--- a/testing/profiles/common/user.js
+++ b/testing/profiles/common/user.js
@@ -79,18 +79,16 @@ user_pref("browser.tabs.disableBackgroun
 // Only load extensions from the application and user profile
 // AddonManager.SCOPE_PROFILE + AddonManager.SCOPE_APPLICATION
 user_pref("extensions.enabledScopes", 5);
 user_pref("extensions.autoDisableScopes", 0);
 // Disable metadata caching for installed add-ons by default
 user_pref("extensions.getAddons.cache.enabled", false);
 // Disable intalling any distribution add-ons
 user_pref("extensions.installDistroAddons", false);
-// XPI extensions are required for test harnesses to load
-user_pref("extensions.defaultProviders.enabled", true);
 user_pref("xpinstall.signatures.required", false);
 user_pref("extensions.legacy.enabled", true);
 
 user_pref("geo.wifi.uri", "http://{server}/tests/dom/tests/mochitest/geolocation/network_geolocation.sjs");
 user_pref("geo.wifi.timeToWaitBeforeSending", 2000);
 user_pref("geo.wifi.scan", false);
 user_pref("geo.wifi.logging.enabled", true);
 

--- a/toolkit/mozapps/extensions/AddonManager.jsm
+++ b/toolkit/mozapps/extensions/AddonManager.jsm
@@ -18,17 +18,16 @@ if ("@mozilla.org/xre/app-info;1" in Cc)
 }
 
 ChromeUtils.import("resource://gre/modules/AppConstants.jsm");
 Cu.importGlobalProperties(["DOMParser", "Element"]);
 
 const MOZ_COMPATIBILITY_NIGHTLY = !["aurora", "beta", "release", "esr"].includes(AppConstants.MOZ_UPDATE_CHANNEL);
 
 const PREF_BLOCKLIST_PINGCOUNTVERSION = "extensions.blocklist.pingCountVersion";
-const PREF_DEFAULT_PROVIDERS_ENABLED  = "extensions.defaultProviders.enabled";
 const PREF_EM_UPDATE_ENABLED          = "extensions.update.enabled";
 const PREF_EM_LAST_APP_VERSION        = "extensions.lastAppVersion";
 const PREF_EM_LAST_PLATFORM_VERSION   = "extensions.lastPlatformVersion";
 const PREF_EM_AUTOUPDATE_DEFAULT      = "extensions.update.autoUpdateDefault";
 const PREF_EM_STRICT_COMPATIBILITY    = "extensions.strictCompatibility";
 const PREF_EM_CHECK_UPDATE_SECURITY   = "extensions.checkUpdateSecurity";
 const PREF_APP_UPDATE_ENABLED         = "app.update.enabled";
 const PREF_APP_UPDATE_AUTO            = "app.update.auto";
@@ -758,38 +757,33 @@ var AddonManagerInternal = {
                                                       gAutoUpdateDefault);
       Services.prefs.addObserver(PREF_EM_AUTOUPDATE_DEFAULT, this);
 
       gWebExtensionsMinPlatformVersion =
         Services.prefs.getCharPref(PREF_MIN_WEBEXT_PLATFORM_VERSION,
                                    gWebExtensionsMinPlatformVersion);
       Services.prefs.addObserver(PREF_MIN_WEBEXT_PLATFORM_VERSION, this);
 
