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diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_extensionPreferencesManager.js b/toolkit/components/extensions/test/xpcshell/test_ext_extensionPreferencesManager.js
new file mode 100644
index 0000000000..2f6e3dbe14
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+++ b/toolkit/components/extensions/test/xpcshell/test_ext_extensionPreferencesManager.js
@@ -0,0 +1,877 @@
+/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set sts=2 sw=2 et tw=80: */
+"use strict";
+
+ChromeUtils.defineESModuleGetters(this, {
+  ExtensionPreferencesManager:
+    "resource://gre/modules/ExtensionPreferencesManager.sys.mjs",
+  ExtensionSettingsStore:
+    "resource://gre/modules/ExtensionSettingsStore.sys.mjs",
+  Preferences: "resource://gre/modules/Preferences.sys.mjs",
+});
+
+var { PromiseUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/PromiseUtils.sys.mjs"
+);
+
+const { createAppInfo, promiseShutdownManager, promiseStartupManager } =
+  AddonTestUtils;
+
+AddonTestUtils.init(this);
+
+createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");
+
+let lastSetPref;
+
+const STORE_TYPE = "prefs";
+
+// Test settings to use with the preferences manager.
+const SETTINGS = {
+  multiple_prefs: {
+    prefNames: ["my.pref.1", "my.pref.2", "my.pref.3"],
+
+    initalValues: ["value1", "value2", "value3"],
+
+    valueFn(pref, value) {
+      return `${pref}-${value}`;
+    },
+
+    setCallback(value) {
+      let prefs = {};
+      for (let pref of this.prefNames) {
+        prefs[pref] = this.valueFn(pref, value);
+      }
+      return prefs;
+    },
+  },
+
+  singlePref: {
+    prefNames: ["my.single.pref"],
+
+    initalValues: ["value1"],
+
+    onPrefsChanged(item) {
+      lastSetPref = item;
+    },
+
+    valueFn(pref, value) {
+      return value;
+    },
+
+    setCallback(value) {
+      return { [this.prefNames[0]]: this.valueFn(null, value) };
+    },
+  },
+};
+
+ExtensionPreferencesManager.addSetting(
+  "multiple_prefs",
+  SETTINGS.multiple_prefs
+);
+ExtensionPreferencesManager.addSetting("singlePref", SETTINGS.singlePref);
+
+// Set initial values for prefs.
+for (let setting in SETTINGS) {
+  setting = SETTINGS[setting];
+  for (let i = 0; i < setting.prefNames.length; i++) {
+    Preferences.set(setting.prefNames[i], setting.initalValues[i]);
+  }
+}
+
+function checkPrefs(settingObj, value, msg) {
+  for (let pref of settingObj.prefNames) {
+    equal(Preferences.get(pref), settingObj.valueFn(pref, value), msg);
+  }
+}
+
+function checkOnPrefsChanged(setting, value, msg) {
+  if (value) {
+    deepEqual(lastSetPref, value, msg);
+    lastSetPref = null;
+  } else {
+    ok(!lastSetPref, msg);
+  }
+}
+
+add_task(async function test_preference_manager() {
+  await promiseStartupManager();
+
+  // Create an array of test framework extension wrappers to install.
+  let testExtensions = [
+    ExtensionTestUtils.loadExtension({
+      useAddonManager: "temporary",
+      manifest: {},
+    }),
+    ExtensionTestUtils.loadExtension({
+      useAddonManager: "temporary",
+      manifest: {},
+    }),
+  ];
+
+  for (let extension of testExtensions) {
+    await extension.startup();
+  }
+
+  // Create an array actual Extension objects which correspond to the
+  // test framework extension wrappers.
+  let extensions = testExtensions.map(extension => extension.extension);
+
+  for (let setting in SETTINGS) {
+    let settingObj = SETTINGS[setting];
+    let newValue1 = "newValue1";
+    let levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+      extensions[1].id,
+      setting
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        null,
+        "onPrefsChanged has not been called yet"
+      );
+    }
+    equal(
+      levelOfControl,
+      "controllable_by_this_extension",
+      "getLevelOfControl returns correct levelOfControl with no settings set."
