SCOPE_USER) {
         addDirectoryInstallLocation(KEY_APP_SYSTEM_USER, "XREUSysExt",
@@ -6462,49 +6447,6 @@ class MutableDirectoryInstallLocation extends DirectoryInstallLocation {
   }
 }
 
-/**
- * An object which identifies a built-in install location for add-ons, such
- * as default system add-ons.
- *
- * This location should point either to an omni jar, or a directory in a local build.
- */
-class BuiltInInstallLocation extends DirectoryInstallLocation {
-  /**
-   * Read the manifest of allowed add-ons and build a mapping between ID and URI
-   * for each.
-   */
-  _readAddons() {
-    let manifest;
-    try {
-      let url = Services.io.newURI(BUILT_IN_ADDONS_URI);
-      let data = Cu.readURI(url);
-      manifest = JSON.parse(data);
-    } catch (e) {
-      logger.warn("List of valid built-in add-ons could not be parsed.", e);
-      return;
-    }
-
-    if (!("system" in manifest)) {
-      logger.warn("No list of valid system add-ons found.");
-      return;
-    }
-
-    for (let id of manifest.system) {
-      let file = new FileUtils.File(this._directory.path);
-      file.append(`${id}.xpi`);
-
-      // Only attempt to load unpacked directory if unofficial build.
-      if (!AppConstants.MOZILLA_OFFICIAL && !file.exists()) {
-        file = new FileUtils.File(this._directory.path);
-        file.append(`${id}`);
-      }
-
-      this._IDToFileMap[id] = file;
-      XPIProvider._addURIMapping(id, file);
-    }
-  }
-}
-
 /**
  * An object which identifies a directory install location for system add-ons
  * updates.
diff --git a/toolkit/mozapps/extensions/test/xpcshell/head_addons.js b/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
index a916b14103e2..e80bd63d5a18 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
@@ -24,8 +24,6 @@ const PREF_SYSTEM_ADDON_SET           = "extensions.systemAddonSet";
 const PREF_SYSTEM_ADDON_UPDATE_URL    = "extensions.systemAddon.update.url";
 const PREF_APP_UPDATE_ENABLED         = "app.update.enabled";
 const PREF_ALLOW_NON_MPC              = "extensions.allow-non-mpc-extensions";
-const PREF_DISABLE_SECURITY = ("security.turn_off_all_security_so_that_" +
-                               "viruses_can_take_over_this_computer");
 
 // Forcibly end the test if it runs longer than 15 minutes
 const TIMEOUT_MS = 900000;
@@ -76,7 +74,6 @@ const {
   createUpdateRDF,
   getFileForAddon,
   manuallyUninstall,
-  overrideBuiltIns,
   promiseAddonEvent,
   promiseCompleteAllInstalls,
   promiseCompleteInstall,
@@ -162,7 +159,6 @@ const promiseAddonByID = AddonManager.getAddonByID;
 const promiseAddonsByIDs = AddonManager.getAddonsByIDs;
 const promiseAddonsWithOperationsByTypes = AddonManager.getAddonsWithOperationsByTypes;
 
-let gStartup = null;
 var gPort = null;
 var gUrlToFileMap = {};
 
@@ -1533,14 +1529,12 @@ async function setupSystemAddonConditions(setup, distroDir) {
   do_print("Clearing existing database.");
   Services.prefs.clearUserPref(PREF_SYSTEM_ADDON_SET);
   distroDir.leafName = "empty";
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system4@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
   await promiseShutdownManager();
 
   do_print("Setting up conditions.");
   await setup.setup();
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system4@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   // Make sure the initial state is correct
@@ -1586,9 +1580,7 @@ async function verifySystemAddonState(initialState, finalState = undefined, alre
   await checkInstalledSystemAddons(...finalState, distroDir);
 
   // Check that the new state is active after a restart
-  shutdownManager();
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system4@tests.mozilla.org", "system5@tests.mozilla.org"] });
-  await promiseStartupManager();
+  await promiseRestartManager();
   await checkInstalledSystemAddons(finalState, distroDir);
 }
 
