--- a/browser/base/content/test/static/browser_all_files_referenced.js
+++ b/browser/base/content/test/static/browser_all_files_referenced.js
@@ -117,19 +117,16 @@ var whitelist = [
 
   // browser/extensions/pdfjs/content/web/viewer.js#7450
   {file: "resource://pdf.js/web/debugger.js"},
 
   // These are used in content processes. They are actually referenced.
   {file: "resource://shield-recipe-client-content/shield-content-frame.js"},
   {file: "resource://shield-recipe-client-content/shield-content-process.js"},
 
-  // New L10n API that is not yet used in production
-  {file: "chrome://global/content/l10n.js"},
-
   // Starting from here, files in the whitelist are bugs that need fixing.
   // Bug 1339424 (wontfix?)
   {file: "chrome://browser/locale/taskbar.properties",
    platforms: ["linux", "macosx"]},
   // Bug 1316187
   {file: "chrome://global/content/customizeToolbar.xul"},
   // Bug 1343837
   {file: "chrome://global/content/findUtils.js"},

--- a/browser/components/preferences/in-content/main.js
+++ b/browser/components/preferences/in-content/main.js
@@ -1181,22 +1181,38 @@ var gMainPane = {
     }
   },
 
   buildContentProcessCountMenuList() {
     if (Services.appinfo.browserTabsRemoteAutostart) {
       let processCountPref = document.getElementById("dom.ipc.processCount");
       let defaultProcessCount = processCountPref.defaultValue;
       let bundlePreferences = document.getElementById("bundlePreferences");
-      let label = bundlePreferences.getFormattedString("defaultContentProcessCount",
-        [defaultProcessCount]);
+
       let contentProcessCount =
         document.querySelector(`#contentProcessCount > menupopup >
                                 menuitem[value="${defaultProcessCount}"]`);
-      contentProcessCount.label = label;
+
+      // New localization API experiment (October 2017).
+      // See bug 1402061 for details.
+      //
+      // The `intl.l10n.fluent.disabled` pref can be used
+      // to opt-out of the experiment in case of any
+      // unforseen problems. The legacy API will then
+      // be used.
+      if (Services.prefs.getBoolPref("intl.l10n.fluent.disabled", false)) {
+        let label = bundlePreferences.getFormattedString("defaultContentProcessCount",
+          [defaultProcessCount]);
+        contentProcessCount.label = label;
+      } else {
+        document.l10n.setAttributes(
+          contentProcessCount,
+          "default-content-process-count",
+          { num: defaultProcessCount });
+      }
 
       document.getElementById("limitContentProcess").disabled = false;
       document.getElementById("contentProcessCount").disabled = false;
       document.getElementById("contentProcessCountEnabledDescription").hidden = false;
       document.getElementById("contentProcessCountDisabledDescription").hidden = true;
     } else {
       document.getElementById("limitContentProcess").disabled = true;
       document.getElementById("contentProcessCount").disabled = true;

--- a/browser/components/preferences/in-content/main.xul
+++ b/browser/components/preferences/in-content/main.xul
@@ -1,14 +1,17 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 <!-- General panel -->
 
+<link rel="localization" href="browser/preferences/main.ftl"/>
+<script type="text/javascript" src="chrome://global/content/l10n.js"></script>
+
 <script type="application/javascript"
         src="chrome://browser/content/preferences/in-content/main.js"/>
 
 #ifdef MOZ_UPDATER
   <script type="application/javascript" src="chrome://browser/content/aboutDialog-appUpdater.js"/>
 #endif
 
 <script type="application/javascript"