+    );
+
+    let prefsChanged = await ExtensionPreferencesManager.setSetting(
+      extensions[1].id,
+      setting,
+      newValue1
+    );
+    ok(prefsChanged, "setSetting returns true when the pref(s) have been set.");
+    checkPrefs(
+      settingObj,
+      newValue1,
+      "setSetting sets the prefs for the first extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        { id: extensions[1].id, value: newValue1, key: setting },
+        "onPrefsChanged is called when pref changes"
+      );
+    }
+    levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+      extensions[1].id,
+      setting
+    );
+    equal(
+      levelOfControl,
+      "controlled_by_this_extension",
+      "getLevelOfControl returns correct levelOfControl when a pref has been set."
+    );
+
+    let checkSetting = await ExtensionPreferencesManager.getSetting(setting);
+    equal(
+      checkSetting.value,
+      newValue1,
+      "getSetting returns the expected value."
+    );
+
+    let newValue2 = "newValue2";
+    prefsChanged = await ExtensionPreferencesManager.setSetting(
+      extensions[0].id,
+      setting,
+      newValue2
+    );
+    ok(
+      !prefsChanged,
+      "setSetting returns false when the pref(s) have not been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue1,
+      "setSetting does not set the pref(s) for an earlier extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        null,
+        "onPrefsChanged isn't called without control change"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.disableSetting(
+      extensions[0].id,
+      setting
+    );
+    ok(
+      !prefsChanged,
+      "disableSetting returns false when the pref(s) have not been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue1,
+      "disableSetting does not change the pref(s) for the non-top extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        null,
+        "onPrefsChanged isn't called without control change on disable"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.enableSetting(
+      extensions[0].id,
+      setting
+    );
+    ok(
+      !prefsChanged,
+      "enableSetting returns false when the pref(s) have not been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue1,
+      "enableSetting does not change the pref(s) for the non-top extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        null,
+        "onPrefsChanged isn't called without control change on enable"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.removeSetting(
+      extensions[0].id,
+      setting
+    );
+    ok(
+      !prefsChanged,
+      "removeSetting returns false when the pref(s) have not been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue1,
+      "removeSetting does not change the pref(s) for the non-top extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        null,
+        "onPrefsChanged isn't called without control change on remove"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.setSetting(
+      extensions[0].id,
+      setting,
+      newValue2
+    );
+    ok(
+      !prefsChanged,
+      "setSetting returns false when the pref(s) have not been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue1,
+      "setSetting does not set the pref(s) for an earlier extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        null,
+        "onPrefsChanged isn't called without control change again"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.disableSetting(
+      extensions[1].id,
+      setting
+    );
+    ok(
+      prefsChanged,
+      "disableSetting returns true when the pref(s) have been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue2,
+      "disableSetting sets the pref(s) to the next value when disabling the top extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        { id: extensions[0].id, key: setting, value: newValue2 },
+        "onPrefsChanged is called when control changes on disable"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.enableSetting(
+      extensions[1].id,
+      setting
+    );
+    ok(
+      prefsChanged,
+      "enableSetting returns true when the pref(s) have been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue1,
+      "enableSetting sets the pref(s) to the previous value(s)."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        { id: extensions[1].id, key: setting, value: newValue1 },
+        "onPrefsChanged is called when control changes on enable"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.removeSetting(
+      extensions[1].id,
+      setting
+    );
+    ok(
+      prefsChanged,
+      "removeSetting returns true when the pref(s) have been set."
+    );
+    checkPrefs(
+      settingObj,
+      newValue2,
+      "removeSetting sets the pref(s) to the next value when removing the top extension."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        { id: extensions[0].id, key: setting, value: newValue2 },
+        "onPrefsChanged is called when control changes on remove"
+      );
+    }
+
+    prefsChanged = await ExtensionPreferencesManager.removeSetting(
+      extensions[0].id,
+      setting
+    );
+    ok(
+      prefsChanged,
+      "removeSetting returns true when the pref(s) have been set."
+    );
+    if (settingObj.onPrefsChanged) {
+      checkOnPrefsChanged(
+        setting,
+        { key: setting, initialValue: { "my.single.pref": "value1" } },
+        "onPrefsChanged is called when control is entirely removed"
+      );
+    }
+    for (let i = 0; i < settingObj.prefNames.length; i++) {
+      equal(
+        Preferences.get(settingObj.prefNames[i]),
+        settingObj.initalValues[i],
+        "removeSetting sets the pref(s) to the initial value(s) when removing the last extension."
