From 26a029d407be480d791972afb5975cf62c9360a6 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Fri, 19 Apr 2024 02:47:55 +0200
Subject: Adding upstream version 124.0.1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 intl/l10n/test/gtest/TestLocalization.cpp          | 118 +++++
 intl/l10n/test/gtest/moz.build                     |  11 +
 intl/l10n/test/mochitest/chrome.toml               |  10 +
 .../localization/test_formatMessages.html          | 101 ++++
 .../mochitest/localization/test_formatValue.html   |  81 +++
 .../mochitest/localization/test_formatValues.html  |  85 ++++
 intl/l10n/test/test_datetimeformat.js              |  76 +++
 intl/l10n/test/test_l10nregistry.js                | 563 +++++++++++++++++++++
 intl/l10n/test/test_l10nregistry_fuzzed.js         | 205 ++++++++
 intl/l10n/test/test_l10nregistry_sync.js           | 536 ++++++++++++++++++++
 intl/l10n/test/test_localization.js                | 319 ++++++++++++
 intl/l10n/test/test_localization_sync.js           | 289 +++++++++++
 intl/l10n/test/test_messagecontext.js              |  47 ++
 intl/l10n/test/test_missing_variables.js           |  35 ++
 intl/l10n/test/test_pseudo.js                      | 131 +++++
 intl/l10n/test/xpcshell.toml                       |  21 +
 16 files changed, 2628 insertions(+)
 create mode 100644 intl/l10n/test/gtest/TestLocalization.cpp
 create mode 100644 intl/l10n/test/gtest/moz.build
 create mode 100644 intl/l10n/test/mochitest/chrome.toml
 create mode 100644 intl/l10n/test/mochitest/localization/test_formatMessages.html
 create mode 100644 intl/l10n/test/mochitest/localization/test_formatValue.html
 create mode 100644 intl/l10n/test/mochitest/localization/test_formatValues.html
 create mode 100644 intl/l10n/test/test_datetimeformat.js
 create mode 100644 intl/l10n/test/test_l10nregistry.js
 create mode 100644 intl/l10n/test/test_l10nregistry_fuzzed.js
 create mode 100644 intl/l10n/test/test_l10nregistry_sync.js
 create mode 100644 intl/l10n/test/test_localization.js
 create mode 100644 intl/l10n/test/test_localization_sync.js
 create mode 100644 intl/l10n/test/test_messagecontext.js
 create mode 100644 intl/l10n/test/test_missing_variables.js
 create mode 100644 intl/l10n/test/test_pseudo.js
 create mode 100644 intl/l10n/test/xpcshell.toml

(limited to 'intl/l10n/test')

diff --git a/intl/l10n/test/gtest/TestLocalization.cpp b/intl/l10n/test/gtest/TestLocalization.cpp
new file mode 100644
index 0000000000..804c228c98
--- /dev/null
+++ b/intl/l10n/test/gtest/TestLocalization.cpp
@@ -0,0 +1,118 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+#include "gtest/gtest.h"
+#include "mozilla/intl/Localization.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+using namespace mozilla::intl;
+
+TEST(Intl_Localization, FormatValueSyncMissing)
+{
+  nsTArray<nsCString> resIds = {
+      "toolkit/global/handlerDialog.ftl"_ns,
+  };
+  RefPtr<Localization> l10n = Localization::Create(resIds, true);
+
+  auto l10nId = "non-existing-l10n-id"_ns;
+  IgnoredErrorResult rv;
+  nsAutoCString result;
+
+  l10n->FormatValueSync(l10nId, {}, result, rv);
+  ASSERT_FALSE(rv.Failed());
+  ASSERT_TRUE(result.IsEmpty());
+}
+
+TEST(Intl_Localization, FormatValueSync)
+{
+  nsTArray<nsCString> resIds = {
+      "toolkit/global/handlerDialog.ftl"_ns,
+  };
+  RefPtr<Localization> l10n = Localization::Create(resIds, true);
+
+  auto l10nId = "permission-dialog-unset-description"_ns;
+  IgnoredErrorResult rv;
+  nsAutoCString result;
+
+  l10n->FormatValueSync(l10nId, {}, result, rv);
+  ASSERT_FALSE(rv.Failed());
+  ASSERT_FALSE(result.IsEmpty());
+}
+
+TEST(Intl_Localization, FormatValueSyncWithArgs)
+{
+  nsTArray<nsCString> resIds = {
+      "toolkit/global/handlerDialog.ftl"_ns,
+  };
+  RefPtr<Localization> l10n = Localization::Create(resIds, true);
+
+  auto l10nId = "permission-dialog-description"_ns;
+
+  auto l10nArgs = dom::Optional<intl::L10nArgs>();
+  l10nArgs.Construct();
+
+  auto dirArg = l10nArgs.Value().Entries().AppendElement();
+  dirArg->mKey = "scheme"_ns;
+  dirArg->mValue.SetValue().SetAsUTF8String().Assign("Foo"_ns);
+
+  IgnoredErrorResult rv;
+  nsAutoCString result;
+
+  l10n->FormatValueSync(l10nId, l10nArgs, result, rv);
+  ASSERT_FALSE(rv.Failed());
+  ASSERT_TRUE(result.Find("Foo"_ns) > -1);
+}
+
+TEST(Intl_Localization, FormatMessagesSync)
+{
+  nsTArray<nsCString> resIds = {
+      "toolkit/global/handlerDialog.ftl"_ns,
+  };
+  RefPtr<Localization> l10n = Localization::Create(resIds, true);
+
+  dom::Sequence<dom::OwningUTF8StringOrL10nIdArgs> l10nIds;
+  auto* l10nId = l10nIds.AppendElement(fallible);
+  ASSERT_TRUE(l10nId);
+  l10nId->SetAsUTF8String().Assign("permission-dialog-unset-description"_ns);
+
+  IgnoredErrorResult rv;
+  nsTArray<dom::Nullable<dom::L10nMessage>> result;
+
+  l10n->FormatMessagesSync(l10nIds, result, rv);
+  ASSERT_FALSE(rv.Failed());
+  ASSERT_FALSE(result.IsEmpty());
+}
+
+TEST(Intl_Localization, FormatMessagesSyncWithArgs)
+{
+  nsTArray<nsCString> resIds = {
+      "toolkit/global/handlerDialog.ftl"_ns,
+  };
+  RefPtr<Localization> l10n = Localization::Create(resIds, true);
+
+  dom::Sequence<dom::OwningUTF8StringOrL10nIdArgs> l10nIds;
+  L10nIdArgs& key0 = l10nIds.AppendElement(fallible)->SetAsL10nIdArgs();
+  key0.mId.Assign("permission-dialog-description"_ns);
+  auto arg = key0.mArgs.SetValue().Entries().AppendElement();
+  arg->mKey = "scheme"_ns;
+  arg->mValue.SetValue().SetAsUTF8String().Assign("Foo"_ns);
+
+  L10nIdArgs& key1 = l10nIds.AppendElement(fallible)->SetAsL10nIdArgs();
+  key1.mId.Assign("chooser-window"_ns);
+
+  IgnoredErrorResult rv;
+  nsTArray<dom::Nullable<dom::L10nMessage>> result;
+
+  l10n->FormatMessagesSync(l10nIds, result, rv);
+  ASSERT_FALSE(rv.Failed());
+  ASSERT_TRUE(result.Length() == 2);
+  ASSERT_TRUE(result.ElementAt(0).Value().mValue.Find("Foo"_ns) > -1);
+
+  auto fmtAttr = result.ElementAt(1).Value().mAttributes.Value();
+  ASSERT_TRUE(fmtAttr.Length() == 2);
+  ASSERT_FALSE(fmtAttr.ElementAt(0).mName.IsEmpty());
+  ASSERT_FALSE(fmtAttr.ElementAt(0).mValue.IsEmpty());
+}
diff --git a/intl/l10n/test/gtest/moz.build b/intl/l10n/test/gtest/moz.build
new file mode 100644
index 0000000000..6d04077a50
--- /dev/null
+++ b/intl/l10n/test/gtest/moz.build
@@ -0,0 +1,11 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+UNIFIED_SOURCES += [
+    "TestLocalization.cpp",
+]
+
+FINAL_LIBRARY = "xul-gtest"
diff --git a/intl/l10n/test/mochitest/chrome.toml b/intl/l10n/test/mochitest/chrome.toml
new file mode 100644
index 0000000000..0e0f500f5d
--- /dev/null
+++ b/intl/l10n/test/mochitest/chrome.toml
@@ -0,0 +1,10 @@
+[DEFAULT]
+
+["localization/test_formatMessages.html"]
+skip-if = ["debug"] # Intentionally triggers a debug assert for missing Fluent arguments.
+
+["localization/test_formatValue.html"]
+skip-if = ["debug"] # Intentionally triggers a debug assert for missing Fluent arguments.
+
+["localization/test_formatValues.html"]
+skip-if = ["debug"] # Intentionally triggers a debug assert for missing Fluent arguments.
diff --git a/intl/l10n/test/mochitest/localization/test_formatMessages.html b/intl/l10n/test/mochitest/localization/test_formatMessages.html
new file mode 100644
index 0000000000..6e5a9f3f41
--- /dev/null
+++ b/intl/l10n/test/mochitest/localization/test_formatMessages.html
@@ -0,0 +1,101 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test Localization.prototype.formatMessages API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css">
+  <script type="application/javascript">
+  "use strict";
+  const mockSource = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}/", [
+    {
+      path: "/localization/en-US/mock.ftl",
+      source: `
+key1 = Value
+  .title = Title 1
+  .accesskey = K
+key2 =
+  .label = This is a label for { $user }
+`
+    }
+  ]);
+  let registry = new L10nRegistry({
+    bundleOptions: {
+      useIsolating: false
+    }
+  });
+  registry.registerSources([mockSource]);
+
+  function getAttributeByName(attributes, name) {
+    return attributes.find(attr => attr.name === name);
+  }
+
+  (async () => {
+    SimpleTest.waitForExplicitFinish();
+
+    const loc = new Localization(
+      ['mock.ftl'],
+      false,
+      registry,
+    );
+
+    {
+      // Simple mix works.
+      let msgs = await loc.formatMessages([
+        {id: "key1"},
+        {id: "key2", args: { user: "Amy"}},
+      ]);
+      {
+        is(msgs[0].value, "Value");
+        let attr0 = getAttributeByName(msgs[0].attributes, "title");
+        is(attr0.name, "title");
+        is(attr0.value, "Title 1");
+        let attr1 = getAttributeByName(msgs[0].attributes, "accesskey");
+        is(attr1.name, "accesskey");
+        is(attr1.value, "K");
+      }
+
+      {
+        is(msgs[1].value, null);
+        let attr0 = getAttributeByName(msgs[1].attributes, "label");
+        is(attr0.name, "label");
+        is(attr0.value, "This is a label for Amy");
+      }
+    }
+
+    {
+      // Missing arguments cause exception in automation.