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_nodisable_hidden.js b/toolkit/mozapps/extensions/test/xpcshell/test_nodisable_hidden.js
index a927004cda42..428bbc0571bb 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_nodisable_hidden.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_nodisable_hidden.js
@@ -18,7 +18,7 @@ createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
 // normal add-ons can be user disabled.
 add_task(async function() {
 
-  writeInstallRDFToXPI({
+  writeInstallRDFToDir({
     id: NORMAL_ID,
     version: "1.0",
     bootstrap: true,
@@ -62,7 +62,7 @@ add_task(async function() {
 // system add-ons can never be user disabled.
 add_task(async function() {
 
-  writeInstallRDFToXPI({
+  writeInstallRDFToDir({
     id: SYSTEM_ID,
     version: "1.0",
     bootstrap: true,
@@ -74,8 +74,6 @@ add_task(async function() {
     name: "Test disabling hidden add-ons, hidden system add-on case.",
   }, distroDir, SYSTEM_ID);
 
-  await overrideBuiltIns({ "system": [SYSTEM_ID] });
-
   startupManager();
 
   let addon = await promiseAddonByID(SYSTEM_ID);
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_system_allowed.js b/toolkit/mozapps/extensions/test/xpcshell/test_system_allowed.js
deleted file mode 100644
index eb5cdbf595fc..000000000000
--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_allowed.js
+++ /dev/null
@@ -1,48 +0,0 @@
-// Tests that only allowed built-in system add-ons are loaded on startup.
-
-BootstrapMonitor.init();
-
-// Build the test set
-var distroDir = FileUtils.getDir("ProfD", ["sysfeatures"], true);
-do_get_file("data/system_addons/system1_1.xpi").copyTo(distroDir, "system1@tests.mozilla.org.xpi");
-do_get_file("data/system_addons/system2_1.xpi").copyTo(distroDir, "system2@tests.mozilla.org.xpi");
-do_get_file("data/system_addons/system3_1.xpi").copyTo(distroDir, "system3@tests.mozilla.org.xpi");
-registerDirectory("XREAppFeat", distroDir);
-
-createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "0");
-
-// Ensure that only allowed add-ons are loaded.
-add_task(async function test_allowed_addons() {
-  // 1 and 2 are allowed, 3 is not.
-  let validAddons = { "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org"]};
-  await overrideBuiltIns(validAddons);
-
-  await promiseStartupManager();
-
-  let addon = await AddonManager.getAddonByID("system1@tests.mozilla.org");
-  notEqual(addon, null);
-
-  addon = await AddonManager.getAddonByID("system2@tests.mozilla.org");
-  notEqual(addon, null);
-
-  addon = await AddonManager.getAddonByID("system3@tests.mozilla.org");
-  do_check_eq(addon, null);
-  equal(addon, null);
-
-  // 3 is now allowed, 1 and 2 are not.
-  validAddons = { "system": ["system3@tests.mozilla.org"]};
-  await overrideBuiltIns(validAddons);
-
-  await promiseRestartManager();
-
-  addon = await AddonManager.getAddonByID("system1@tests.mozilla.org");
-  equal(addon, null);
-
-  addon = await AddonManager.getAddonByID("system2@tests.mozilla.org");
-  equal(addon, null);
-
-  addon = await AddonManager.getAddonByID("system3@tests.mozilla.org");
-  notEqual(addon, null);
-
-  await promiseShutdownManager();
-});
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_system_delay_update.js b/toolkit/mozapps/extensions/test/xpcshell/test_system_delay_update.js
index 74196fb0983e..7732a1aa7193 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_delay_update.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_delay_update.js
@@ -14,6 +14,7 @@ const DEFER_ID = "system_delay_defer@tests.mozilla.org";
 const DEFER_ALSO_ID = "system_delay_defer_also@tests.mozilla.org";
 const NORMAL_ID = "system1@tests.mozilla.org";
 
+
 const TEST_IGNORE_PREF = "delaytest.ignore";
 
 const distroDir = FileUtils.getDir("ProfD", ["sysfeatures"], true);
@@ -117,8 +118,6 @@ add_task(async function() {
   do_get_file("data/system_addons/system_delay_ignore.xpi").copyTo(distroDir, "system_delay_ignore@tests.mozilla.org.xpi");
   do_get_file("data/system_addons/system1_1.xpi").copyTo(distroDir, "system1@tests.mozilla.org.xpi");
 