-      let defaultProvidersEnabled = Services.prefs.getBoolPref(PREF_DEFAULT_PROVIDERS_ENABLED, true);
-      AddonManagerPrivate.recordSimpleMeasure("default_providers", defaultProvidersEnabled);
-
       // Ensure all default providers have had a chance to register themselves
-      if (defaultProvidersEnabled) {
-        for (let url of DEFAULT_PROVIDERS) {
-          try {
-            let scope = {};
-            ChromeUtils.import(url, scope);
-            // Sanity check - make sure the provider exports a symbol that
-            // has a 'startup' method
-            let syms = Object.keys(scope);
-            if ((syms.length < 1) ||
-                (typeof scope[syms[0]].startup != "function")) {
-              logger.warn("Provider " + url + " has no startup()");
-              AddonManagerPrivate.recordException("AMI", "provider " + url, "no startup()");
-            }
-            logger.debug("Loaded provider scope for " + url + ": " + Object.keys(scope).toSource());
-          } catch (e) {
-            AddonManagerPrivate.recordException("AMI", "provider " + url + " load failed", e);
-            logger.error("Exception loading default provider \"" + url + "\"", e);
+      for (let url of DEFAULT_PROVIDERS) {
+        try {
+          let scope = {};
+          ChromeUtils.import(url, scope);
+          // Sanity check - make sure the provider exports a symbol that
+          // has a 'startup' method
+          let syms = Object.keys(scope);
+          if ((syms.length < 1) ||
+              (typeof scope[syms[0]].startup != "function")) {
+            logger.warn("Provider " + url + " has no startup()");
+            AddonManagerPrivate.recordException("AMI", "provider " + url, "no startup()");
           }
+          logger.debug("Loaded provider scope for " + url + ": " + Object.keys(scope).toSource());
+        } catch (e) {
+          AddonManagerPrivate.recordException("AMI", "provider " + url + " load failed", e);
+          logger.error("Exception loading default provider \"" + url + "\"", e);
         }
       }
 
       // Load any providers registered in the category manager
       let catman = Cc["@mozilla.org/categorymanager;1"].
                    getService(Ci.nsICategoryManager);
       let entries = catman.enumerateCategory(CATEGORY_PROVIDER_MODULE);
       while (entries.hasMoreElements()) {

--- a/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/head_addons.js
@@ -67,17 +67,16 @@ ChromeUtils.defineModuleGetter(this, "Pr
 ChromeUtils.defineModuleGetter(this, "TestUtils",
                                "resource://testing-common/TestUtils.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(this, "aomStartup",
                                    "@mozilla.org/addons/addon-manager-startup;1",
                                    "amIAddonManagerStartup");
 
 const {
-  awaitPromise,
   createAppInfo,
   createHttpServer,
   createInstallRDF,
   createTempWebExtensionFile,
   createUpdateRDF,
   getFileForAddon,
   manuallyInstall,
   manuallyUninstall,
@@ -662,41 +661,16 @@ function do_check_compatibilityoverride(
 }
 
 function do_check_icons(aActual, aExpected) {
   for (var size in aExpected) {
     Assert.equal(aActual[size], aExpected[size]);
   }
 }
 
-function startupManager() {
-  // promiseStartupManager() does not actually do any asynchronous
-  // work so we don't need to wait for it here.  Unfortunately, wrapping
-  // this wait awaitPromise() creates other unrelated failures, rather
-  // than going through those we should just get rid of this wrapper,
-  // see bug 1459998.
-  promiseStartupManager();
-}
-
-/**
- * Restarts the add-on manager as if the host application was restarted.
- *
- * @param  aNewVersion
- *         An optional new version to use for the application. Passing this
- *         will change nsIXULAppInfo.version and make the startup appear as if
- *         the application version has changed.
- */
-function restartManager(aNewVersion) {
-  awaitPromise(promiseRestartManager(aNewVersion));
-}
-
-function shutdownManager() {
-  awaitPromise(promiseShutdownManager());
-}
-
 function isThemeInAddonsList(aDir, aId) {
   return AddonTestUtils.addonsList.hasTheme(aDir, aId);
 }
 
 function isExtensionInAddonsList(aDir, aId) {
   return AddonTestUtils.addonsList.hasExtension(aDir, aId);
 }
 