+      try {
+        let msgs = await loc.formatMessages([
+          {id: "key1"},
+          {id: "key2"},
+        ]);
+        ok(false, "Missing argument didn't cause an exception.");
+      } catch (e) {
+        is(e.message,
+          "[fluent][resolver] errors in en-US/key2: Resolver error: Unknown variable: $user",
+          "Missing key causes an exception.");
+      }
+    }
+
+    {
+      // Missing keys cause exception in automation.
+      try {
+        let msgs = await loc.formatMessages([
+          { id: "key1" },
+          { id: "key4" },
+        ]);
+        ok(false, "Missing key didn't cause an exception.");
+      } catch (e) {
+        is(e.message,
+           "[fluent] Missing message in locale en-US: key4",
+           "Missing key causes an exception.");
+      }
+    }
+    SimpleTest.finish();
+  })();
+  </script>
+</head>
+<body>
+</body>
+</html>
diff --git a/intl/l10n/test/mochitest/localization/test_formatValue.html b/intl/l10n/test/mochitest/localization/test_formatValue.html
new file mode 100644
index 0000000000..e1ee02fa7a
--- /dev/null
+++ b/intl/l10n/test/mochitest/localization/test_formatValue.html
@@ -0,0 +1,81 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test Localization.prototype.formatValue API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css">
+  <script type="application/javascript">
+  "use strict";
+  const mockSource = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}/", [
+    {
+      path: "/localization/en-US/mock.ftl",
+      source: `
+key1 = Value
+key2 = Value { $user }
+key3 = Value { $count }
+`
+    }
+  ]);
+  let registry = new L10nRegistry({
+    bundleOptions: {
+      useIsolating: false
+    }
+  });
+  registry.registerSources([mockSource]);
+
+  (async () => {
+    SimpleTest.waitForExplicitFinish();
+
+    const loc = new Localization(
+      ['mock.ftl'],
+      false,
+      registry,
+    );
+
+    {
+      // Simple value works.
+      let val = await loc.formatValue("key1");
+      is(val, "Value");
+    }
+
+    {
+      // Value with a string argument works.
+      let val = await loc.formatValue("key2", { user: "John" });
+      is(val, "Value John");
+    }
+
+    {
+      // Value with a number argument works.
+      let val = await loc.formatValue("key3", { count: -3.21 });
+      is(val, "Value -3.21");
+    }
+
+    {
+      // Verify that in automation, a missing
+      // argument causes an exception.
+      try {
+        let val = await loc.formatValue("key3");
+        ok(false, "Missing argument didn't cause an exception.");
+      } catch (e) {
+        is(e.message,
+          "[fluent][resolver] errors in en-US/key3: Resolver error: Unknown variable: $count",
+          "Missing key causes an exception.");
+      }
+    }
+
+    {
+      // Incorrect argument type works.
+      // Due to how WebIDL handles union types, it'll convert
+      // the argument to a string `[object Object]`.
+      let val = await loc.formatValue("key2", { user: { name: true } });
+      is(val, "Value [object Object]");
+    }
+
+    SimpleTest.finish();
+  })();
+  </script>
+</head>
+<body>
+</body>
+</html>
diff --git a/intl/l10n/test/mochitest/localization/test_formatValues.html b/intl/l10n/test/mochitest/localization/test_formatValues.html
new file mode 100644
index 0000000000..819d418b2e
--- /dev/null
+++ b/intl/l10n/test/mochitest/localization/test_formatValues.html
@@ -0,0 +1,85 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test Localization.prototype.formatValues API</title>
+  <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="chrome://mochikit/content/tests/SimpleTest/test.css">
+  <script type="application/javascript">
+  "use strict";
+  const mockSource = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}/", [
+    {
+      path: "/localization/en-US/mock.ftl",
+      source: `
+key1 = Value
+key2 = Value { $user }
+key3 = Value { $count }
+`
+    }
+  ]);
+  let registry = new L10nRegistry({
+    bundleOptions: {
+      useIsolating: false
+    }
+  });
+  registry.registerSources([mockSource]);
+
+  (async () => {
+    SimpleTest.waitForExplicitFinish();
+
+    const loc = new Localization(
+      ['mock.ftl'],
+      false,
+      registry,
+    );
+
+    {
+      // Simple mix works.
+      let vals = await loc.formatValues([
+        {id: "key1"},
+        {id: "key2", args: { user: "Amy"}},
+        {id: "key3", args: { count: -32.12 }},
+      ]);
+      is(vals[0], "Value");
+      is(vals[1], "Value Amy");
+      is(vals[2], "Value -32.12");
+    }
+
+    {
+      // Missing arguments cause exception in automation.
+      try {
+        let vals = await loc.formatValues([
+          {id: "key1"},
+          {id: "key2"},
+          {id: "key3", args: { count: -32.12 }},
+        ]);
+        ok(false, "Missing argument didn't cause an exception.");
+      } catch (e) {
+        is(e.message,
+          "[fluent][resolver] errors in en-US/key2: Resolver error: Unknown variable: $user",
+          "Missing key causes an exception.");
+      }
+    }
+
+    {
+      // Missing keys cause exception in automation.
+      try {
+        let vals = await loc.formatValues([
+          { id: "key1" },
+          { id: "key4", args: { count: -32.12 } },
+        ]);
+        ok(false, "Missing key didn't cause an exception.");
+      } catch (e) {
+        is(e.message,
+	   "[fluent] Missing message in locale en-US: key4",
+           "Missing key causes an exception.");
+      }
+    }
+
+    SimpleTest.finish();
+  })();
+  </script>
+</head>
+<body>
+</body>
+</html>
diff --git a/intl/l10n/test/test_datetimeformat.js b/intl/l10n/test/test_datetimeformat.js
new file mode 100644
index 0000000000..4c0794424b
--- /dev/null
+++ b/intl/l10n/test/test_datetimeformat.js
@@ -0,0 +1,76 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const FIREFOX_RELEASE_TIMESTAMP = 1032800850000;
+const FIREFOX_RELEASE_DATE = new Date(FIREFOX_RELEASE_TIMESTAMP);
+
+add_task(function test_date_time_format() {
+  const bundle = new FluentBundle(["en-US"]);
+
+  bundle.addResource(
+    new FluentResource(`
+dt-arg = Just the arg is: {$dateArg}
+dt-bare = The bare date is: { DATETIME($dateArg) }
+dt-month-year = Months and year are not time-zone dependent here: { DATETIME($dateArg, month: "long") }
+dt-bad = This is a bad month: { DATETIME($dateArg, month: "oops") }
+# TODO - Bug 1707728:
+dt-timezone = The timezone: { DATETIME($dateArg, timezone: "America/New_York") }
+dt-unknown = Unknown: { DATETIME($dateArg, unknown: "unknown") }
+dt-style = Style formatting: { DATETIME($dateArg, dateStyle: "short", timeStyle: "short") }
+    `)
+  );
+
+  function testMessage(id, dateArg, expectedMessage) {
+    const message = bundle.formatPattern(bundle.getMessage(id).value, {
+      dateArg,
+    });
+
+    if (typeof expectedMessage === "object") {
+      // Assume regex.
+      ok(
+        expectedMessage.test(message),
+        `"${message}" matches regex: ${expectedMessage.toString()}`
+      );
+    } else {
+      // Assume string.
+      equal(message, expectedMessage);
+    }
+  }
+
+  // TODO - Bug 1707728 - Some of these are implemented as regexes since time zones are not
+  // supported in fluent messages as of yet. They could be simplified if a time zone were
+  // specified.
+  testMessage(
+    "dt-arg",
+    FIREFOX_RELEASE_DATE,
+    /^Just the arg is: (Sun|Mon|Tue) Sep \d+ 2002 \d+:\d+:\d+ .* \(.*\)$/
+  );
+  testMessage(
+    "dt-bare",
+    FIREFOX_RELEASE_TIMESTAMP,
+    /^The bare date is: Sep \d+, 2002, \d+:\d+:\d+ (AM|PM)$/
+  );
+  testMessage(
+    "dt-month-year",
+    FIREFOX_RELEASE_TIMESTAMP,
+    "Months and year are not time-zone dependent here: September"
+  );
+  testMessage(
+    "dt-bad",
+    FIREFOX_RELEASE_TIMESTAMP,
+    /^This is a bad month: Sep \d+, 2002, \d+:\d+:\d+ (AM|PM)$/
+  );
+  testMessage(
+    "dt-unknown",
+    FIREFOX_RELEASE_TIMESTAMP,
+    /^Unknown: Sep \d+, 2002, \d+:\d+:\d+ (AM|PM)$/
+  );
+  testMessage(
+    "dt-style",
+    FIREFOX_RELEASE_TIMESTAMP,
+    /^Style formatting: \d+\/\d+\/\d+, \d+:\d+ (AM|PM)$/
+  );
+
+// TODO - Bug 1707728
+// testMessage("dt-timezone", ...);
+});
diff --git a/intl/l10n/test/test_l10nregistry.js b/intl/l10n/test/test_l10nregistry.js
new file mode 100644
index 0000000000..cbbb1e7316
--- /dev/null
+++ b/intl/l10n/test/test_l10nregistry.js
@@ -0,0 +1,563 @@
+/* Any copyrighequal dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const {setTimeout} = ChromeUtils.importESModule("resource://gre/modules/Timer.sys.mjs");
+
+const l10nReg = new L10nRegistry();
+
+add_task(function test_methods_presence() {
+  equal(typeof l10nReg.generateBundles, "function");
+  equal(typeof l10nReg.generateBundlesSync, "function");
+  equal(typeof l10nReg.getAvailableLocales, "function");
+  equal(typeof l10nReg.registerSources, "function");
+  equal(typeof l10nReg.removeSources, "function");
+  equal(typeof l10nReg.updateSources, "function");
+});
+
+/**
+ * Test that passing empty resourceIds list works.
+ */
+add_task(async function test_empty_resourceids() {
+  const fs = [];
+
+  const source = L10nFileSource.createMock("test", "", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const bundles = l10nReg.generateBundles(["en-US"], []);
+
+  const done = (await bundles.next()).done;
+
+  equal(done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * Test that passing empty sources list works.