--- a/browser/components/preferences/in-content/tests/browser.ini
+++ b/browser/components/preferences/in-content/tests/browser.ini
@@ -80,8 +80,9 @@ skip-if = e10s
 support-files =
   subdialog.xul
   subdialog2.xul
 [browser_telemetry.js]
 # Skip this test on Android as FHR and Telemetry are separate systems there.
 skip-if = !healthreport || !telemetry || (os == 'linux' && debug) || (os == 'android')
 [browser_containers_name_input.js]
 run-if = nightly_build # Containers is enabled only on Nightly
+[browser_fluent.js]

new file mode 100644
--- /dev/null
+++ b/browser/components/preferences/in-content/tests/browser_fluent.js
@@ -0,0 +1,48 @@
+/* eslint-disable mozilla/no-cpows-in-tests */
+
+function whenMainPaneLoadedFinished() {
+  return new Promise(function(resolve, reject) {
+    const topic = "main-pane-loaded";
+    Services.obs.addObserver(function observer(aSubject) {
+      Services.obs.removeObserver(observer, topic);
+      resolve();
+    }, topic);
+  });
+}
+
+// Temporary test for an experimental new localization API.
+// See bug 1402069 for details.
+add_task(async function() {
+  // The string is used only when `browserTabsRemoteAutostart` is true
+  if (!Services.appinfo.browserTabsRemoteAutostart) {
+    ok(true, "fake test to avoid harness complaining");
+    return;
+  }
+
+  await Promise.all([
+    openPreferencesViaOpenPreferencesAPI("general", {leaveOpen: true}),
+    whenMainPaneLoadedFinished(),
+  ]);
+
+  let doc = gBrowser.contentDocument;
+  await doc.l10n.ready;
+
+  let processCountPref = doc.getElementById("dom.ipc.processCount");
+  let defaultProcessCount = processCountPref.defaultValue;
+
+  let [ msg ] = await doc.l10n.formatMessages([
+    ["default-content-process-count", { num: defaultProcessCount }]
+  ]);
+
+  let elem = doc.querySelector(
+    `#contentProcessCount > menupopup > menuitem[value="${defaultProcessCount}"]`);
+
+  Assert.deepEqual(msg, {
+    value: null,
+    attrs: [
+      ["label", elem.getAttribute("label")]
+    ]
+  });
+
+  await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+});

--- a/browser/installer/package-manifest.in
+++ b/browser/installer/package-manifest.in
@@ -41,16 +41,17 @@
 @APPNAME@/Contents/PkgInfo
 @RESPATH@/firefox.icns
 @RESPATH@/document.icns
 @RESPATH@/@LPROJ_ROOT@.lproj/*
 #endif
 
 [@AB_CD@]
 @RESPATH@/dictionaries/*
+@RESPATH@/browser/localization/*
 #if defined(XP_WIN) || defined(XP_LINUX)
 @RESPATH@/fonts/*
 #endif
 @RESPATH@/hyphenation/*
 @RESPATH@/browser/@PREF_DIR@/firefox-l10n.js
 #ifdef HAVE_MAKENSISU
 @BINPATH@/uninstall/helper.exe
 #endif

new file mode 100644
--- /dev/null
+++ b/browser/locales/en-US/browser/preferences/main.ftl
@@ -0,0 +1,4 @@
+// Variables:
+//   $num - default value of the `dom.ipc.processCount` pref.
+default-content-process-count
+    .label = { $num } (default)

--- a/browser/locales/jar.mn
+++ b/browser/locales/jar.mn
@@ -2,16 +2,19 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 # Note: This file should only contain locale entries. All
 # override and resource entries should go to browser/base/jar.mn to avoid
 # having to create the same entry for each locale.
 
+[localization] @AB_CD@.jar:
+  browser                                          (%browser/**/*.ftl)
+
 @AB_CD@.jar:
 % locale browser @AB_CD@ %locale/browser/
 *   locale/browser/bookmarks.html                  (generic/profile/bookmarks.html.in)
     locale/browser/aboutDialog.dtd                 (%chrome/browser/aboutDialog.dtd)
     locale/browser/aboutPrivateBrowsing.dtd        (%chrome/browser/aboutPrivateBrowsing.dtd)
     locale/browser/aboutPrivateBrowsing.properties (%chrome/browser/aboutPrivateBrowsing.properties)
     locale/browser/aboutRobots.dtd                 (%chrome/browser/aboutRobots.dtd)
     locale/browser/aboutHome.dtd                   (%chrome/browser/aboutHome.dtd)