@@ -726,19 +700,16 @@ function check_startup_changes(aType, aI
  * @param   aDir
  *          The directory to add the install.rdf to
  * @param   aId
  *          An optional string to override the default installation aId
  * @param   aExtraFile
  *          An optional dummy file to create in the directory
  * @return  An nsIFile for the directory in which the add-on is installed.
  */
-function writeInstallRDFToDir(aData, aDir, aId = aData.id, aExtraFile = null) {
-  return awaitPromise(promiseWriteInstallRDFToDir(aData, aDir, aId, aExtraFile));
-}
 async function promiseWriteInstallRDFToDir(aData, aDir, aId = aData.id, aExtraFile = null) {
   let files = {
     "install.rdf": AddonTestUtils.createInstallRDF(aData),
   };
   if (typeof aExtraFile === "object")
     Object.assign(files, aExtraFile);
   else
     files[aExtraFile] = "";
@@ -762,17 +733,17 @@ async function promiseWriteInstallRDFToD
  * @param   aDir
  *          The install directory to add the extension to
  * @param   aId
  *          An optional string to override the default installation aId
  * @param   aExtraFile
  *          An optional dummy file to create in the extension
  * @return  A file pointing to where the extension was installed
  */
-function writeInstallRDFToXPI(aData, aDir, aId = aData.id, aExtraFile = null) {
+async function promiseWriteInstallRDFToXPI(aData, aDir, aId = aData.id, aExtraFile = null) {
   let files = {
     "install.rdf": AddonTestUtils.createInstallRDF(aData),
   };
   if (typeof aExtraFile === "object")
     Object.assign(files, aExtraFile);
   else
   if (aExtraFile)
     files[aExtraFile] = "";
@@ -782,19 +753,16 @@ function writeInstallRDFToXPI(aData, aDi
 
   var file = aDir.clone();
   file.append(`${aId}.xpi`);
 
   AddonTestUtils.writeFilesToZip(file.path, files);
 
   return file;
 }
-async function promiseWriteInstallRDFToXPI(aData, aDir, aId = aData.id, aExtraFile = null) {
-  return writeInstallRDFToXPI(aData, aDir, aId, aExtraFile);
-}
 
 /**
  * Writes an install.rdf manifest into an extension using the properties passed
  * in a JS object. The objects should contain a property for each property to
  * appear in the RDF. The object may contain an array of objects with id,
  * minVersion and maxVersion in the targetApplications property to give target
  * application compatibility.
  *
@@ -803,23 +771,16 @@ async function promiseWriteInstallRDFToX
  * @param   aDir
  *          The install directory to add the extension to
  * @param   aId
  *          An optional string to override the default installation aId
  * @param   aExtraFile
  *          An optional dummy file to create in the extension
  * @return  A file pointing to where the extension was installed
  */
-function writeInstallRDFForExtension(aData, aDir, aId, aExtraFile) {
-  if (TEST_UNPACKED) {
-    return writeInstallRDFToDir(aData, aDir, aId, aExtraFile);
-  }
-  return writeInstallRDFToXPI(aData, aDir, aId, aExtraFile);
-}
-
 function promiseWriteInstallRDFForExtension(aData, aDir, aId, aExtraFile) {
   if (TEST_UNPACKED) {
     return promiseWriteInstallRDFToDir(aData, aDir, aId, aExtraFile);
   }
   return promiseWriteInstallRDFToXPI(aData, aDir, aId, aExtraFile);
 }
 
 /**
@@ -1266,24 +1227,16 @@ async function saveJSON(aData, aFile) {
 function callback_soon(aFunction) {
   return function(...args) {
     executeSoon(function() {
       aFunction.apply(null, args);
     }, aFunction.name ? "delayed callback " + aFunction.name : "delayed callback");
   };
 }
 