+ */
+add_task(async function test_empty_sources() {
+  const fs = [];
+  const bundles = l10nReg.generateBundlesSync(["en-US"], fs);
+
+  const done = (await bundles.next()).done;
+
+  equal(done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test tests generation of a proper context for a single
+ * source scenario
+ */
+add_task(async function test_methods_calling() {
+  const fs = [
+    { path: "/localization/en-US/browser/menu.ftl", source: "key = Value" }
+  ];
+  const source = L10nFileSource.createMock("test", "", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const bundles = l10nReg.generateBundles(["en-US"], ["/browser/menu.ftl"]);
+
+  const bundle = (await bundles.next()).value;
+
+  equal(bundle.hasMessage("key"), true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that the public methods return expected values
+ * for the single source scenario
+ */
+add_task(async function test_has_one_source() {
+  const fs = [
+    {path: "./app/data/locales/en-US/test.ftl", source: "key = value en-US"}
+  ];
+  let oneSource = L10nFileSource.createMock("app", "", ["en-US"], "./app/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource]);
+
+
+  // has one source
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("app"), true);
+
+
+  // returns a single context
+
+  let bundles = l10nReg.generateBundles(["en-US"], ["test.ftl"]);
+  let bundle0 = (await bundles.next()).value;
+  equal(bundle0.hasMessage("key"), true);
+
+  equal((await bundles.next()).done, true);
+
+
+  // returns no contexts for missing locale
+
+  bundles = l10nReg.generateBundles(["pl"], ["test.ftl"]);
+
+  equal((await bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that public methods return expected values
+ * for the dual source scenario.
+ */
+add_task(async function test_has_two_sources() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "key = platform value" },
+    { path: "./app/data/locales/pl/test.ftl", source: "key = app value" }
+  ];
+  let oneSource = L10nFileSource.createMock("platform", "", ["en-US"], "./platform/data/locales/{locale}/", fs);
+  let secondSource = L10nFileSource.createMock("app", "", ["pl"], "./app/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource, secondSource]);
+
+  // has two sources
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("app"), true);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns correct contexts for en-US
+
+  let bundles = l10nReg.generateBundles(["en-US"], ["test.ftl"]);
+  let bundle0 = (await bundles.next()).value;
+
+  equal(bundle0.hasMessage("key"), true);
+  let msg = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg.value), "platform value");
+
+  equal((await bundles.next()).done, true);
+
+
+  // returns correct contexts for [pl, en-US]
+
+  bundles = l10nReg.generateBundles(["pl", "en-US"], ["test.ftl"]);
+  bundle0 = (await bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "app value");
+
+  let bundle1 = (await bundles.next()).value;
+  equal(bundle1.locales[0], "en-US");
+  equal(bundle1.hasMessage("key"), true);
+  let msg1 = bundle1.getMessage("key");
+  equal(bundle1.formatPattern(msg1.value), "platform value");
+
+  equal((await bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that behavior specific to the L10nFileSource
+ * works correctly.
+ *
+ * In particular it tests that L10nFileSource correctly returns
+ * missing files as `false` instead of `undefined`.
+ */
+add_task(function test_indexed() {
+  let oneSource = new L10nFileSource("langpack-pl", "app", ["pl"], "/data/locales/{locale}/", {}, [
+    "/data/locales/pl/test.ftl",
+  ]);
+  equal(oneSource.hasFile("pl", "test.ftl"), "present");
+  equal(oneSource.hasFile("pl", "missing.ftl"), "missing");
+});
+
+/**
+ * This test checks if the correct order of contexts is used for
+ * scenarios where a new file source is added on top of the default one.
+ */
+add_task(async function test_override() {
+  const fs = [
+    { path: "/app/data/locales/pl/test.ftl", source: "key = value" },
+    { path: "/data/locales/pl/test.ftl", source: "key = addon value"},
+  ];
+  let fileSource = L10nFileSource.createMock("app", "", ["pl"], "/app/data/locales/{locale}/", fs);
+  let oneSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", fs);
+  l10nReg.registerSources([fileSource, oneSource]);
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("langpack-pl"), true);
+
+  let bundles = l10nReg.generateBundles(["pl"], ["test.ftl"]);
+  let bundle0 = (await bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "addon value");
+
+  let bundle1 = (await bundles.next()).value;
+  equal(bundle1.locales[0], "pl");
+  equal(bundle1.hasMessage("key"), true);
+  let msg1 = bundle1.getMessage("key");
+  equal(bundle1.formatPattern(msg1.value), "value");
+
+  equal((await bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that new contexts are returned
+ * after source update.
+ */
+add_task(async function test_updating() {
+  const fs = [
+    { path: "/data/locales/pl/test.ftl", source: "key = value" }
+  ];
+  let oneSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource]);
+
+  let bundles = l10nReg.generateBundles(["pl"], ["test.ftl"]);
+  let bundle0 = (await bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "value");
+
+
+  const newSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", [
+    { path: "/data/locales/pl/test.ftl", source: "key = new value" }
+  ]);
+  l10nReg.updateSources([newSource]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  bundles = l10nReg.generateBundles(["pl"], ["test.ftl"]);
+  bundle0 = (await bundles.next()).value;
+  msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "new value");
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that generated contexts return correct values
+ * after sources are being removed.
+ */
+add_task(async function test_removing() {
+  const fs = [
+    { path: "/app/data/locales/pl/test.ftl", source: "key = value" },
+    { path: "/data/locales/pl/test.ftl", source: "key = addon value" },
+  ];
+
+  let fileSource = L10nFileSource.createMock("app", "", ["pl"], "/app/data/locales/{locale}/", fs);
+  let oneSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", fs);
+  l10nReg.registerSources([fileSource, oneSource]);
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("langpack-pl"), true);
+
+  let bundles = l10nReg.generateBundles(["pl"], ["test.ftl"]);
+  let bundle0 = (await bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "addon value");
+
+  let bundle1 = (await bundles.next()).value;
+  equal(bundle1.locales[0], "pl");
+  equal(bundle1.hasMessage("key"), true);
+  let msg1 = bundle1.getMessage("key");
+  equal(bundle1.formatPattern(msg1.value), "value");
+
+  equal((await bundles.next()).done, true);
+
+  // Remove langpack
+
+  l10nReg.removeSources(["langpack-pl"]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("langpack-pl"), false);
+
+  bundles = l10nReg.generateBundles(["pl"], ["test.ftl"]);
+  bundle0 = (await bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "value");
+
+  equal((await bundles.next()).done, true);
+
+  // Remove app source
+
+  l10nReg.removeSources(["app"]);
+
+  equal(l10nReg.getSourceNames().length, 0);
+
+  bundles = l10nReg.generateBundles(["pl"], ["test.ftl"]);
+  equal((await bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that the logic works correctly when there's a missing
+ * file in the FileSource scenario.
+ */
+add_task(async function test_missing_file() {
+  const fs = [
+    { path: "./app/data/locales/en-US/test.ftl", source: "key = value en-US" },
+    { path: "./platform/data/locales/en-US/test.ftl", source: "key = value en-US" },
+    { path: "./platform/data/locales/en-US/test2.ftl", source: "key2 = value2 en-US" },
+  ];
+  let oneSource = L10nFileSource.createMock("app", "", ["en-US"], "./app/data/locales/{locale}/", fs);
+  let twoSource = L10nFileSource.createMock("platform", "", ["en-US"], "./platform/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource, twoSource]);
+
+  // has two sources
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("app"), true);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns a single context
+
+  let bundles = l10nReg.generateBundles(["en-US"], ["test.ftl", "test2.ftl"]);
+
+  // First permutation:
+  //   [platform, platform] - both present
+  let bundle1 = (await bundles.next());
+  equal(bundle1.value.hasMessage("key"), true);
+
+  // Second permutation skipped:
+  //   [platform, app] - second missing
+  // Third permutation:
+  //   [app, platform] - both present
+  let bundle2 = (await bundles.next());
+  equal(bundle2.value.hasMessage("key"), true);
+
+  // Fourth permutation skipped:
+  //   [app, app] - second missing
+  equal((await bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+add_task(async function test_hasSource() {
+  equal(l10nReg.hasSource("nonsense"), false, "Non-existing source doesn't exist");
+  equal(l10nReg.hasSource("app"), false, "hasSource returns true before registering a source");
+  let oneSource = new L10nFileSource("app", "app", ["en-US"], "/{locale}/");
+  l10nReg.registerSources([oneSource]);
+  equal(l10nReg.hasSource("app"), true, "hasSource returns true after registering a source");
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that we handle correctly a scenario where a source
+ * is being removed while the iterator operates.
+ */
+add_task(async function test_remove_source_mid_iter_cycle() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "key = platform value" },
+    { path: "./app/data/locales/pl/test.ftl", source: "key = app value" },
+  ];
+  let oneSource = L10nFileSource.createMock("platform", "", ["en-US"], "./platform/data/locales/{locale}/", fs);
+  let secondSource = L10nFileSource.createMock("app", "", ["pl"], "./app/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource, secondSource]);
+
+  let bundles = l10nReg.generateBundles(["en-US", "pl"], ["test.ftl"]);
+
+  let bundle0 = await bundles.next();
+
+  // The registry has a copy of the file sources, so it will be unaffected.
+  l10nReg.removeSources(["app"]);
+
+  let bundle1 = await bundles.next();
+
+  equal((await bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+add_task(async function test_metasources() {
+  let fs = [
+    { path: "/localization/en-US/browser/menu1.ftl", source: "key1 = Value" },
+    { path: "/localization/en-US/browser/menu2.ftl", source: "key2 = Value" },
+    { path: "/localization/en-US/browser/menu3.ftl", source: "key3 = Value" },
+    { path: "/localization/en-US/browser/menu4.ftl", source: "key4 = Value" },
+    { path: "/localization/en-US/browser/menu5.ftl", source: "key5 = Value" },
+    { path: "/localization/en-US/browser/menu6.ftl", source: "key6 = Value" },
+    { path: "/localization/en-US/browser/menu7.ftl", source: "key7 = Value" },
+    { path: "/localization/en-US/browser/menu8.ftl", source: "key8 = Value" },
+  ];
+
+  const browser = L10nFileSource.createMock("browser", "app", ["en-US"], "/localization/{locale}", fs);
+  const toolkit = L10nFileSource.createMock("toolkit", "app", ["en-US"], "/localization/{locale}", fs);
+  const browser2 = L10nFileSource.createMock("browser2", "langpack", ["en-US"], "/localization/{locale}", fs);
+  const toolkit2 = L10nFileSource.createMock("toolkit2", "langpack", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([toolkit, browser, toolkit2, browser2]);
+
+  let res = [
+    "/browser/menu1.ftl",
+    "/browser/menu2.ftl",
+    "/browser/menu3.ftl",
+    "/browser/menu4.ftl",
+    "/browser/menu5.ftl",
+    "/browser/menu6.ftl",
+    "/browser/menu7.ftl",
+    {path: "/browser/menu8.ftl", optional: false},
+  ];
+
+  const bundles = l10nReg.generateBundles(["en-US"], res);
+
+  let nbundles = 0;
+  while (!(await bundles.next()).done) {
+    nbundles += 1;
+  }
+
+  // If metasources are working properly, we'll generate 2^8 = 256 bundles for
+  // each metasource giving 512 bundles in total. Otherwise, we generate
+  // 4^8 = 65536 bundles.
