--- a/browser/components/newtab/aboutNewTabService.js
+++ b/browser/components/newtab/aboutNewTabService.js
@@ -178,30 +178,22 @@ AboutNewTabService.prototype = {
    * Figure out what path under prerendered to use based on current state.
    */
   updatePrerenderedPath() {
     // Debug files are specially packaged in a non-localized directory
     let path;
     if (this._activityStreamDebug) {
       path = "static";
     } else {
-      // Use the exact match locale if it's packaged
-      const locale = Services.locale.getRequestedLocale();
-      if (ACTIVITY_STREAM_LOCALES.has(locale)) {
-        path = locale;
-      } else {
-        // Fall back to a shared-language packaged locale
-        const language = locale.split("-")[0];
-        if (ACTIVITY_STREAM_LOCALES.has(language)) {
-          path = language;
-        } else {
-          // Just use the default locale as a final fallback
-          path = "en-US";
-        }
-      }
+      let locales = Services.locale.negotiateLanguages(
+        Services.locale.getAppLocalesAsLangTags(),
+        ACTIVITY_STREAM_LOCALES,
+        Services.locale.defaultLocale
+      );
+      path = locales[0];
     }
     this._activityStreamPath = `${path}/`;
   },
 
   /*
    * Returns the default URL.
    *
    * This URL only depends on the browser.newtabpage.activity-stream.enabled pref. Overriding

--- a/browser/extensions/activity-stream/lib/ActivityStream.jsm
+++ b/browser/extensions/activity-stream/lib/ActivityStream.jsm
@@ -287,17 +287,17 @@ this.ActivityStream = class ActivityStre
     if (Services.prefs.prefHasUserValue(GEO_PREF)) {
       this.geo = Services.prefs.getStringPref(GEO_PREF);
     } else if (this.geo !== "") {
       // Watch for geo changes and use a dummy value for now
       Services.prefs.addObserver(GEO_PREF, this);
       this.geo = "";
     }
 
-    this.locale = Services.locale.getRequestedLocale();
+    this.locale = Services.locale.getAppLocaleAsLangTag();
 
     // Update the pref config of those with dynamic values
     for (const pref of PREFS_CONFIG.keys()) {
       const prefConfig = PREFS_CONFIG.get(pref);
       if (!prefConfig.getValue) {
         continue;
       }
 

--- a/browser/extensions/activity-stream/lib/TelemetryFeed.jsm
+++ b/browser/extensions/activity-stream/lib/TelemetryFeed.jsm
@@ -254,17 +254,17 @@ this.TelemetryFeed = class TelemetryFeed
    *
    * @param  {string} id The portID of the session, if a session is relevant (optional)
    * @return {obj}    A telemetry ping
    */
   createPing(portID) {
     const appInfo = this.store.getState().App;
     const ping = {
       addon_version: appInfo.version,
-      locale: Services.locale.getRequestedLocale(),
+      locale: Services.locale.getAppLocaleAsLangTag(),
       user_prefs: this.userPreferences
     };
 
     // If the ping is part of a user session, add session-related info
     if (portID) {
       const session = this.sessions.get(portID) || this.addSession(portID);
       Object.assign(ping, {session_id: session.session_id});
 

--- a/browser/extensions/activity-stream/test/unit/lib/ActivityStream.test.js
+++ b/browser/extensions/activity-stream/test/unit/lib/ActivityStream.test.js
@@ -166,39 +166,39 @@ describe("ActivityStream", () => {
 
       as._updateDynamicPrefs();
 
       assert.isFalse(PREFS_CONFIG.get("feeds.section.topstories").value);
     });
     it("should be false with expected geo and unexpected locale", () => {
       sandbox.stub(global.Services.prefs, "prefHasUserValue").returns(true);
       sandbox.stub(global.Services.prefs, "getStringPref").returns("US");
-      sandbox.stub(global.Services.locale, "getRequestedLocale").returns("no-LOCALE");
+      sandbox.stub(global.Services.locale, "getAppLocalesAsLangTags").returns("no-LOCALE");
 