-function writeProxyFileToDir(aDir, aAddon, aId) {
-  awaitPromise(promiseWriteProxyFileToDir(aDir, aAddon, aId));
-
-  let file = aDir.clone();
-  file.append(aId);
-  return file;
-}
-
 async function serveSystemUpdate(xml, perform_update, testserver) {
   testserver.registerPathHandler("/data/update.xml", (request, response) => {
     response.write(xml);
   });
 
   try {
     await perform_update();
   } finally {
@@ -1500,31 +1453,31 @@ async function getSystemAddonDirectories
  * @param {nsIFile} distroDir - the system add-on distribution directory (the "features" dir in the app directory)
  */
 async function setupSystemAddonConditions(setup, distroDir) {
   info("Clearing existing database.");
   Services.prefs.clearUserPref(PREF_SYSTEM_ADDON_SET);
   distroDir.leafName = "empty";
 
   let updateList = [];
-  awaitPromise(overrideBuiltIns({ "system": updateList }));
+  await overrideBuiltIns({ "system": updateList });
   await promiseStartupManager();
   await promiseShutdownManager();
 
   info("Setting up conditions.");
   await setup.setup();
 
   if (distroDir) {
     if (distroDir.path.endsWith("hidden")) {
       updateList = ["system1@tests.mozilla.org", "system2@tests.mozilla.org"];
     } else if (distroDir.path.endsWith("prefilled")) {
       updateList = ["system2@tests.mozilla.org", "system3@tests.mozilla.org"];
     }
   }
-  awaitPromise(overrideBuiltIns({ "system": updateList }));
+  await overrideBuiltIns({ "system": updateList });
   await promiseStartupManager();
 
   // Make sure the initial state is correct
   info("Checking initial state.");
   await checkInstalledSystemAddons(setup.initialState, distroDir);
 }
 
 /**
@@ -1571,17 +1524,17 @@ async function verifySystemAddonState(in
 
   if (distroDir) {
     if (distroDir.path.endsWith("hidden")) {
       updateList = ["system1@tests.mozilla.org", "system2@tests.mozilla.org"];
     } else if (distroDir.path.endsWith("prefilled")) {
       updateList = ["system2@tests.mozilla.org", "system3@tests.mozilla.org"];
     }
   }
-  awaitPromise(overrideBuiltIns({ "system": updateList }));
+  await overrideBuiltIns({ "system": updateList });
   await promiseStartupManager();
   await checkInstalledSystemAddons(finalState, distroDir);
 }
 
 /**
  * Run system add-on tests and compare the results against a set of expected conditions.
  *
  * @param {String} setupName - name of the current setup conditions.

--- a/toolkit/mozapps/extensions/test/xpcshell/test_bootstrap_const.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_bootstrap_const.js
@@ -1,16 +1,17 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "1");
-startupManager();
 
 add_task(async function() {
+  await promiseStartupManager();
+
   let sawInstall = false;
   Services.obs.addObserver(function() {
     sawInstall = true;
   }, "addon-install");
 
   await promiseInstallAllFiles([do_get_addon("test_bootstrap_const")]);
 
   ok(sawInstall);

deleted file mode 100644
--- a/toolkit/mozapps/extensions/test/xpcshell/test_default_providers_pref.js
+++ /dev/null
@@ -1,13 +0,0 @@
-/* Any copyright is dedicated to the Public Domain.
-   http://creativecommons.org/publicdomain/zero/1.0/ */
-
-// Tests the extensions.defaultProviders.enabled pref which turns
-// off the default XPIProvider and LightweightThemeManager.
-
-async function run_test() {
-  Services.prefs.setBoolPref("extensions.defaultProviders.enabled", false);
-  createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "1.9.2");
-  await promiseStartupManager();
-  Assert.ok(!AddonManager.isInstallEnabled("application/x-xpinstall"));
-  Services.prefs.clearUserPref("extensions.defaultProviders.enabled");
-}