+  equal(nbundles, 512);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that when a required resource is missing for a locale,
+ * we do not produce a bundle for that locale.
+ */
+add_task(async function test_missing_required_resource() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "test-key = en-US value" },
+    { path: "./platform/data/locales/pl/missing-in-en-US.ftl", source: "missing-key = pl value" },
+    { path: "./platform/data/locales/pl/test.ftl", source: "test-key = pl value" },
+  ];
+  let source = L10nFileSource.createMock("platform", "", ["en-US", "pl"], "./platform/data/locales/{locale}/", fs);
+  l10nReg.registerSources([source]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns correct contexts for [en-US, pl]
+
+  let bundles = l10nReg.generateBundlesSync(["en-US", "pl"], ["test.ftl", "missing-in-en-US.ftl"]);
+  let bundle0 = (await bundles.next()).value;
+
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("test-key"), true);
+  equal(bundle0.hasMessage("missing-key"), true);
+
+  let msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "pl value");
+
+  let msg1 = bundle0.getMessage("missing-key");
+  equal(bundle0.formatPattern(msg1.value), "pl value");
+
+  equal((await bundles.next()).done, true);
+
+
+  // returns correct contexts for [pl, en-US]
+
+  bundles = l10nReg.generateBundlesSync(["pl", "en-US"], ["test.ftl", {path: "missing-in-en-US.ftl", optional: false}]);
+  bundle0 = (await bundles.next()).value;
+
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("test-key"), true);
+
+  msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "pl value");
+
+  msg1 = bundle0.getMessage("missing-key");
+  equal(bundle0.formatPattern(msg1.value), "pl value");
+
+  equal((await bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that when an optional resource is missing, we continue
+ * to produce a bundle for that locale. The bundle will have missing entries
+ * with regard to the missing optional resource.
+ */
+add_task(async function test_missing_optional_resource() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "test-key = en-US value" },
+    { path: "./platform/data/locales/pl/missing-in-en-US.ftl", source: "missing-key = pl value" },
+    { path: "./platform/data/locales/pl/test.ftl", source: "test-key = pl value" },
+  ];
+  let source = L10nFileSource.createMock("platform", "", ["en-US", "pl"], "./platform/data/locales/{locale}/", fs);
+  l10nReg.registerSources([source]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns correct contexts for [en-US, pl]
+
+  let bundles = l10nReg.generateBundlesSync(["en-US", "pl"], ["test.ftl", { path: "missing-in-en-US.ftl", optional: true }]);
+  let bundle0 = (await bundles.next()).value;
+
+  equal(bundle0.locales[0], "en-US");
+  equal(bundle0.hasMessage("test-key"), true);
+  equal(bundle0.hasMessage("missing-key"), false);
+
+  let msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "en-US value");
+
+  equal(bundle0.getMessage("missing-key"), null);
+
+  let bundle1 = (await bundles.next()).value;
+
+  equal(bundle1.locales[0], "pl");
+  equal(bundle1.hasMessage("test-key"), true);
+  equal(bundle1.hasMessage("missing-key"), true);
+
+  msg0 = bundle1.getMessage("test-key");
+  equal(bundle1.formatPattern(msg0.value), "pl value");
+
+  msg1 = bundle1.getMessage("missing-key");
+  equal(bundle1.formatPattern(msg1.value), "pl value");
+
+  equal((await bundles.next()).done, true);
+
+  // returns correct contexts for [pl, en-US]
+
+  bundles = l10nReg.generateBundlesSync(["pl", "en-US"], ["test.ftl", { path: "missing-in-en-US.ftl", optional: true }]);
+  bundle0 = (await bundles.next()).value;
+
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("test-key"), true);
+  equal(bundle0.hasMessage("missing-key"), true);
+
+  msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "pl value");
+
+  msg1 = bundle0.getMessage("missing-key");
+  equal(bundle0.formatPattern(msg1.value), "pl value");
+
+  bundle1 = (await bundles.next()).value;
+
+  equal(bundle1.locales[0], "en-US");
+  equal(bundle1.hasMessage("test-key"), true);
+  equal(bundle1.hasMessage("missing-key"), false);
+
+  msg0 = bundle1.getMessage("test-key");
+  equal(bundle1.formatPattern(msg0.value), "en-US value");
+
+  equal(bundle1.getMessage("missing-key"), null);
+
+  // cleanup
+  l10nReg.clearSources();
+});
diff --git a/intl/l10n/test/test_l10nregistry_fuzzed.js b/intl/l10n/test/test_l10nregistry_fuzzed.js
new file mode 100644
index 0000000000..f99c3b61e5
--- /dev/null
+++ b/intl/l10n/test/test_l10nregistry_fuzzed.js
@@ -0,0 +1,205 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * This test is a fuzzing test for the L10nRegistry API. It was written to find
+ * a hard to reproduce bug in the L10nRegistry code. If it fails, place the seed
+ * from the failing run in the code directly below to make it consistently reproducible.
+ */
+let seed = Math.floor(Math.random() * 1e9);
+
+console.log(`Starting a fuzzing run with seed: ${seed}.`);
+console.log("To reproduce this test locally, re-run it locally with:");
+console.log(`let seed = ${seed};`);
+
+/**
+ * A simple non-robust psuedo-random number generator.
+ *
+ * It is implemented using a Lehmer random number generator.
+ * https://en.wikipedia.org/wiki/16,807
+ *
+ * @returns {number} Ranged [0, 1)
+ */
+function prng() {
+  const multiplier = 16807;
+  const prime = 2147483647;
+  seed = seed * multiplier % prime
+  return (seed - 1) / prime
+}
+
+/**
+ * Generate a name like "mock-dmsxfodrqboljmxdeayt".
+ * @returns {string}
+ */
+function generateRandomName() {
+  let name = 'mock-'
+  const letters = "abcdefghijklmnopqrstuvwxyz";
+  for (let i = 0; i < 20; i++) {
+    name += letters[Math.floor(prng() * letters.length)];
+  }
+  return name;
+}
+
+/**
+ * Picks one item from an array.
+ *
+ * @param {Array<T>}
+ * @returns {T}
+ */
+function pickOne(list) {
+  return list[Math.floor(prng() * list.length)]
+}
+
+/**
+ * Picks a random subset from an array.
+ *
+ * @param {Array<T>}
+ * @returns {Array<T>}
+ */
+function pickN(list, count) {
+  list = list.slice();
+  const result = [];
+  for (let i = 0; i < count && i < list.length; i++) {
+    // Pick a random item.
+    const index = Math.floor(prng() * list.length);
+
+    // Swap item to the end.
+    const a = list[index];
+    const b = list[list.length - 1];
+    list[index] = b;
+    list[list.length - 1] = a
+
+    // Now that the random item is on the end, pop it off and add it to the results.
+    result.push(list.pop());
+  }
+
+  return result
+}
+
+/**
+ * Generate a random number
+ * @param {number} min
+ * @param {number} max
+ * @returns {number}
+ */
+function random(min, max) {
+  const delta = max - min;
+  return min + delta * prng();
+}
+
+/**
+ * Generate a random number generator with a distribution more towards the lower end.
+ * @param {number} min
+ * @param {number} max
+ * @returns {number}
+ */
+function randomPow(min, max) {
+  const delta = max - min;
+  const r = prng()
+  return min + delta * r * r;
+}
+
+add_task(async function test_fuzzing_sources() {
+    const iterations = 100;
+    const maxSources = 10;
+  
+    const metasources = ["app", "langpack", ""];
+    const availableLocales = ["en", "en-US", "pl", "en-CA", "es-AR", "es-ES"];
+
+    const l10nReg = new L10nRegistry();
+
+    for (let i = 0; i < iterations; i++) {
+      console.log("----------------------------------------------------------------------");
+      console.log("Iteration", i);
+      let sourceCount = randomPow(0, maxSources);
+
+      const mocks = [];
+      const fs = [];
+
+      const locales = new Set();
+      const filenames = new Set();
+
+      for (let j = 0; j < sourceCount; j++) {
+        const locale = pickOne(availableLocales);
+        locales.add(locale);
+
+        let metasource = pickOne(metasources);
+        if (metasource === "langpack") {
+          metasource = `${metasource}-${locale}`
+        }
+  
+        const dir = generateRandomName();
+        const filename = generateRandomName() + j + ".ftl";
+        const path = `${dir}/${locale}/${filename}`
+        const name = metasource || "app";
+        const source = "key = value";
+
+        filenames.add(filename);
+
+        console.log("Add source", { name, metasource, path, source });
+        fs.push({ path, source });
+
+        mocks.push([
+          name, // name
+          metasource, // metasource,
+          [locale], // locales,
+          dir + "/{locale}/",
+          fs
+        ])
+      }
+
+      l10nReg.registerSources(mocks.map(args => L10nFileSource.createMock(...args)));
+
+      const bundleLocales = pickN([...locales], random(1, 4));
+      const bundleFilenames = pickN([...filenames], random(1, 10));
+
+      console.log("generateBundles", {bundleLocales, bundleFilenames});
+      const bundles = l10nReg.generateBundles(
+        bundleLocales,
+        bundleFilenames
+      );
+
+      function next() {
+        console.log("Getting next bundle");
+        const bundle = bundles.next()
+        console.log("Next bundle obtained", bundle);
+        return bundle;
+      }
+
+      const ops = [
+        // Increase the frequency of next being called.
+        next,
+        next,
+        next,
+        () => {
+          const newMocks = [];
+          for (const mock of pickN(mocks, random(0, 3))) {
+            const newMock = mock.slice();
+            newMocks.push(newMock)
+          }
+          console.log("l10nReg.updateSources");
+          l10nReg.updateSources(newMocks.map(mock => L10nFileSource.createMock(...mock)));
+        },
+        () => {
+          console.log("l10nReg.clearSources");
+          l10nReg.clearSources();
+        }
+      ];
+
+      console.log("Start the operation loop");
+      while (true) {
+        console.log("Next operation");
+        const op = pickOne(ops);
+        const result = await op();
+        if (result?.done) {
+          // The iterator completed.
+          break;
+        }
+      }
+
+      console.log("Clear sources");
+      l10nReg.clearSources();
+    }
+
+    ok(true, "The L10nRegistry fuzzing did not crash.")