       as._updateDynamicPrefs();
 
       assert.isFalse(PREFS_CONFIG.get("feeds.section.topstories").value);
     });
     it("should be true with expected geo and locale", () => {
       sandbox.stub(global.Services.prefs, "prefHasUserValue").returns(true);
       sandbox.stub(global.Services.prefs, "getStringPref").returns("US");
-      sandbox.stub(global.Services.locale, "getRequestedLocale").returns("en-US");
+      sandbox.stub(global.Services.locale, "getAppLocalesAsLangTags").returns("en-US");
 
       as._updateDynamicPrefs();
 
       assert.isTrue(PREFS_CONFIG.get("feeds.section.topstories").value);
     });
     it("should be false after expected geo and locale then unexpected", () => {
       sandbox.stub(global.Services.prefs, "prefHasUserValue").returns(true);
       sandbox.stub(global.Services.prefs, "getStringPref")
         .onFirstCall()
         .returns("US")
         .onSecondCall()
         .returns("NOGEO");
-      sandbox.stub(global.Services.locale, "getRequestedLocale").returns("en-US");
+      sandbox.stub(global.Services.locale, "getAppLocalesAsLangTags").returns("en-US");
 
       as._updateDynamicPrefs();
       as._updateDynamicPrefs();
 
       assert.isFalse(PREFS_CONFIG.get("feeds.section.topstories").value);
     });
   });
   describe("_updateDynamicPrefs topstories delayed default value", () => {
@@ -219,17 +219,17 @@ describe("ActivityStream", () => {
 
       as._updateDynamicPrefs();
       clock.tick(1);
 
       assert.isFalse(PREFS_CONFIG.get("feeds.section.topstories").value);
     });
     it("should set true with expected geo and locale", () => {
       sandbox.stub(global.Services.prefs, "getStringPref").returns("US");
-      sandbox.stub(global.Services.locale, "getRequestedLocale").returns("en-US");
+      sandbox.stub(global.Services.locale, "getAppLocalesAsLangTags").returns("en-US");
 
       as._updateDynamicPrefs();
       clock.tick(1);
 
       assert.isTrue(PREFS_CONFIG.get("feeds.section.topstories").value);
     });
   });
   describe("telemetry reporting on init failure", () => {

--- a/browser/extensions/activity-stream/test/unit/lib/TopStoriesFeed.test.js
+++ b/browser/extensions/activity-stream/test/unit/lib/TopStoriesFeed.test.js
@@ -29,17 +29,17 @@ describe("Top Stories Feed", () => {
     "provider_icon": "provider-icon",
     "provider_description": "provider_desc"
   };
 
   beforeEach(() => {
     FakePrefs.prototype.prefs.apiKeyPref = "test-api-key";
 
     globals = new GlobalOverrider();
-    globals.set("Services", {locale: {getRequestedLocale: () => "en-CA"}, obs: {addObserver: () => {}, removeObserver: () => {}}});
+    globals.set("Services", {locale: {getAppLocalesAsLangTags: () => ["en-CA"]}, obs: {addObserver: () => {}, removeObserver: () => {}}});
     globals.set("PlacesUtils", {history: {}});
     clock = sinon.useFakeTimers();
     shortURLStub = sinon.stub().callsFake(site => site.url);
     sectionsManagerStub = {
       onceInitialized: sinon.stub().callsFake(callback => callback()),
       enableSection: sinon.spy(),
       disableSection: sinon.spy(),
       updateSection: sinon.spy(),
@@ -126,17 +126,17 @@ describe("Top Stories Feed", () => {
     it("should report error for invalid configuration", () => {
       globals.sandbox.spy(global.Components.utils, "reportError");
       sectionsManagerStub.sections.set("topstories", {options: {api_key_pref: "invalid"}});
       instance.init();
 