-  await overrideBuiltIns({ "system": [IGNORE_ID, NORMAL_ID] })
-
   startupManager();
   let updateList = [
     { id: IGNORE_ID, version: "2.0", path: "system_delay_ignore_2.xpi" },
@@ -182,8 +181,6 @@ add_task(async function() {
   do_get_file("data/system_addons/system_delay_complete.xpi").copyTo(distroDir, "system_delay_complete@tests.mozilla.org.xpi");
   do_get_file("data/system_addons/system1_1.xpi").copyTo(distroDir, "system1@tests.mozilla.org.xpi");
 
-  await overrideBuiltIns({ "system": [COMPLETE_ID, NORMAL_ID] })
-
   startupManager();
 
   let updateList = [
@@ -268,8 +265,6 @@ add_task(async function() {
   do_get_file("data/system_addons/system_delay_defer.xpi").copyTo(distroDir, "system_delay_defer@tests.mozilla.org.xpi");
   do_get_file("data/system_addons/system1_1.xpi").copyTo(distroDir, "system1@tests.mozilla.org.xpi");
 
-  await overrideBuiltIns({ "system": [DEFER_ID, NORMAL_ID] })
-
   startupManager();
 
   let updateList = [
@@ -359,8 +354,6 @@ add_task(async function() {
   do_get_file("data/system_addons/system_delay_defer.xpi").copyTo(distroDir, "system_delay_defer@tests.mozilla.org.xpi");
   do_get_file("data/system_addons/system_delay_defer_also.xpi").copyTo(distroDir, "system_delay_defer_also@tests.mozilla.org.xpi");
 
-  await overrideBuiltIns({ "system": [DEFER_ID, DEFER_ALSO_ID] })
-
   startupManager();
 
   let updateList = [
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_system_repository.js b/toolkit/mozapps/extensions/test/xpcshell/test_system_repository.js
index d3b8865a6c44..2a32b579e833 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_repository.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_repository.js
@@ -30,8 +30,6 @@ add_task(async function test_app_addons() {
     do_throw("Unexpected request to server.");
   });
 
-  awaitPromise(overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org"] }))
-
   startupManager();
 
   await new Promise((resolve) => {
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_system_reset.js b/toolkit/mozapps/extensions/test/xpcshell/test_system_reset.js
index 9034f4b4ec90..921b0fc64f37 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_reset.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_reset.js
@@ -95,7 +95,6 @@ async function check_installed(conditions) {
 
 // Test with a missing features directory
 add_task(async function test_missing_app_dir() {
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager();
 
   let conditions = [
@@ -115,7 +114,6 @@ add_task(async function test_missing_app_dir() {
 add_task(async function test_new_version() {
   gAppInfo.version = "1";
   distroDir.leafName = "app1";
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager();
 
   let conditions = [
@@ -135,7 +133,6 @@ add_task(async function test_new_version() {
 add_task(async function test_upgrade() {
   gAppInfo.version = "2";
   distroDir.leafName = "app2";
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager();
 
   let conditions = [
@@ -155,7 +152,6 @@ add_task(async function test_upgrade() {
 add_task(async function test_downgrade() {
   gAppInfo.version = "1";
   distroDir.leafName = "app1";
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager();
 