+});
diff --git a/intl/l10n/test/test_l10nregistry_sync.js b/intl/l10n/test/test_l10nregistry_sync.js
new file mode 100644
index 0000000000..178a24ff6d
--- /dev/null
+++ b/intl/l10n/test/test_l10nregistry_sync.js
@@ -0,0 +1,536 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const {setTimeout} = ChromeUtils.importESModule("resource://gre/modules/Timer.sys.mjs");
+
+const l10nReg = new L10nRegistry();
+
+add_task(function test_methods_presence() {
+  equal(typeof l10nReg.generateBundles, "function");
+  equal(typeof l10nReg.generateBundlesSync, "function");
+  equal(typeof l10nReg.getAvailableLocales, "function");
+  equal(typeof l10nReg.registerSources, "function");
+  equal(typeof l10nReg.removeSources, "function");
+  equal(typeof l10nReg.updateSources, "function");
+});
+
+/**
+ * Test that passing empty resourceIds list works.
+ */
+add_task(function test_empty_resourceids() {
+  const fs = [];
+
+  const source = L10nFileSource.createMock("test", "", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const bundles = l10nReg.generateBundlesSync(["en-US"], []);
+
+  const done = (bundles.next()).done;
+
+  equal(done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * Test that passing empty sources list works.
+ */
+add_task(function test_empty_sources() {
+  const fs = [];
+  const bundles = l10nReg.generateBundlesSync(["en-US"], fs);
+
+  const done = (bundles.next()).done;
+
+  equal(done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test tests generation of a proper context for a single
+ * source scenario
+ */
+add_task(function test_methods_calling() {
+  const fs = [
+    { path: "/localization/en-US/browser/menu.ftl", source: "key = Value" }
+  ];
+  const source = L10nFileSource.createMock("test", "", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const bundles = l10nReg.generateBundlesSync(["en-US"], ["/browser/menu.ftl"]);
+
+  const bundle = (bundles.next()).value;
+
+  equal(bundle.hasMessage("key"), true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that the public methods return expected values
+ * for the single source scenario
+ */
+add_task(function test_has_one_source() {
+  const fs = [
+    {path: "./app/data/locales/en-US/test.ftl", source: "key = value en-US"}
+  ];
+  let oneSource = L10nFileSource.createMock("app", "", ["en-US"], "./app/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource]);
+
+
+  // has one source
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("app"), true);
+
+
+  // returns a single context
+
+  let bundles = l10nReg.generateBundlesSync(["en-US"], ["test.ftl"]);
+  let bundle0 = (bundles.next()).value;
+  equal(bundle0.hasMessage("key"), true);
+
+  equal((bundles.next()).done, true);
+
+
+  // returns no contexts for missing locale
+
+  bundles = l10nReg.generateBundlesSync(["pl"], ["test.ftl"]);
+
+  equal((bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that public methods return expected values
+ * for the dual source scenario.
+ */
+add_task(function test_has_two_sources() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "key = platform value" },
+    { path: "./app/data/locales/pl/test.ftl", source: "key = app value" }
+  ];
+  let oneSource = L10nFileSource.createMock("platform", "", ["en-US"], "./platform/data/locales/{locale}/", fs);
+  let secondSource = L10nFileSource.createMock("app", "", ["pl"], "./app/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource, secondSource]);
+
+  // has two sources
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("app"), true);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns correct contexts for en-US
+
+  let bundles = l10nReg.generateBundlesSync(["en-US"], ["test.ftl"]);
+  let bundle0 = (bundles.next()).value;
+
+  equal(bundle0.hasMessage("key"), true);
+  let msg = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg.value), "platform value");
+
+  equal((bundles.next()).done, true);
+
+
+  // returns correct contexts for [pl, en-US]
+
+  bundles = l10nReg.generateBundlesSync(["pl", "en-US"], ["test.ftl"]);
+  bundle0 = (bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "app value");
+
+  let bundle1 = (bundles.next()).value;
+  equal(bundle1.locales[0], "en-US");
+  equal(bundle1.hasMessage("key"), true);
+  let msg1 = bundle1.getMessage("key");
+  equal(bundle1.formatPattern(msg1.value), "platform value");
+
+  equal((bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test checks if the correct order of contexts is used for
+ * scenarios where a new file source is added on top of the default one.
+ */
+add_task(function test_override() {
+  const fs = [
+    { path: "/app/data/locales/pl/test.ftl", source: "key = value" },
+    { path: "/data/locales/pl/test.ftl", source: "key = addon value"},
+  ];
+  let fileSource = L10nFileSource.createMock("app", "", ["pl"], "/app/data/locales/{locale}/", fs);
+  let oneSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", fs);
+  l10nReg.registerSources([fileSource, oneSource]);
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("langpack-pl"), true);
+
+  let bundles = l10nReg.generateBundlesSync(["pl"], ["test.ftl"]);
+  let bundle0 = (bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "addon value");
+
+  let bundle1 = (bundles.next()).value;
+  equal(bundle1.locales[0], "pl");
+  equal(bundle1.hasMessage("key"), true);
+  let msg1 = bundle1.getMessage("key");
+  equal(bundle1.formatPattern(msg1.value), "value");
+
+  equal((bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that new contexts are returned
+ * after source update.
+ */
+add_task(function test_updating() {
+  const fs = [
+    { path: "/data/locales/pl/test.ftl", source: "key = value" }
+  ];
+  let oneSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource]);
+
+  let bundles = l10nReg.generateBundlesSync(["pl"], ["test.ftl"]);
+  let bundle0 = (bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "value");
+
+
+  const newSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", [
+    { path: "/data/locales/pl/test.ftl", source: "key = new value" }
+  ]);
+  l10nReg.updateSources([newSource]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  bundles = l10nReg.generateBundlesSync(["pl"], ["test.ftl"]);
+  bundle0 = (bundles.next()).value;
+  msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "new value");
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that generated contexts return correct values
+ * after sources are being removed.
+ */
+add_task(function test_removing() {
+  const fs = [
+    { path: "/app/data/locales/pl/test.ftl", source: "key = value" },
+    { path: "/data/locales/pl/test.ftl", source: "key = addon value" },
+  ];
+
+  let fileSource = L10nFileSource.createMock("app", "", ["pl"], "/app/data/locales/{locale}/", fs);
+  let oneSource = L10nFileSource.createMock("langpack-pl", "", ["pl"], "/data/locales/{locale}/", fs);
+  l10nReg.registerSources([fileSource, oneSource]);
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("langpack-pl"), true);
+
+  let bundles = l10nReg.generateBundlesSync(["pl"], ["test.ftl"]);
+  let bundle0 = (bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  let msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "addon value");
+
+  let bundle1 = (bundles.next()).value;
+  equal(bundle1.locales[0], "pl");
+  equal(bundle1.hasMessage("key"), true);
+  let msg1 = bundle1.getMessage("key");
+  equal(bundle1.formatPattern(msg1.value), "value");
+
+  equal((bundles.next()).done, true);
+
+  // Remove langpack
+
+  l10nReg.removeSources(["langpack-pl"]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("langpack-pl"), false);
+
+  bundles = l10nReg.generateBundlesSync(["pl"], ["test.ftl"]);
+  bundle0 = (bundles.next()).value;
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("key"), true);
+  msg0 = bundle0.getMessage("key");
+  equal(bundle0.formatPattern(msg0.value), "value");
+
+  equal((bundles.next()).done, true);
+
+  // Remove app source
+
+  l10nReg.removeSources(["app"]);
+
+  equal(l10nReg.getSourceNames().length, 0);
+
+  bundles = l10nReg.generateBundlesSync(["pl"], ["test.ftl"]);
+  equal((bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that the logic works correctly when there's a missing
+ * file in the FileSource scenario.
+ */
+add_task(function test_missing_file() {
+  const fs = [
+    { path: "./app/data/locales/en-US/test.ftl", source: "key = value en-US" },
+    { path: "./platform/data/locales/en-US/test.ftl", source: "key = value en-US" },
+    { path: "./platform/data/locales/en-US/test2.ftl", source: "key2 = value2 en-US" },
+  ];
+  let oneSource = L10nFileSource.createMock("app", "", ["en-US"], "./app/data/locales/{locale}/", fs);
+  let twoSource = L10nFileSource.createMock("platform", "", ["en-US"], "./platform/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource, twoSource]);
+
+  // has two sources
+
+  equal(l10nReg.getSourceNames().length, 2);
+  equal(l10nReg.hasSource("app"), true);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns a single context
+
+  let bundles = l10nReg.generateBundlesSync(["en-US"], ["test.ftl", "test2.ftl"]);
+
+  // First permutation:
+  //   [platform, platform] - both present
+  let bundle1 = (bundles.next());
+  equal(bundle1.value.hasMessage("key"), true);
+
+  // Second permutation skipped:
+  //   [platform, app] - second missing
+  // Third permutation:
+  //   [app, platform] - both present
+  let bundle2 = (bundles.next());
+  equal(bundle2.value.hasMessage("key"), true);
+
+  // Fourth permutation skipped:
+  //   [app, app] - second missing
+  equal((bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that we handle correctly a scenario where a source
+ * is being removed while the iterator operates.
+ */
+add_task(function test_remove_source_mid_iter_cycle() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "key = platform value" },
+    { path: "./app/data/locales/pl/test.ftl", source: "key = app value" },
+  ];
+  let oneSource = L10nFileSource.createMock("platform", "", ["en-US"], "./platform/data/locales/{locale}/", fs);
+  let secondSource = L10nFileSource.createMock("app", "", ["pl"], "./app/data/locales/{locale}/", fs);
+  l10nReg.registerSources([oneSource, secondSource]);
+
+  let bundles = l10nReg.generateBundlesSync(["en-US", "pl"], ["test.ftl"]);
+
+  let bundle0 = bundles.next();
+
+  l10nReg.removeSources(["app"]);
+
+  equal((bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+add_task(async function test_metasources() {
+  let fs = [
+    { path: "/localization/en-US/browser/menu1.ftl", source: "key1 = Value" },
+    { path: "/localization/en-US/browser/menu2.ftl", source: "key2 = Value" },
+    { path: "/localization/en-US/browser/menu3.ftl", source: "key3 = Value" },
+    { path: "/localization/en-US/browser/menu4.ftl", source: "key4 = Value" },
+    { path: "/localization/en-US/browser/menu5.ftl", source: "key5 = Value" },
+    { path: "/localization/en-US/browser/menu6.ftl", source: "key6 = Value" },
+    { path: "/localization/en-US/browser/menu7.ftl", source: "key7 = Value" },
+    { path: "/localization/en-US/browser/menu8.ftl", source: "key8 = Value" },
+  ];
+
+  const browser = L10nFileSource.createMock("browser", "app", ["en-US"], "/localization/{locale}", fs);
+  const toolkit = L10nFileSource.createMock("toolkit", "app", ["en-US"], "/localization/{locale}", fs);
+  const browser2 = L10nFileSource.createMock("browser2", "langpack", ["en-US"], "/localization/{locale}", fs);
+  const toolkit2 = L10nFileSource.createMock("toolkit2", "langpack", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([toolkit, browser, toolkit2, browser2]);
+
+  let res = [
+    "/browser/menu1.ftl",
+    "/browser/menu2.ftl",
+    "/browser/menu3.ftl",
+    "/browser/menu4.ftl",
+    "/browser/menu5.ftl",
+    "/browser/menu6.ftl",
+    "/browser/menu7.ftl",
+    {path: "/browser/menu8.ftl", optional: false},
+  ];
+
+  const bundles = l10nReg.generateBundlesSync(["en-US"], res);
+
+  let nbundles = 0;
+  while (!bundles.next().done) {
+    nbundles += 1;
+  }
+
+  // If metasources are working properly, we'll generate 2^8 = 256 bundles for
+  // each metasource giving 512 bundles in total. Otherwise, we generate
+  // 4^8 = 65536 bundles.