+      );
+    }
+
+    checkSetting = await ExtensionPreferencesManager.getSetting(setting);
+    equal(
+      checkSetting,
+      null,
+      "getSetting returns null when nothing has been set."
+    );
+  }
+
+  // Tests for unsetAll.
+  let newValue3 = "newValue3";
+  for (let setting in SETTINGS) {
+    let settingObj = SETTINGS[setting];
+    await ExtensionPreferencesManager.setSetting(
+      extensions[0].id,
+      setting,
+      newValue3
+    );
+    checkPrefs(settingObj, newValue3, "setSetting set the pref.");
+  }
+
+  let setSettings = await ExtensionSettingsStore.getAllForExtension(
+    extensions[0].id,
+    STORE_TYPE
+  );
+  deepEqual(
+    setSettings,
+    Object.keys(SETTINGS),
+    "Expected settings were set for extension."
+  );
+  await ExtensionPreferencesManager.disableAll(extensions[0].id);
+
+  for (let setting in SETTINGS) {
+    let settingObj = SETTINGS[setting];
+    for (let i = 0; i < settingObj.prefNames.length; i++) {
+      equal(
+        Preferences.get(settingObj.prefNames[i]),
+        settingObj.initalValues[i],
+        "disableAll unset the pref."
+      );
+    }
+  }
+
+  setSettings = await ExtensionSettingsStore.getAllForExtension(
+    extensions[0].id,
+    STORE_TYPE
+  );
+  deepEqual(
+    setSettings,
+    Object.keys(SETTINGS),
+    "disableAll retains the settings."
+  );
+
+  await ExtensionPreferencesManager.enableAll(extensions[0].id);
+  for (let setting in SETTINGS) {
+    let settingObj = SETTINGS[setting];
+    checkPrefs(settingObj, newValue3, "enableAll re-set the pref.");
+  }
+
+  await ExtensionPreferencesManager.removeAll(extensions[0].id);
+
+  for (let setting in SETTINGS) {
+    let settingObj = SETTINGS[setting];
+    for (let i = 0; i < settingObj.prefNames.length; i++) {
+      equal(
+        Preferences.get(settingObj.prefNames[i]),
+        settingObj.initalValues[i],
+        "removeAll unset the pref."
+      );
+    }
+  }
+
+  setSettings = await ExtensionSettingsStore.getAllForExtension(
+    extensions[0].id,
+    STORE_TYPE
+  );
+  deepEqual(setSettings, [], "removeAll removed all settings.");
+
+  // Tests for preventing automatic changes to manually edited prefs.
+  for (let setting in SETTINGS) {
+    let apiValue = "newValue";
+    let manualValue = "something different";
+    let settingObj = SETTINGS[setting];
+    let extension = extensions[1];
+    await ExtensionPreferencesManager.setSetting(
+      extension.id,
+      setting,
+      apiValue
+    );
+
+    let checkResetPrefs = method => {
+      let prefNames = settingObj.prefNames;
+      for (let i = 0; i < prefNames.length; i++) {
+        if (i === 0) {
+          equal(
+            Preferences.get(prefNames[0]),
+            manualValue,
+            `${method} did not change a manually set pref.`
+          );
+        } else {
+          equal(
+            Preferences.get(prefNames[i]),
+            settingObj.valueFn(prefNames[i], apiValue),
+            `${method} did not change another pref when a pref was manually set.`
+          );
+        }
+      }
+    };
+
+    // Manually set the preference to a different value.
+    Preferences.set(settingObj.prefNames[0], manualValue);
+
+    await ExtensionPreferencesManager.disableAll(extension.id);
+    checkResetPrefs("disableAll");
+
+    await ExtensionPreferencesManager.enableAll(extension.id);
+    checkResetPrefs("enableAll");
+
+    await ExtensionPreferencesManager.removeAll(extension.id);
+    checkResetPrefs("removeAll");
+  }
+
+  // Test with an uninitialized pref.
