diff --git a/devtools/server/tests/unit/head_dbg.js b/devtools/server/tests/unit/head_dbg.js
index c0b38ba87522..d1a43b387213 100644
--- a/devtools/server/tests/unit/head_dbg.js
+++ b/devtools/server/tests/unit/head_dbg.js
@@ -51,6 +51,13 @@ const { addDebuggerToGlobal } = ChromeUtils.import(
   "resource://gre/modules/jsdebugger.jsm"
 );
 
+const { AddonTestUtils } = ChromeUtils.import(
+  "resource://testing-common/AddonTestUtils.jsm"
+);
+const { getAppInfo } = ChromeUtils.import(
+  "resource://testing-common/AppInfo.jsm"
+);
+
 const systemPrincipal = Cc["@mozilla.org/systemprincipal;1"].createInstance(
   Ci.nsIPrincipal
 );
@@ -59,17 +66,22 @@ var { loadSubScript, loadSubScriptWithOptions } = Services.scriptloader;
 
 /**
  * Initializes any test that needs to work with add-ons.
+ *
+ * Should be called once per test script that needs to use AddonTestUtils (and
+ * not once per test task!).
  */
-function startupAddonsManager() {
+async function startupAddonsManager() {
   // Create a directory for extensions.
   const profileDir = do_get_profile().clone();
   profileDir.append("extensions");
 
-  const internalManager = Cc["@mozilla.org/addons/integration;1"]
-    .getService(Ci.nsIObserver)
-    .QueryInterface(Ci.nsITimerCallback);
+  /* global globalThis */
+  /* See Bug 1595810 to add globalThis to eslint */
+  AddonTestUtils.init(globalThis);
+  AddonTestUtils.overrideCertDB();
+  AddonTestUtils.appInfo = getAppInfo();
 
-  internalManager.observe(null, "addons-startup", null);
+  await AddonTestUtils.promiseStartupManager();
 }
 
 async function createTargetForFakeTab(title) {
diff --git a/devtools/server/tests/unit/test_addon_events.js b/devtools/server/tests/unit/test_addon_events.js
index c87dd1766024..102add582104 100644
--- a/devtools/server/tests/unit/test_addon_events.js
+++ b/devtools/server/tests/unit/test_addon_events.js
@@ -5,13 +5,12 @@
 "use strict";
 
 const { AddonManager } = require("resource://gre/modules/AddonManager.jsm");
-
-startupAddonsManager();
-
 add_task(async function testReloadExitedAddon() {
   // Force sync plugin loading to avoid spurious install events from plugins.
   Services.prefs.setBoolPref("plugin.load_flash_only", false);
 
+  await startupAddonsManager();
+
   DebuggerServer.init();
   DebuggerServer.registerAllActors();
 
diff --git a/devtools/server/tests/unit/test_addon_reload.js b/devtools/server/tests/unit/test_addon_reload.js
index b7307f78aa3a..8f3ad1d4fb55 100644
--- a/devtools/server/tests/unit/test_addon_reload.js
+++ b/devtools/server/tests/unit/test_addon_reload.js
@@ -6,8 +6,6 @@
 
 const { AddonManager } = require("resource://gre/modules/AddonManager.jsm");
 
-startupAddonsManager();
-
 function promiseAddonEvent(event) {
   return new Promise(resolve => {
     const listener = {
@@ -49,6 +47,8 @@ function getSupportFile(path) {
 }
 
 add_task(async function testReloadExitedAddon() {
+  await startupAddonsManager();
+
   DebuggerServer.init();
   DebuggerServer.registerAllActors();
 
diff --git a/devtools/server/tests/unit/test_addons_actor.js b/devtools/server/tests/unit/test_addons_actor.js
index 0f6fd854de27..a8bb13b05b94 100644
--- a/devtools/server/tests/unit/test_addons_actor.js
+++ b/devtools/server/tests/unit/test_addons_actor.js
@@ -4,8 +4,6 @@
 
 "use strict";
 
-startupAddonsManager();
-
 async function connect() {
   DebuggerServer.init();
   DebuggerServer.registerAllActors();
@@ -17,6 +15,12 @@ async function connect() {
   return [client, addons];
 }
 
+// The AddonsManager test helper can only be called once per test script.
+// This `setup` task will run first.
+add_task(async function setup() {
+  await startupAddonsManager();
+});
+
 add_task(async function testSuccessfulInstall() {
   const [client, addons] = await connect();
 
diff --git a/devtools/server/tests/unit/test_extension_storage_actor.js b/devtools/server/tests/unit/test_extension_storage_actor.js
index b20d13053a17..e12ea9bff822 100644
--- a/devtools/server/tests/unit/test_extension_storage_actor.js
+++ b/devtools/server/tests/unit/test_extension_storage_actor.js
@@ -5,10 +5,6 @@
 
 "use strict";
 
-const { AddonTestUtils } = ChromeUtils.import(
-  "resource://testing-common/AddonTestUtils.jsm"
-);
-
 const { FileUtils } = ChromeUtils.import(
   "resource://gre/modules/FileUtils.jsm"
 );