+  equal(nbundles, 512);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that when a required resource is missing for a locale,
+ * we do not produce a bundle for that locale.
+ */
+add_task(function test_missing_required_resource() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "test-key = en-US value" },
+    { path: "./platform/data/locales/pl/missing-in-en-US.ftl", source: "missing-key = pl value" },
+    { path: "./platform/data/locales/pl/test.ftl", source: "test-key = pl value" },
+  ];
+  let source = L10nFileSource.createMock("platform", "", ["en-US", "pl"], "./platform/data/locales/{locale}/", fs);
+  l10nReg.registerSources([source]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns correct contexts for [en-US, pl]
+
+  let bundles = l10nReg.generateBundlesSync(["en-US", "pl"], ["test.ftl", "missing-in-en-US.ftl"]);
+  let bundle0 = (bundles.next()).value;
+
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("test-key"), true);
+  equal(bundle0.hasMessage("missing-key"), true);
+
+  let msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "pl value");
+
+  let msg1 = bundle0.getMessage("missing-key");
+  equal(bundle0.formatPattern(msg1.value), "pl value");
+
+  equal((bundles.next()).done, true);
+
+
+  // returns correct contexts for [pl, en-US]
+
+  bundles = l10nReg.generateBundlesSync(["pl", "en-US"], ["test.ftl", {path: "missing-in-en-US.ftl", optional: false}]);
+  bundle0 = (bundles.next()).value;
+
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("test-key"), true);
+
+  msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "pl value");
+
+  msg1 = bundle0.getMessage("missing-key");
+  equal(bundle0.formatPattern(msg1.value), "pl value");
+
+  equal((bundles.next()).done, true);
+
+  // cleanup
+  l10nReg.clearSources();
+});
+
+/**
+ * This test verifies that when an optional resource is missing, we continue
+ * to produce a bundle for that locale. The bundle will have missing entries
+ * with regard to the missing optional resource.
+ */
+add_task(function test_missing_optional_resource() {
+  const fs = [
+    { path: "./platform/data/locales/en-US/test.ftl", source: "test-key = en-US value" },
+    { path: "./platform/data/locales/pl/missing-in-en-US.ftl", source: "missing-key = pl value" },
+    { path: "./platform/data/locales/pl/test.ftl", source: "test-key = pl value" },
+  ];
+  let source = L10nFileSource.createMock("platform", "", ["en-US", "pl"], "./platform/data/locales/{locale}/", fs);
+  l10nReg.registerSources([source]);
+
+  equal(l10nReg.getSourceNames().length, 1);
+  equal(l10nReg.hasSource("platform"), true);
+
+
+  // returns correct contexts for [en-US, pl]
+
+  let bundles = l10nReg.generateBundlesSync(["en-US", "pl"], ["test.ftl", { path: "missing-in-en-US.ftl", optional: true }]);
+  let bundle0 = (bundles.next()).value;
+
+  equal(bundle0.locales[0], "en-US");
+  equal(bundle0.hasMessage("test-key"), true);
+  equal(bundle0.hasMessage("missing-key"), false);
+
+  let msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "en-US value");
+
+  equal(bundle0.getMessage("missing-key"), null);
+
+  let bundle1 = (bundles.next()).value;
+
+  equal(bundle1.locales[0], "pl");
+  equal(bundle1.hasMessage("test-key"), true);
+  equal(bundle1.hasMessage("missing-key"), true);
+
+  msg0 = bundle1.getMessage("test-key");
+  equal(bundle1.formatPattern(msg0.value), "pl value");
+
+  msg1 = bundle1.getMessage("missing-key");
+  equal(bundle1.formatPattern(msg1.value), "pl value");
+
+  equal((bundles.next()).done, true);
+
+  // returns correct contexts for [pl, en-US]
+
+  bundles = l10nReg.generateBundlesSync(["pl", "en-US"], ["test.ftl", { path: "missing-in-en-US.ftl", optional: true }]);
+  bundle0 = (bundles.next()).value;
+
+  equal(bundle0.locales[0], "pl");
+  equal(bundle0.hasMessage("test-key"), true);
+  equal(bundle0.hasMessage("missing-key"), true);
+
+  msg0 = bundle0.getMessage("test-key");
+  equal(bundle0.formatPattern(msg0.value), "pl value");
+
+  msg1 = bundle0.getMessage("missing-key");
+  equal(bundle0.formatPattern(msg1.value), "pl value");
+
+  bundle1 = (bundles.next()).value;
+
+  equal(bundle1.locales[0], "en-US");
+  equal(bundle1.hasMessage("test-key"), true);
+  equal(bundle1.hasMessage("missing-key"), false);
+
+  msg0 = bundle1.getMessage("test-key");
+  equal(bundle1.formatPattern(msg0.value), "en-US value");
+
+  equal(bundle1.getMessage("missing-key"), null);
+
+  // cleanup
+  l10nReg.clearSources();
+});
diff --git a/intl/l10n/test/test_localization.js b/intl/l10n/test/test_localization.js
new file mode 100644
index 0000000000..1b89a0549a
--- /dev/null
+++ b/intl/l10n/test/test_localization.js
@@ -0,0 +1,319 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { AppConstants } = ChromeUtils.importESModule("resource://gre/modules/AppConstants.sys.mjs");
+
+// Disable `xpc::IsInAutomation()` so incomplete locales do not generate
+// errors.
+Services.prefs.setBoolPref(
+  "security.turn_off_all_security_so_that_viruses_can_take_over_this_computer",
+  false
+);
+
+add_task(function test_methods_presence() {
+  strictEqual(typeof Localization.prototype.formatValues, "function");
+  strictEqual(typeof Localization.prototype.formatMessages, "function");
+  strictEqual(typeof Localization.prototype.formatValue, "function");
+});
+
+add_task(async function test_methods_calling() {
+  const l10nReg = new L10nRegistry();
+  const fs = [
+    { path: "/localization/de/browser/menu.ftl", source: `
+key-value1 = [de] Value2
+` },
+    { path: "/localization/en-US/browser/menu.ftl", source: `
+key-value1 = [en] Value2
+key-value2 = [en] Value3
+key-attr =
+    .label = [en] Label 3
+` },
+  ];
+  const originalRequested = Services.locale.requestedLocales;
+
+  const source = L10nFileSource.createMock("test", "app", ["de", "en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const l10n = new Localization([
+    "/browser/menu.ftl",
+  ], false, l10nReg, ["de", "en-US"]);
+
+  {
+    let values = await l10n.formatValues([
+      {id: "key-value1"},
+      {id: "key-value2"},
+      {id: "key-missing"},
+      {id: "key-attr"}
+    ]);
+
+    strictEqual(values[0], "[de] Value2");
+    strictEqual(values[1], "[en] Value3");
+    strictEqual(values[2], null);
+    strictEqual(values[3], null);
+  }
+
+  {
+    let values = await l10n.formatValues([
+      "key-value1",
+      "key-value2",
+      "key-missing",
+      "key-attr"
+    ]);
+
+    strictEqual(values[0], "[de] Value2");
+    strictEqual(values[1], "[en] Value3");
+    strictEqual(values[2], null);
+    strictEqual(values[3], null);
+  }
+
+  {
+    strictEqual(await l10n.formatValue("key-missing"), null);
+    strictEqual(await l10n.formatValue("key-value1"), "[de] Value2");
+    strictEqual(await l10n.formatValue("key-value2"), "[en] Value3");
+    strictEqual(await l10n.formatValue("key-attr"), null);
+  }
+
+  {
+    let messages = await l10n.formatMessages([
+      {id: "key-value1"},
+      {id: "key-missing"},
+      {id: "key-value2"},
+      {id: "key-attr"},
+    ]);
+
+    strictEqual(messages[0].value, "[de] Value2");
+    strictEqual(messages[1], null);
+    strictEqual(messages[2].value, "[en] Value3");
+    strictEqual(messages[3].value, null);
+  }
+});
+
+add_task(async function test_builtins() {
+  const l10nReg = new L10nRegistry();
+  const known_platforms = {
+    "linux": "linux",
+    "win": "windows",
+    "macosx": "macos",
+    "android": "android",
+  };
+
+  const fs = [
+    { path: "/localization/en-US/test.ftl", source: `
+key = { PLATFORM() ->
+        ${ Object.values(known_platforms).map(
+              name => `      [${ name }] ${ name.toUpperCase() } Value\n`).join("") }
+       *[other] OTHER Value
+    }` },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const l10n = new Localization([
+    "/test.ftl",
+  ], false, l10nReg, ["en-US"]);
+
+  let values = await l10n.formatValues([{id: "key"}]);
+
+  ok(values[0].includes(
+    `${ known_platforms[AppConstants.platform].toUpperCase() } Value`));
+});
+
+add_task(async function test_add_remove_resourceIds() {
+  const l10nReg = new L10nRegistry();
+  const fs = [
+    { path: "/localization/en-US/browser/menu.ftl", source: "key1 = Value1" },
+    { path: "/localization/en-US/toolkit/menu.ftl", source: "key2 = Value2" },
+  ];
+
+
+  const source = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const l10n = new Localization(["/browser/menu.ftl"], false, l10nReg, ["en-US"]);
+
+  let values = await l10n.formatValues([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], null);
+
+  l10n.addResourceIds(["/toolkit/menu.ftl"]);
+
+  values = await l10n.formatValues([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], "Value2");
+
+  values = await l10n.formatValues(["key1", {id: "key2"}]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], "Value2");
+
+  values = await l10n.formatValues([{id: "key1"}, "key2"]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], "Value2");
+
+  l10n.removeResourceIds(["/browser/menu.ftl"]);
+
+  values = await l10n.formatValues([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], null);
+  strictEqual(values[1], "Value2");
+});
+
+add_task(async function test_switch_to_async() {
+  const l10nReg = new L10nRegistry();
+
+  const fs = [
+    { path: "/localization/en-US/browser/menu.ftl", source: "key1 = Value1" },
+    { path: "/localization/en-US/toolkit/menu.ftl", source: "key2 = Value2" },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const l10n = new Localization(["/browser/menu.ftl"], true, l10nReg, ["en-US"]);
+
+  let values = l10n.formatValuesSync([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], null);
+
+  l10n.setAsync();
+
+  Assert.throws(() => {
+    l10n.formatValuesSync([{ id: "key1" }, { id: "key2" }]);
+  }, /Can't use formatValuesSync when state is async./);
+
+  l10n.addResourceIds(["/toolkit/menu.ftl"]);
+
+  values = await l10n.formatValues([{id: "key1"}, {id: "key2"}]);
+  let values2 = await l10n.formatValues([{id: "key1"}, {id: "key2"}]);
+
+  deepEqual(values, values2);
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], "Value2");
+
+  l10n.removeResourceIds(["/browser/menu.ftl"]);
+
+  values = await l10n.formatValues([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], null);
+  strictEqual(values[1], "Value2");
+});
+
+/**
+ * This test verifies that when a required resource is missing,
+ * we fallback entirely to the next locale for all entries.