+  let setting = "singlePref";
+  let settingObj = SETTINGS[setting];
+  let pref = settingObj.prefNames[0];
+  let newValue = "newValue";
+  Preferences.reset(pref);
+  await ExtensionPreferencesManager.setSetting(
+    extensions[1].id,
+    setting,
+    newValue
+  );
+  equal(
+    Preferences.get(pref),
+    settingObj.valueFn(pref, newValue),
+    "Uninitialized pref is set."
+  );
+  await ExtensionPreferencesManager.removeSetting(extensions[1].id, setting);
+  ok(!Preferences.has(pref), "removeSetting removed the pref.");
+
+  // Test levelOfControl with a locked pref.
+  setting = "multiple_prefs";
+  let prefToLock = SETTINGS[setting].prefNames[0];
+  Preferences.lock(prefToLock, 1);
+  ok(Preferences.locked(prefToLock), `Preference ${prefToLock} is locked.`);
+  let levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+    extensions[1].id,
+    setting
+  );
+  equal(
+    levelOfControl,
+    "not_controllable",
+    "getLevelOfControl returns correct levelOfControl when a pref is locked."
+  );
+
+  for (let extension of testExtensions) {
+    await extension.unload();
+  }
+
+  await promiseShutdownManager();
+});
+
+add_task(async function test_preference_manager_set_when_disabled() {
+  await promiseStartupManager();
+
+  let id = "@set-disabled-pref";
+  let extension = ExtensionTestUtils.loadExtension({
+    useAddonManager: "temporary",
+    manifest: {
+      browser_specific_settings: { gecko: { id } },
+    },
+  });
+
+  await extension.startup();
+
+  // We test both a default pref and a user-set pref.  Get the default
+  // value off the pref we'll use.  We fake the default pref by setting
+  // a value on it before creating the setting.
+  Services.prefs.setBoolPref("bar", true);
+
+  function isUndefinedPref(pref) {
+    try {
+      Services.prefs.getStringPref(pref);
+      return false;
+    } catch (e) {
+      return true;
+    }
+  }
+  ok(isUndefinedPref("foo"), "test pref is not set");
+
+  await ExtensionSettingsStore.initialize();
+  let lastItemChange = PromiseUtils.defer();
+  ExtensionPreferencesManager.addSetting("some-pref", {
+    prefNames: ["foo", "bar"],
+    onPrefsChanged(item) {
+      lastItemChange.resolve(item);
+      lastItemChange = PromiseUtils.defer();
+    },
+    setCallback(value) {
+      return { [this.prefNames[0]]: value, [this.prefNames[1]]: false };
+    },
+  });
+
+  await ExtensionPreferencesManager.setSetting(id, "some-pref", "my value");
+
+  let item = ExtensionSettingsStore.getSetting("prefs", "some-pref");
+  equal(item.value, "my value", "The value has been set");
+  equal(
+    Services.prefs.getStringPref("foo"),
+    "my value",
+    "The user pref has been set"
+  );
+  equal(
+    Services.prefs.getBoolPref("bar"),
+    false,
+    "The default pref has been set"
+  );
+
+  await ExtensionPreferencesManager.disableSetting(id, "some-pref");
+
+  // test that a disabled setting has been returned to the default value.  In this
+  // case the pref is not a default pref, so it will be undefined.
+  item = ExtensionSettingsStore.getSetting("prefs", "some-pref");
+  equal(item.value, undefined, "The value is back to default");
+  equal(item.initialValue.foo, undefined, "The initialValue is correct");
+  ok(isUndefinedPref("foo"), "user pref is not set");
+  equal(
+    Services.prefs.getBoolPref("bar"),
+    true,
+    "The default pref has been restored to the default"
+  );
+
+  // test that setSetting() will enable a disabled setting
+  await ExtensionPreferencesManager.setSetting(id, "some-pref", "new value");
+
+  item = ExtensionSettingsStore.getSetting("prefs", "some-pref");
+  equal(item.value, "new value", "The value is set again");
+  equal(
+    Services.prefs.getStringPref("foo"),
+    "new value",
+    "The user pref is set again"
+  );
+  equal(
+    Services.prefs.getBoolPref("bar"),
+    false,
+    "The default pref has been set again"
+  );
+
+  // Force settings to be serialized and reloaded to mimick what happens
+  // with settings through a restart of Firefox.  Bug 1576266.