   let conditions = [
@@ -199,7 +195,6 @@ add_task(async function test_updated() {
   };
   Services.prefs.setCharPref(PREF_SYSTEM_ADDON_SET, JSON.stringify(addonSet));
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -217,7 +212,6 @@ add_task(async function test_updated() {
 // default system add-ons
 add_task(async function safe_mode_disabled() {
   gAppInfo.inSafeMode = true;
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -234,7 +228,6 @@ add_task(async function safe_mode_disabled() {
 // Leaving safe mode should re-enable the updated system add-ons
 add_task(async function normal_mode_enabled() {
   gAppInfo.inSafeMode = false;
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -254,7 +247,6 @@ add_task(async function test_skips_additional() {
   let file = do_get_file("data/system_addons/system4_1.xpi");
   file.copyTo(updatesDir, "system4@tests.mozilla.org.xpi");
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -275,7 +267,6 @@ add_task(async function test_revert() {
   // With the add-on physically gone from disk we won't see uninstall events
   BootstrapMonitor.clear("system2@tests.mozilla.org");
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   // With system add-on 2 gone the updated set is now invalid so it reverts to
@@ -296,7 +287,6 @@ add_task(async function test_reuse() {
   let file = do_get_file("data/system_addons/system2_2.xpi");
   file.copyTo(updatesDir, "system2@tests.mozilla.org.xpi");
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -314,7 +304,6 @@ add_task(async function test_reuse() {
 add_task(async function test_corrupt_pref() {
   Services.prefs.setCharPref(PREF_SYSTEM_ADDON_SET, "foo");
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -351,7 +340,6 @@ add_task(async function test_bad_profile_cert() {
   };
   Services.prefs.setCharPref(PREF_SYSTEM_ADDON_SET, JSON.stringify(addonSet));
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -369,7 +357,6 @@ add_task(async function test_bad_profile_cert() {
 add_task(async function test_bad_app_cert() {
   gAppInfo.version = "3";
   distroDir.leafName = "app3";
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager();
 
   // Since we updated the app version, the system addon set should be reset as well.
@@ -420,7 +407,6 @@ add_task(async function test_updated_bad_update_set() {
   };
   Services.prefs.setCharPref(PREF_SYSTEM_ADDON_SET, JSON.stringify(addonSet));
 
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager(false);
 
   let conditions = [
@@ -431,3 +417,4 @@ add_task(async function test_updated_bad_update_set() {
 
   await promiseShutdownManager();
 });
+
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_system_update.js b/toolkit/mozapps/extensions/test/xpcshell/test_system_update.js
index f7ae5f2da4db..ec507c4e121e 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_update.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_update.js
@@ -298,7 +298,6 @@ const TESTS = {
 
 add_task(async function setup() {
   // Initialise the profile
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system4@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager();
   await promiseShutdownManager();
 });
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_system_update_fail.js b/toolkit/mozapps/extensions/test/xpcshell/test_system_update_fail.js
index 9167d65a062f..55e20044a1ca 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_update_fail.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_update_fail.js
@@ -166,7 +166,6 @@ const TESTS = {
 
 add_task(async function setup() {
   // Initialise the profile
-  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org", "system4@tests.mozilla.org", "system5@tests.mozilla.org"] });
   startupManager();
   await promiseShutdownManager();
 });
diff --git a/toolkit/mozapps/extensions/test/xpcshell/test_update.js b/toolkit/mozapps/extensions/test/xpcshell/test_update.js
index 3897b7254c15..afffe38da270 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_update.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_update.js
@@ -1359,14 +1359,12 @@ function check_test_7_cache() {
 
 // Test that the update check returns nothing for addons in locked install
 // locations.
-add_test(async function run_test_locked_install() {
+add_test(function run_test_locked_install() {
   const lockedDir = gProfD.clone();
   lockedDir.append("locked_extensions");
   registerDirectory("XREAppFeat", lockedDir);
-
-  await promiseShutdownManager();
-
-  writeInstallRDFToXPI({
+  restartManager();
+  writeInstallRDFForExtension({
     id: "addon13@tests.mozilla.org",
     version: "1.0",
     updateURL: "http://localhost:" + gPort + "/data/test_update.rdf",
@@ -1377,11 +1375,7 @@ add_test(async function run_test_locked_install() {
     }],
     name: "Test Addon 13",
   }, lockedDir);
-
-  let validAddons = { "system": ["addon13@tests.mozilla.org"] };
-  await overrideBuiltIns(validAddons);
-
-  await promiseStartupManager();
+  restartManager();
 
   AddonManager.getAddonByID("addon13@tests.mozilla.org", function(a13) {
     do_check_neq(a13, null);
diff --git a/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini b/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
index 4dd7e11eb5ea..07ef8e6d8624 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
+++ b/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
@@ -51,6 +51,5 @@ tags = webextensions
 [test_system_delay_update.js]
 [test_schema_change.js]
 [test_registerchrome.js]
-[test_system_allowed.js]
 
 [include:xpcshell-shared.ini]