+ */
+add_task(async function test_format_from_missing_required_resource() {
+  const l10nReg = new L10nRegistry();
+
+  const fs = [
+    { path: "/localization/de/browser/menu.ftl", source: `
+key-value = [de] Value1
+` },
+    { path: "/localization/de/browser/missing-in-en-US.ftl", source: `
+key-missing = [de] MissingValue
+` },
+    { path: "/localization/en-US/browser/menu.ftl", source: `
+key-value = [en] Value1
+` },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["de", "en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  // returns correct contexts for [en-US, de]
+
+  let l10n = new Localization([
+    "/browser/menu.ftl",
+    "/browser/missing-in-en-US.ftl",
+  ], false, l10nReg, ["en-US", "de"]);
+
+  {
+    let values = await l10n.formatValues([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[de] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+
+  // returns correct contexts for [de, en-US]
+
+  l10n = new Localization([
+    "/browser/menu.ftl",
+    {path: "/browser/missing-in-en-US.ftl", optional: false},
+  ], false, l10nReg, ["de", "en-US"]);
+
+  {
+    let values = await l10n.formatValues([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[de] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+});
+
+/**
+ * This test verifies that when an optional resource is missing, we continue
+ * to populate entires from other resources in the same locale, and we only
+ * fallback entries from the missing optional resource to the next locale.
+ */
+add_task(async function test_format_from_missing_optional_resource() {
+  const l10nReg = new L10nRegistry();
+
+  const fs = [
+    { path: "/localization/de/browser/menu.ftl", source: `
+key-value = [de] Value1
+` },
+    { path: "/localization/de/browser/missing-in-en-US.ftl", source: `
+key-missing = [de] MissingValue
+` },
+    { path: "/localization/en-US/browser/menu.ftl", source: `
+key-value = [en] Value1
+` },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["de", "en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  // returns correct contexts for [en-US, de]
+
+  let l10n = new Localization([
+    {path: "/browser/menu.ftl", optional: false},
+    {path: "/browser/missing-in-en-US.ftl", optional: true},
+  ], false, l10nReg, ["en-US", "de"]);
+
+  {
+    let values = await l10n.formatValues([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[en] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+
+  // returns correct contexts for [de, en-US]
+
+  l10n = new Localization([
+    {path: "/browser/menu.ftl", optional: false},
+    {path: "/browser/missing-in-en-US.ftl", optional: true},
+  ], false, l10nReg, ["de", "en-US"]);
+
+  {
+    let values = await l10n.formatValues([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[de] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+});
diff --git a/intl/l10n/test/test_localization_sync.js b/intl/l10n/test/test_localization_sync.js
new file mode 100644
index 0000000000..fa2186bc07
--- /dev/null
+++ b/intl/l10n/test/test_localization_sync.js
@@ -0,0 +1,289 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { AppConstants } = ChromeUtils.importESModule("resource://gre/modules/AppConstants.sys.mjs");
+
+// Disable `xpc::IsInAutomation()` so incomplete locales do not generate
+// errors.
+Services.prefs.setBoolPref(
+  "security.turn_off_all_security_so_that_viruses_can_take_over_this_computer",
+  false
+);
+
+add_task(function test_methods_calling() {
+  const l10nReg = new L10nRegistry();
+
+  const fs = [
+    { path: "/localization/de/browser/menu.ftl", source: `
+key-value1 = [de] Value2
+` },
+    { path: "/localization/en-US/browser/menu.ftl", source: `
+key-value1 = [en] Value2
+key-value2 = [en] Value3
+key-attr =
+    .label = [en] Label 3
+` },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["de", "en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  const l10n = new Localization([
+    "/browser/menu.ftl",
+  ], true, l10nReg, ["de", "en-US"]);
+
+
+  {
+    let values = l10n.formatValuesSync([
+      {id: "key-value1"},
+      {id: "key-value2"},
+      {id: "key-missing"},
+      {id: "key-attr"}
+    ]);
+
+    strictEqual(values[0], "[de] Value2");
+    strictEqual(values[1], "[en] Value3");
+    strictEqual(values[2], null);
+    strictEqual(values[3], null);
+  }
+
+  {
+    let values = l10n.formatValuesSync([
+      "key-value1",
+      "key-value2",
+      "key-missing",
+      "key-attr"
+    ]);
+
+    strictEqual(values[0], "[de] Value2");
+    strictEqual(values[1], "[en] Value3");
+    strictEqual(values[2], null);
+    strictEqual(values[3], null);
+  }
+
+  {
+    strictEqual(l10n.formatValueSync("key-missing"), null);
+    strictEqual(l10n.formatValueSync("key-value1"), "[de] Value2");
+    strictEqual(l10n.formatValueSync("key-value2"), "[en] Value3");
+    strictEqual(l10n.formatValueSync("key-attr"), null);
+  }
+
+  {
+    let messages = l10n.formatMessagesSync([
+      {id: "key-value1"},
+      {id: "key-missing"},
+      {id: "key-value2"},
+      {id: "key-attr"},
+    ]);
+
+    strictEqual(messages[0].value, "[de] Value2");
+    strictEqual(messages[1], null);
+    strictEqual(messages[2].value, "[en] Value3");
+    strictEqual(messages[3].value, null);
+  }
+});
+
+add_task(function test_builtins() {
+  const known_platforms = {
+    "linux": "linux",
+    "win": "windows",
+    "macosx": "macos",
+    "android": "android",
+  };
+
+  const fs = [
+    { path: "/localization/en-US/test.ftl", source: `
+key = { PLATFORM() ->
+        ${ Object.values(known_platforms).map(
+              name => `      [${ name }] ${ name.toUpperCase() } Value\n`).join("") }
+       *[other] OTHER Value
+    }` },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}", fs);
+  const l10nReg = new L10nRegistry();
+  l10nReg.registerSources([source]);
+
+  const l10n = new Localization([
+    "/test.ftl",
+  ], true, l10nReg, ["en-US"]);
+
+  let values = l10n.formatValuesSync([{id: "key"}]);
+
+  ok(values[0].includes(
+    `${ known_platforms[AppConstants.platform].toUpperCase() } Value`));
+});
+
+add_task(function test_add_remove_resourceIds() {
+  const fs = [
+    { path: "/localization/en-US/browser/menu.ftl", source: "key1 = Value1" },
+    { path: "/localization/en-US/toolkit/menu.ftl", source: "key2 = Value2" },
+  ];
+  const originalRequested = Services.locale.requestedLocales;
+
+  const source = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}", fs);
+  const l10nReg = new L10nRegistry();
+  l10nReg.registerSources([source]);
+
+  const l10n = new Localization(["/browser/menu.ftl"], true, l10nReg, ["en-US"]);
+
+  let values = l10n.formatValuesSync([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], null);
+
+  l10n.addResourceIds(["/toolkit/menu.ftl"]);
+
+  values = l10n.formatValuesSync([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], "Value2");
+
+  values = l10n.formatValuesSync(["key1", {id: "key2"}]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], "Value2");
+
+  values = l10n.formatValuesSync([{id: "key1"}, "key2"]);
+
+  strictEqual(values[0], "Value1");
+  strictEqual(values[1], "Value2");
+
+  l10n.removeResourceIds(["/browser/menu.ftl"]);
+
+  values = l10n.formatValuesSync([{id: "key1"}, {id: "key2"}]);
+
+  strictEqual(values[0], null);
+  strictEqual(values[1], "Value2");
+});
+
+add_task(function test_calling_sync_methods_in_async_mode_fails() {
+  const l10n = new Localization(["/browser/menu.ftl"], false);
+
+  Assert.throws(() => {
+    l10n.formatValuesSync([{ id: "key1" }, { id: "key2" }]);
+  }, /Can't use formatValuesSync when state is async./);
+
+  Assert.throws(() => {
+    l10n.formatValueSync("key1");
+  }, /Can't use formatValueSync when state is async./);
+
+  Assert.throws(() => {
+    l10n.formatMessagesSync([{ id: "key1"}]);
+  }, /Can't use formatMessagesSync when state is async./);
+});
+
+/**
+ * This test verifies that when a required resource is missing,
+ * we fallback entirely to the next locale for all entries.
+ */
+add_task(function test_format_from_missing_required_resource() {
+  const l10nReg = new L10nRegistry();
+
+  const fs = [
+    { path: "/localization/de/browser/menu.ftl", source: `
+key-value = [de] Value1
+` },
+    { path: "/localization/de/browser/missing-in-en-US.ftl", source: `
+key-missing = [de] MissingValue
+` },
+    { path: "/localization/en-US/browser/menu.ftl", source: `
+key-value = [en] Value1
+` },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["de", "en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  // returns correct contexts for [en-US, de]
+
+  let l10n = new Localization([
+    "/browser/menu.ftl",
+    "/browser/missing-in-en-US.ftl",
+  ], true, l10nReg, ["en-US", "de"]);
+
+  {
+    let values = l10n.formatValuesSync([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[de] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+
+  // returns correct contexts for [de, en-US]
+
+  l10n = new Localization([
+    "/browser/menu.ftl",
+    {path: "/browser/missing-in-en-US.ftl", optional: false},
+  ], true, l10nReg, ["de", "en-US"]);
+
+  {
+    let values = l10n.formatValuesSync([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[de] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+});
+
+/**
+ * This test verifies that when an optional resource is missing
+ * we continue to populate entires from other resources in the same locale
+ * and only fallback entries from the optional resource to the next locale.