       assert.called(Components.utils.reportError);
     });
     it("should report error for missing api key", () => {
-      let fakeServices = {prefs: {getCharPref: sinon.spy()}, locale: {getRequestedLocale: sinon.spy()}};
+      let fakeServices = {prefs: {getCharPref: sinon.spy()}, locale: {getAppLocalesAsLangTags: sinon.spy()}};
       globals.set("Services", fakeServices);
       globals.sandbox.spy(global.Components.utils, "reportError");
       sectionsManagerStub.sections.set("topstories", {
         options: {
           "stories_endpoint": "https://somedomain.org/stories?key=$apiKey",
           "topics_endpoint": "https://somedomain.org/topics?key=$apiKey"
         }
       });

--- a/browser/extensions/activity-stream/test/unit/lib/UserDomainAffinityProvider.test.js
+++ b/browser/extensions/activity-stream/test/unit/lib/UserDomainAffinityProvider.test.js
@@ -32,17 +32,17 @@ const PARAMETER_SETS = {
 
 describe("User Domain Affinity Provider", () => {
   let UserDomainAffinityProvider;
   let instance;
   let globals;
 
   beforeEach(() => {
     globals = new GlobalOverrider();
-    globals.set("Services", {locale: {getRequestedLocale: () => "en-CA"}, io: {newURI: u => ({host: "www.somedomain.org"})}});
+    globals.set("Services", {locale: {getAppLocalesAsLangTags: () => ["en-CA"]}, io: {newURI: u => ({host: "www.somedomain.org"})}});
     globals.set("PlacesUtils", {
       history: {
         getNewQuery: () => ({"TIME_RELATIVE_NOW": 1}),
         getNewQueryOptions: () => ({}),
         executeQuery: () => ({root: {childCount: 1, getChild: index => ({uri: "www.somedomain.org", accessCount: 1})}})
       }
     });
     global.Components.classes["@mozilla.org/browser/nav-history-service;1"] = {

--- a/browser/extensions/activity-stream/test/unit/unit-entry.js
+++ b/browser/extensions/activity-stream/test/unit/unit-entry.js
@@ -39,21 +39,22 @@ overrider.set({
   ContentSearchUIController: function() {}, // NB: This is a function/constructor
   dump() {},
   fetch() {},
   // eslint-disable-next-line object-shorthand
   Image: function() {}, // NB: This is a function/constructor
   Preferences: FakePrefs,
   Services: {
     locale: {
-      getAppLocalesAsLangTags() {},
-      getRequestedLocale() {
-        return "en-US";
+      getAppLocalesAsLangTags() {
+        return ["en-US"];
       },
-      negotiateLanguages() {}
+      negotiateLanguages() {
+        return ["en-US"];
+      }
     },
     urlFormatter: {formatURL: str => str},
     mm: {
       addMessageListener: (msg, cb) => cb(),
       removeMessageListener() {}
     },
     appShell: {hiddenDOMWindow: {performance: new FakePerformance()}},
     obs: {

--- a/browser/extensions/formautofill/bootstrap.js
+++ b/browser/extensions/formautofill/bootstrap.js
@@ -52,17 +52,17 @@ function addUpgradeListener(instanceID) 
   });
 }
 
 function isAvailable() {
   let availablePref = Services.prefs.getCharPref("extensions.formautofill.available");
   if (availablePref == "on") {
     return true;
   } else if (availablePref == "detect") {
-    let locale = Services.locale.getRequestedLocale();
+    let locale = Services.locale.getAppLocaleAsLangTag();
     let region = Services.prefs.getCharPref("browser.search.region", "");
     let supportedCountries = Services.prefs.getCharPref("extensions.formautofill.supportedCountries")
                                            .split(",");
     return locale == "en-US" && supportedCountries.includes(region);
   }
   return false;
 }