--- a/toolkit/mozapps/extensions/test/xpcshell/test_dependencies.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_dependencies.js
@@ -1,17 +1,16 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
 const profileDir = gProfD.clone();
 profileDir.append("extensions");
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "1");
-startupManager();
 
 const BOOTSTRAP = String.raw`
   Components.utils.import("resource://gre/modules/Services.jsm");
 
   function startup(data) {
     Services.obs.notifyObservers(null, "test-addon-bootstrap-startup", data.id);
   }
   function shutdown(data) {
@@ -41,16 +40,18 @@ const ADDONS = [
     dependencies: ["addon2@dependency-test.mozilla.org"],
   },
 ];
 
 let addonFiles = [];
 
 let events = [];
 add_task(async function setup() {
+  await promiseStartupManager();
+
   let startupObserver = (subject, topic, data) => {
     events.push(["startup", data]);
   };
   let shutdownObserver = (subject, topic, data) => {
     events.push(["shutdown", data]);
   };
 
   Services.obs.addObserver(startupObserver, "test-addon-bootstrap-startup");

--- a/toolkit/mozapps/extensions/test/xpcshell/test_install_from_sources.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_install_from_sources.js
@@ -1,28 +1,29 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
 const ID = "bootstrap1@tests.mozilla.org";
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
-startupManager();
 
 BootstrapMonitor.init();
 
 // Partial list of bootstrap reasons from XPIProvider.jsm
 const BOOTSTRAP_REASONS = {
   ADDON_INSTALL: 5,
   ADDON_UPGRADE: 7,
   ADDON_DOWNGRADE: 8,
 };
 
 // Install an unsigned add-on with no existing add-on present.
 // Restart and make sure it is still around.
 add_task(async function() {
+  await promiseStartupManager();
+
   let extInstallCalled = false;
   AddonManager.addInstallListener({
     onExternalInstall: (aInstall) => {
       Assert.equal(aInstall.id, ID);
       Assert.equal(aInstall.version, "1.0");
       extInstallCalled = true;
     },
   });

--- a/toolkit/mozapps/extensions/test/xpcshell/test_legacy.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_legacy.js
@@ -1,15 +1,16 @@
 
 const LEGACY_PREF = "extensions.legacy.enabled";
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "1");
-startupManager();
 
 add_task(async function test_disable() {
+  await promiseStartupManager();
+
   let legacy = [
     {
       id: "bootstrap@tests.mozilla.org",
       name: "Bootstrap add-on",
       version: "1.0",
       bootstrap: true,
     },
   ];

--- a/toolkit/mozapps/extensions/test/xpcshell/test_pass_symbol.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_pass_symbol.js
@@ -3,22 +3,23 @@
  */
 
 const PASS_PREF = "symboltest.instanceid.pass";
 const FAIL_BOGUS_PREF = "symboltest.instanceid.fail_bogus";
 const FAIL_ID_PREF = "symboltest.instanceid.fail_bogus";
 const ADDON_ID = "test_symbol@tests.mozilla.org";
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
-startupManager();
 
 BootstrapMonitor.init();
 
 // symbol is passed when add-on is installed
 add_task(async function() {
+  await promiseStartupManager();
+
   PromiseTestUtils.expectUncaughtRejection(/no addon found for symbol/);
 
   for (let pref of [PASS_PREF, FAIL_BOGUS_PREF, FAIL_ID_PREF])
     Services.prefs.clearUserPref(pref);
 
   await promiseInstallAllFiles([do_get_addon("test_symbol")], true);
 
   let addon = await promiseAddonByID(ADDON_ID);

--- a/toolkit/mozapps/extensions/test/xpcshell/test_proxy.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_proxy.js
@@ -1,21 +1,22 @@
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/
  */
 
 const ID = "proxy1@tests.mozilla.org";
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
-startupManager();
 
 BootstrapMonitor.init();
 
 // Ensure that a proxy file to an add-on with a valid manifest works.
 add_task(async function() {
+  await promiseStartupManager();
+
   let tempdir = gTmpD.clone();
   await promiseWriteInstallRDFToDir({
     id: ID,
     version: "1.0",
     bootstrap: true,
     unpack: true,
     targetApplications: [{
           id: "xpcshell@tests.mozilla.org",