+ */
+add_task(function test_format_from_missing_optional_resource() {
+  const l10nReg = new L10nRegistry();
+
+  const fs = [
+    { path: "/localization/de/browser/menu.ftl", source: `
+key-value = [de] Value1
+` },
+    { path: "/localization/de/browser/missing-in-en-US.ftl", source: `
+key-missing = [de] MissingValue
+` },
+    { path: "/localization/en-US/browser/menu.ftl", source: `
+key-value = [en] Value1
+` },
+  ];
+
+  const source = L10nFileSource.createMock("test", "app", ["de", "en-US"], "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+
+  // returns correct contexts for [en-US, de]
+
+  let l10n = new Localization([
+    {path: "/browser/menu.ftl", optional: false},
+    {path: "/browser/missing-in-en-US.ftl", optional: true},
+  ], true, l10nReg, ["en-US", "de"]);
+
+  {
+    let values = l10n.formatValuesSync([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[en] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+
+  // returns correct contexts for [de, en-US]
+
+  l10n = new Localization([
+    {path: "/browser/menu.ftl", optional: false},
+    {path: "/browser/missing-in-en-US.ftl", optional: true},
+  ], true, l10nReg, ["de", "en-US"]);
+
+  {
+    let values = l10n.formatValuesSync([
+      {id: "key-value"},
+      {id: "key-missing"},
+    ]);
+
+    strictEqual(values[0], "[de] Value1");
+    strictEqual(values[1], "[de] MissingValue");
+  }
+});
diff --git a/intl/l10n/test/test_messagecontext.js b/intl/l10n/test/test_messagecontext.js
new file mode 100644
index 0000000000..7ab17dea90
--- /dev/null
+++ b/intl/l10n/test/test_messagecontext.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function run_test() {
+  test_methods_presence(FluentBundle);
+  test_methods_calling(FluentBundle, FluentResource);
+  test_number_options(FluentBundle, FluentResource);
+
+  ok(true);
+}
+
+function test_methods_presence(FluentBundle) {
+  const bundle = new FluentBundle(["en-US", "pl"]);
+  equal(typeof bundle.addResource, "function");
+  equal(typeof bundle.formatPattern, "function");
+}
+
+function test_methods_calling(FluentBundle, FluentResource) {
+  const bundle = new FluentBundle(["en-US", "pl"], {
+    useIsolating: false,
+  });
+  bundle.addResource(new FluentResource("key = Value"));
+
+  const msg = bundle.getMessage("key");
+  equal(bundle.formatPattern(msg.value), "Value");
+
+  bundle.addResource(new FluentResource("key2 = Hello { $name }"));
+
+  const msg2 = bundle.getMessage("key2");
+  equal(bundle.formatPattern(msg2.value, { name: "Amy" }), "Hello Amy");
+  ok(true);
+}
+
+function test_number_options(FluentBundle, FluentResource) {
+  const bundle = new FluentBundle(["en-US", "pl"], {
+    useIsolating: false,
+  });
+  bundle.addResource(new FluentResource(`
+key = { NUMBER(0.53, style: "percent") } { NUMBER(0.12, style: "percent", minimumFractionDigits: 0) }
+    { NUMBER(-2.5, style: "percent") } { NUMBER(2.91, style: "percent") } { NUMBER("wrong", style: "percent") }
+`));
+
+  const msg = bundle.getMessage("key");
+  equal(bundle.formatPattern(msg.value), "53.00% 12%\n-250.0% 291.00% ");
+
+  ok(true);
+}
diff --git a/intl/l10n/test/test_missing_variables.js b/intl/l10n/test/test_missing_variables.js
new file mode 100644
index 0000000000..8c05f7f94f
--- /dev/null
+++ b/intl/l10n/test/test_missing_variables.js
@@ -0,0 +1,35 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Disable `xpc::IsInAutomation()` so that missing variables don't throw
+// errors.
+Services.prefs.setBoolPref(
+  "security.turn_off_all_security_so_that_viruses_can_take_over_this_computer",
+  false
+);
+
+/**
+ * The following test demonstrates crashing behavior.
+ */
+add_task(function test_missing_variables() {
+  const l10nReg = new L10nRegistry();
+
+  const fs = [
+    { path: "/localization/en-US/browser/test.ftl", source: "welcome-message = Welcome { $user }\n" }
+  ]
+  const locales = ["en-US"];
+  const source = L10nFileSource.createMock("test", "app", locales, "/localization/{locale}", fs);
+  l10nReg.registerSources([source]);
+  const l10n = new Localization(["/browser/test.ftl"], true, l10nReg, locales);
+
+  {
+    const [message] = l10n.formatValuesSync([{ id: "welcome-message", args: { user: "Greg" } }]);
+    equal(message, "Welcome Greg");
+  }
+
+  {
+    // This will crash in debug builds.
+    const [message] = l10n.formatValuesSync([{ id: "welcome-message" }]);
+    equal(message, "Welcome {$user}");
+  }
+});
diff --git a/intl/l10n/test/test_pseudo.js b/intl/l10n/test/test_pseudo.js
new file mode 100644
index 0000000000..23404aab12
--- /dev/null
+++ b/intl/l10n/test/test_pseudo.js
@@ -0,0 +1,131 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+
+const originalValues = {
+  requested: Services.locale.requestedLocales,
+};
+
+const l10nReg = new L10nRegistry();
+
+function getMockRegistry() {
+  const mockSource = L10nFileSource.createMock("test", "app", ["en-US"], "/localization/{locale}/", [
+    {
+      path: "/localization/en-US/mock.ftl",
+      source: `
+key = This is a single message
+   .tooltip = This is a tooltip
+   .accesskey = f
+`
+    }
+  ]);
+  let registry = new L10nRegistry();
+  registry.registerSources([mockSource]);
+  return registry;
+}
+
+function getAttributeByName(attributes, name) {
+  return attributes.find(attr => attr.name === name);
+}
+
+/**
+ * This test verifies that as we switch between
+ * different pseudo strategies, the Localization object
+ * follows and formats using the given strategy.
+ *
+ * We test values and attributes and make sure that
+ * a single-character attributes, commonly used for access keys
+ * don't get transformed.
+ */
+add_task(async function test_pseudo_works() {
+  Services.prefs.setStringPref("intl.l10n.pseudo", "");
+
+  let mockRegistry = getMockRegistry();
+
+  const l10n = new Localization([
+    "mock.ftl",
+  ], false, mockRegistry);
+
+  {
+    // 1. Start with no pseudo
+
+    let message = (await l10n.formatMessages([{id: "key"}]))[0];
+
+    ok(message.value.includes("This is a single message"));
+    let attr0 = getAttributeByName(message.attributes, "tooltip");
+    ok(attr0.value.includes("This is a tooltip"));
+    let attr1 = getAttributeByName(message.attributes, "accesskey");
+    equal(attr1.value, "f");
+  }
+
+  {
+    // 2. Set Accented Pseudo
+
+    Services.prefs.setStringPref("intl.l10n.pseudo", "accented");
+    let message = (await l10n.formatMessages([{id: "key"}]))[0];
+
+    ok(message.value.includes("[Ŧħiş iş aa şiƞɠŀee ḿeeşşaaɠee]"));
+    let attr0 = getAttributeByName(message.attributes, "tooltip");
+    ok(attr0.value.includes("[Ŧħiş iş aa ŧooooŀŧiƥ]"));
+    let attr1 = getAttributeByName(message.attributes, "accesskey");
+    equal(attr1.value, "f");
+  }
+
+  {
+    // 3. Set Bidi Pseudo
+
+    Services.prefs.setStringPref("intl.l10n.pseudo", "bidi");
+    let message = (await l10n.formatMessages([{id: "key"}]))[0];
+
+    ok(message.value.includes("iş a şiƞɠŀe ḿeşşaɠe"));
+    let attr0 = getAttributeByName(message.attributes, "tooltip");
+    ok(attr0.value.includes("Ŧħiş iş a ŧooŀŧiƥ"));
+    let attr1 = getAttributeByName(message.attributes, "accesskey");
+    equal(attr1.value, "f");
+  }
+
+  {
+    // 4. Remove pseudo
+
+    Services.prefs.setStringPref("intl.l10n.pseudo", "");
+    let message = (await l10n.formatMessages([{id: "key"}]))[0];
+
+    ok(message.value.includes("This is a single message"));
+    let attr0 = getAttributeByName(message.attributes, "tooltip");
+    ok(attr0.value.includes("This is a tooltip"));
+    let attr1 = getAttributeByName(message.attributes, "accesskey");
+    equal(attr1.value, "f");
+  }
+
+  Services.locale.requestedLocales = originalValues.requested;
+});
+
+/**
+ * This test verifies that setting a bogus pseudo locale
+ * strategy doesn't break anything.
+ */
+add_task(async function test_unavailable_strategy_works() {
+  Services.prefs.setStringPref("intl.l10n.pseudo", "");
+
+  let mockRegistry = getMockRegistry();
+
+  const l10n = new Localization([
+    "mock.ftl",
+  ], false, mockRegistry);
+
+  {
+    // 1. Set unavailable pseudo strategy
+    Services.prefs.setStringPref("intl.l10n.pseudo", "unknown-strategy");
+
+    let message = (await l10n.formatMessages([{id: "key"}]))[0];
+
+    ok(message.value.includes("This is a single message"));
+    let attr0 = getAttributeByName(message.attributes, "tooltip");
+    ok(attr0.value.includes("This is a tooltip"));
+    let attr1 = getAttributeByName(message.attributes, "accesskey");
+    equal(attr1.value, "f");
+  }
+
+  Services.prefs.setStringPref("intl.l10n.pseudo", "");
+  Services.locale.requestedLocales = originalValues.requested;
+});
diff --git a/intl/l10n/test/xpcshell.toml b/intl/l10n/test/xpcshell.toml
new file mode 100644
index 0000000000..e08a0eae5f
--- /dev/null
+++ b/intl/l10n/test/xpcshell.toml
@@ -0,0 +1,21 @@
+[DEFAULT]
+head = ""
+
+["test_datetimeformat.js"]
+
+["test_l10nregistry.js"]
+
+["test_l10nregistry_fuzzed.js"]
+
+["test_l10nregistry_sync.js"]
+
+["test_localization.js"]
+
+["test_localization_sync.js"]
+
+["test_messagecontext.js"]
+
+["test_missing_variables.js"]
+skip-if = ["debug"] # Intentionally triggers a debug assert for missing Fluent arguments.
+
+["test_pseudo.js"]
-- 
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