+  await ExtensionSettingsStore._reloadFile(true);
+
+  // Now unload the extension to test prefs are reset properly.
+  let promise = lastItemChange.promise;
+  await extension.unload();
+
+  // Test that the pref is unset when an extension is uninstalled.
+  item = await promise;
+  deepEqual(
+    item,
+    { key: "some-pref", initialValue: { bar: true } },
+    "The value has been reset"
+  );
+  ok(isUndefinedPref("foo"), "user pref is not set");
+  equal(
+    Services.prefs.getBoolPref("bar"),
+    true,
+    "The default pref has been restored to the default"
+  );
+  Services.prefs.clearUserPref("bar");
+
+  await promiseShutdownManager();
+});
+
+add_task(async function test_preference_default_upgraded() {
+  await promiseStartupManager();
+
+  let id = "@upgrade-pref";
+  let extension = ExtensionTestUtils.loadExtension({
+    useAddonManager: "temporary",
+    manifest: {
+      browser_specific_settings: { gecko: { id } },
+    },
+  });
+
+  await extension.startup();
+
+  // We set the default value for a pref here so it will be
+  // picked up by EPM.
+  let defaultPrefs = Services.prefs.getDefaultBranch(null);
+  defaultPrefs.setStringPref("bar", "initial default");
+
+  await ExtensionSettingsStore.initialize();
+  ExtensionPreferencesManager.addSetting("some-pref", {
+    prefNames: ["bar"],
+    setCallback(value) {
+      return { [this.prefNames[0]]: value };
+    },
+  });
+
+  await ExtensionPreferencesManager.setSetting(id, "some-pref", "new value");
+  let item = ExtensionSettingsStore.getSetting("prefs", "some-pref");
+  equal(item.value, "new value", "The value is set");
+
+  defaultPrefs.setStringPref("bar", "new default");
+
+  item = ExtensionSettingsStore.getSetting("prefs", "some-pref");
+  equal(item.value, "new value", "The value is still set");
+
+  let prefsChanged = await ExtensionPreferencesManager.removeSetting(
+    id,
+    "some-pref"
+  );
+  ok(prefsChanged, "pref changed on removal of setting.");
+  equal(Preferences.get("bar"), "new default", "default value is correct");
+
+  await extension.unload();
+  await promiseShutdownManager();
+});
+
+add_task(async function test_preference_select() {
+  await promiseStartupManager();
+
+  let extensionData = {
+    useAddonManager: "temporary",
+    manifest: {
+      browser_specific_settings: { gecko: { id: "@one" } },
+    },
+  };
+  let one = ExtensionTestUtils.loadExtension(extensionData);
+
+  await one.startup();
+
+  // We set the default value for a pref here so it will be
+  // picked up by EPM.
+  let defaultPrefs = Services.prefs.getDefaultBranch(null);
+  defaultPrefs.setStringPref("bar", "initial default");
+
+  await ExtensionSettingsStore.initialize();
+  ExtensionPreferencesManager.addSetting("some-pref", {
+    prefNames: ["bar"],
+    setCallback(value) {
+      return { [this.prefNames[0]]: value };
+    },
+  });
+
+  ok(
+    await ExtensionPreferencesManager.setSetting(
+      one.id,
+      "some-pref",
+      "new value"
+    ),
+    "setting was changed"
+  );
+  let item = await ExtensionPreferencesManager.getSetting("some-pref");
+  equal(item.value, "new value", "The value is set");
+
+  // User-set the setting.
+  await ExtensionPreferencesManager.selectSetting(null, "some-pref");
+  item = await ExtensionPreferencesManager.getSetting("some-pref");
+  deepEqual(
+    item,
+    { key: "some-pref", initialValue: {} },
+    "The value is user-set"
+  );
+
+  // Extensions installed before cannot gain control again.
+  let levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+    one.id,
+    "some-pref"
+  );
+  equal(
+    levelOfControl,
+    "not_controllable",
+    "getLevelOfControl returns correct levelOfControl when user-set."
+  );
+
+  // Enabling the top-precedence addon does not take over a user-set setting.