--- a/toolkit/mozapps/extensions/test/xpcshell/test_schema_change.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_schema_change.js
@@ -5,22 +5,23 @@
 BootstrapMonitor.init();
 
 const PREF_DB_SCHEMA = "extensions.databaseSchema";
 
 const profileDir = gProfD.clone();
 profileDir.append("extensions");
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "49");
-startupManager();
 
 /**
  *  Schema change with no application update reloads metadata.
  */
 add_task(async function schema_change() {
+  await promiseStartupManager();
+
   const ID = "schema-change@tests.mozilla.org";
 
   let xpiFile = createTempXPIFile({
     id: ID,
     name: "Test Add-on",
     version: "1.0",
     bootstrap: true,
     targetApplications: [{

--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_repository.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_repository.js
@@ -22,17 +22,17 @@ add_task(async function test_app_addons(
   Services.prefs.setBoolPref(PREF_GETADDONS_CACHE_ENABLED, true);
   Services.prefs.setCharPref(PREF_GETADDONS_BYIDS, `http://localhost:${gServer.identity.primaryPort}/get?%IDS%`);
   Services.prefs.setCharPref(PREF_COMPAT_OVERRIDES, `http://localhost:${gServer.identity.primaryPort}/get?%IDS%`);
 
   gServer.registerPathHandler("/get", (request, response) => {
     do_throw("Unexpected request to server.");
   });
 
-  awaitPromise(overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org"] }));
+  await overrideBuiltIns({ "system": ["system1@tests.mozilla.org", "system2@tests.mozilla.org", "system3@tests.mozilla.org"] });
 
   await promiseStartupManager();
 
   await AddonRepository.cacheAddons(["system1@tests.mozilla.org",
                                      "system2@tests.mozilla.org",
                                      "system3@tests.mozilla.org"]);
 
   let cached = await AddonRepository.getCachedAddonByID("system1@tests.mozilla.org");

--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_update_blank.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_update_blank.js
@@ -102,17 +102,17 @@ const TESTS = {
   // Test that a blank response does nothing
   blank: {
     updateList: null,
   }
 };
 
 add_task(async function setup() {
   // Initialise the profile
-  awaitPromise(overrideBuiltIns({ "system": [] }));
+  await overrideBuiltIns({ "system": [] });
   await promiseStartupManager();
   await promiseShutdownManager();
 });
 
 add_task(async function() {
   for (let setupName of Object.keys(TEST_CONDITIONS)) {
     for (let testName of Object.keys(TESTS)) {
         info("Running test " + setupName + " " + testName);

--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_update_checkSizeHash.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_update_checkSizeHash.js
@@ -136,17 +136,17 @@ const TESTS = {
         { isUpgrade: true, version: "1.0"}
       ]
     }
   }
 };
 
 add_task(async function setup() {
   // Initialise the profile
-  awaitPromise(overrideBuiltIns({ "system": [] }));
+  await overrideBuiltIns({ "system": [] });
   await promiseStartupManager();
   await promiseShutdownManager();
 });
 
 add_task(async function() {
   for (let setupName of Object.keys(TEST_CONDITIONS)) {
     for (let testName of Object.keys(TESTS)) {
         info("Running test " + setupName + " " + testName);

--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_update_newset.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_update_newset.js
@@ -135,17 +135,17 @@ const TESTS = {
         { isUpgrade: true, version: "1.0"}
       ]
     }
   }
 };
 
 add_task(async function setup() {
   // Initialise the profile
-  awaitPromise(overrideBuiltIns({ "system": [] }));
+  await overrideBuiltIns({ "system": [] });
   await promiseStartupManager();
   await promiseShutdownManager();
 });
 
 add_task(async function() {
   for (let setupName of Object.keys(TEST_CONDITIONS)) {
     for (let testName of Object.keys(TESTS)) {
         info("Running test " + setupName + " " + testName);

--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_update_overlapping.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_update_overlapping.js
@@ -137,17 +137,17 @@ const TESTS = {
         { isUpgrade: false, version: null}
       ]
     }
   }
 };
 
 add_task(async function setup() {
   // Initialise the profile
-  awaitPromise(overrideBuiltIns({ "system": [] }));
+  await overrideBuiltIns({ "system": [] });
   await promiseStartupManager();
   await promiseShutdownManager();
 });
 
 add_task(async function() {
   for (let setupName of Object.keys(TEST_CONDITIONS)) {
     for (let testName of Object.keys(TESTS)) {
         info("Running test " + setupName + " " + testName);

--- a/toolkit/mozapps/extensions/test/xpcshell/test_system_update_upgrades.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_system_update_upgrades.js
@@ -135,17 +135,17 @@ const TESTS = {
         { isUpgrade: false, version: null}
       ]
     }
   }
 };
 
 add_task(async function setup() {
   // Initialise the profile
-  awaitPromise(overrideBuiltIns({ "system": [] }));
+  await overrideBuiltIns({ "system": [] });
   await promiseStartupManager();
   await promiseShutdownManager();
 });
 
 add_task(async function() {
   for (let setupName of Object.keys(TEST_CONDITIONS)) {
     for (let testName of Object.keys(TESTS)) {
         info("Running test " + setupName + " " + testName);

--- a/toolkit/mozapps/extensions/test/xpcshell/test_temporary.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_temporary.js
@@ -13,17 +13,16 @@ const sampleRDFManifest = {
     id: "xpcshell@tests.mozilla.org",
     minVersion: "1",
     maxVersion: "1"
   }],
   name: "Test Bootstrap 1 (temporary)",
 };
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
-startupManager();
 
 BootstrapMonitor.init();
 
 // Partial list of bootstrap reasons from XPIProvider.jsm
 const BOOTSTRAP_REASONS = {
   APP_STARTUP: 1,
   ADDON_INSTALL: 5,
   ADDON_UNINSTALL: 6,
@@ -48,16 +47,18 @@ function waitForBootstrapEvent(expectedE
     };
     Services.obs.addObserver(observer, "bootstrapmonitor-event");
   });
 }
 
 // Install a temporary add-on with no existing add-on present.
 // Restart and make sure it has gone away.
 add_task(async function() {
+  await promiseStartupManager();
+
   let extInstallCalled = false;
   AddonManager.addInstallListener({
     onExternalInstall: (aInstall) => {
       Assert.equal(aInstall.id, ID);
       Assert.equal(aInstall.version, "1.0");
       extInstallCalled = true;
     },
   });

--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension.js
@@ -5,21 +5,22 @@
 ChromeUtils.import("resource://gre/modules/AppConstants.jsm");
 
 const ID = "webextension1@tests.mozilla.org";
 
 const profileDir = gProfD.clone();
 profileDir.append("extensions");
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
-startupManager();
 
 const { GlobalManager } = ChromeUtils.import("resource://gre/modules/Extension.jsm", {});
 
 add_task(async function() {
+  await promiseStartupManager();
+
   equal(GlobalManager.extensionMap.size, 0);
 
   await Promise.all([
     promiseInstallAllFiles([do_get_addon("webextension_1")], true),
     promiseWebExtensionStartup()
   ]);
 
   equal(GlobalManager.extensionMap.size, 1);

--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_embedded.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_embedded.js
@@ -3,17 +3,16 @@
  */
 
 BootstrapMonitor.init();
 
 const profileDir = gProfD.clone();
 profileDir.append("extensions");
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "49");
-startupManager();
 