+  await ExtensionPreferencesManager.disableSetting(one.id, "some-pref");
+  await ExtensionPreferencesManager.enableSetting(one.id, "some-pref");
+  item = await ExtensionPreferencesManager.getSetting("some-pref");
+  deepEqual(
+    item,
+    { key: "some-pref", initialValue: {} },
+    "The value is user-set"
+  );
+
+  // Upgrading does not override the user-set setting.
+  extensionData.manifest.version = "2.0";
+  extensionData.manifest.incognito = "not_allowed";
+  await one.upgrade(extensionData);
+  levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+    one.id,
+    "some-pref"
+  );
+  equal(
+    levelOfControl,
+    "not_controllable",
+    "getLevelOfControl returns correct levelOfControl when user-set after addon upgrade."
+  );
+
+  // We can re-select the extension.
+  await ExtensionPreferencesManager.selectSetting(one.id, "some-pref");
+  item = await ExtensionPreferencesManager.getSetting("some-pref");
+  deepEqual(item.value, "new value", "The value is extension set");
+
+  // An extension installed after user-set can take over the setting.
+  await ExtensionPreferencesManager.selectSetting(null, "some-pref");
+  item = await ExtensionPreferencesManager.getSetting("some-pref");
+  deepEqual(
+    item,
+    { key: "some-pref", initialValue: {} },
+    "The value is user-set"
+  );
+
+  let two = ExtensionTestUtils.loadExtension({
+    useAddonManager: "temporary",
+    manifest: {
+      browser_specific_settings: { gecko: { id: "@two" } },
+    },
+  });
+
+  await two.startup();
+  levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+    two.id,
+    "some-pref"
+  );
+  equal(
+    levelOfControl,
+    "controllable_by_this_extension",
+    "getLevelOfControl returns correct levelOfControl when user-set after addon install."
+  );
+
+  await ExtensionPreferencesManager.setSetting(
+    two.id,
+    "some-pref",
+    "another value"
+  );
+  item = ExtensionSettingsStore.getSetting("prefs", "some-pref");
+  equal(item.value, "another value", "The value is set");
+
+  // A new installed extension can override a user selected extension.
+  let three = ExtensionTestUtils.loadExtension({
+    useAddonManager: "temporary",
+    manifest: {
+      browser_specific_settings: { gecko: { id: "@three" } },
+    },
+  });
+
+  // user selects specific extension to take control
+  await ExtensionPreferencesManager.selectSetting(one.id, "some-pref");
+
+  // two cannot control
+  levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+    two.id,
+    "some-pref"
+  );
+  equal(
+    levelOfControl,
+    "not_controllable",
+    "getLevelOfControl returns correct levelOfControl when user-set after addon install."
+  );
+
+  // three can control after install
+  await three.startup();
+  levelOfControl = await ExtensionPreferencesManager.getLevelOfControl(
+    three.id,
+    "some-pref"
+  );
+  equal(
+    levelOfControl,
+    "controllable_by_this_extension",
+    "getLevelOfControl returns correct levelOfControl when user-set after addon install."
+  );
+
+  await ExtensionPreferencesManager.setSetting(
+    three.id,
+    "some-pref",
+    "third value"
+  );
+  item = ExtensionSettingsStore.getSetting("prefs", "some-pref");
+  equal(item.value, "third value", "The value is set");
+
+  // We have returned to precedence based settings.
+  await ExtensionPreferencesManager.removeSetting(three.id, "some-pref");
+  await ExtensionPreferencesManager.removeSetting(two.id, "some-pref");
+  item = await ExtensionPreferencesManager.getSetting("some-pref");
+  equal(item.value, "new value", "The value is extension set");
+
+  await one.unload();
+  await two.unload();
+  await three.unload();
+  await promiseShutdownManager();
+});
+
+add_task(async function test_preference_select() {
+  let prefNames = await ExtensionPreferencesManager.getManagedPrefDetails();
+  // Just check a subset of settings that are in this test file.
+  Assert.ok(prefNames.size > 0, "some prefs exist");
+  for (let settingName in SETTINGS) {
+    let setting = SETTINGS[settingName];
+    for (let prefName of setting.prefNames) {
+      Assert.equal(
+        prefNames.get(prefName),
+        settingName,
+        "setting retrieved prefNames"
+      );
+    }
+  }
+});