 // NOTE: the following import needs to be called after the `createAppInfo`
 // or it will fail Extension.jsm internally imports AddonManager.jsm and
 // AddonManager will raise a ReferenceError exception because it tried to
 // access an undefined `Services.appinfo` object.
 const { Management } = ChromeUtils.import("resource://gre/modules/Extension.jsm", {});
 
 const {
@@ -47,16 +46,17 @@ const EMBEDDED_WEBEXT_MANIFEST = JSON.st
   version: "1.0",
 });
 
 /**
  *  This test case checks that an hasEmbeddedWebExtension addon property
  *  is persisted and restored correctly across restarts.
  */
 add_task(async function has_embedded_webextension_persisted() {
+  await promiseStartupManager();
   const ID = "embedded-webextension-addon-persist@tests.mozilla.org";
 
   const xpiFile = createTempXPIFile({
     id: ID,
     name: "Test Add-on",
     version: "1.0",
     bootstrap: true,
     hasEmbeddedWebExtension: true,

--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_icons.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_icons.js
@@ -4,17 +4,16 @@
 
 const ID = "webextension1@tests.mozilla.org";
 
 const profileDir = gProfD.clone();
 profileDir.append("extensions");
 profileDir.create(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
-startupManager();
 
 async function testSimpleIconsetParsing(manifest) {
   await promiseWriteWebManifestForExtension(manifest, profileDir);
 
   await Promise.all([
     promiseRestartManager(),
     manifest.theme || promiseWebExtensionStartup(ID),
   ]);
@@ -108,16 +107,17 @@ async function testNoIconsParsing(manife
 
   equal(AddonManager.getPreferredIconURL(addon, 128), null);
 
   addon.uninstall();
 }
 
 // Test simple icon set parsing
 add_task(async function() {
+  await promiseStartupManager();
   await testSimpleIconsetParsing({
     name: "Web Extension Name",
     version: "1.0",
     manifest_version: 2,
     applications: {
       gecko: {
         id: ID
       }

--- a/toolkit/mozapps/extensions/test/xpcshell/test_webextension_langpack.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_webextension_langpack.js
@@ -8,17 +8,16 @@ const { Services } = ChromeUtils.import(
 const { L10nRegistry } = ChromeUtils.import("resource://gre/modules/L10nRegistry.jsm", {});
 
 const ID = "langpack-und@test.mozilla.org";
 
 const profileDir = gProfD.clone();
 profileDir.append("extensions");
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "58");
-startupManager();
 
 function promiseLangpackStartup() {
   return new Promise(resolve => {
     const EVENT = "webextension-langpack-startup";
     Services.obs.addObserver(function observer() {
       Services.obs.removeObserver(observer, EVENT);
       resolve();
     }, EVENT);
@@ -26,16 +25,18 @@ function promiseLangpackStartup() {
 }
 
 /**
  * This is a basic life-cycle test which verifies that
  * the language pack registers and unregisters correct
  * languages at various stages.
  */
 add_task(async function() {
+  await promiseStartupManager();
+
   // Make sure that `und` locale is not installed.
   equal(L10nRegistry.getAvailableLocales().includes("und"), false);
   equal(Services.locale.getAvailableLocales().includes("und"), false);
 
   let [, {addon}] = await Promise.all([
     promiseLangpackStartup(),
     promiseInstallFile(do_get_addon("langpack_1"), true),
   ]);

--- a/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
+++ b/toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini
@@ -80,17 +80,16 @@ run-if = addon_signing
 [test_cacheflush.js]
 [test_childprocess.js]
 [test_compatoverrides.js]
 [test_corrupt.js]
 [test_corruptfile.js]
 [test_crash_annotation_quoting.js]
 [test_db_path.js]
 head =
-[test_default_providers_pref.js]
 [test_delay_update.js]
 [test_delay_update_webextension.js]
 skip-if = appname == "thunderbird"
 tags = webextensions
 [test_dependencies.js]
 [test_dictionary_webextension.js]
 [test_distribution.js]
 [test_duplicateplugins.js]