Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

102 files changed, 12170 insertions, 0 deletions

diff --git a/uriloader/exthandler/tests/HandlerServiceTestUtils.sys.mjs b/uriloader/exthandler/tests/HandlerServiceTestUtils.sys.mjs
new file mode 100644
index 0000000000..0e52cc2568
--- /dev/null
+++ b/uriloader/exthandler/tests/HandlerServiceTestUtils.sys.mjs
@@ -0,0 +1,253 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * Shared functions for tests related to invoking external handler applications.
+ */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+import { Assert } from "resource://testing-common/Assert.sys.mjs";
+
+const lazy = {};
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gExternalProtocolService",
+  "@mozilla.org/uriloader/external-protocol-service;1",
+  "nsIExternalProtocolService"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gMIMEService",
+  "@mozilla.org/mime;1",
+  "nsIMIMEService"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gHandlerService",
+  "@mozilla.org/uriloader/handler-service;1",
+  "nsIHandlerService"
+);
+
+export var HandlerServiceTestUtils = {
+  /**
+   * Retrieves the names of all the MIME types and protocols configured in the
+   * handler service instance currently under testing.
+   *
+   * @return Array of strings like "example/type" or "example-scheme", sorted
+   *         alphabetically regardless of category.
+   */
+  getAllHandlerInfoTypes() {
+    return Array.from(
+      lazy.gHandlerService.enumerate(),
+      info => info.type
+    ).sort();
+  },
+
+  /**
+   * Retrieves all the configured handlers for MIME types and protocols.
+   *
+   * @note The nsIHandlerInfo instances returned by the "enumerate" method
+   *       cannot be used for testing because they incorporate information from
+   *       the operating system and also from the default nsIHandlerService
+   *       instance, independently from what instance is under testing.
+   *
+   * @return Array of nsIHandlerInfo instances sorted by their "type" property.
+   */
+  getAllHandlerInfos() {
+    return this.getAllHandlerInfoTypes().map(type => this.getHandlerInfo(type));
+  },
+
+  /**
+   * Retrieves an nsIHandlerInfo for the given MIME type or protocol, which
+   * incorporates information from the operating system and also from the
+   * handler service instance currently under testing.
+   *
+   * @note If the handler service instance currently under testing is not the
+   *       default one and the requested type is a MIME type, the returned
+   *       nsIHandlerInfo will include information from the default
+   *       nsIHandlerService instance. This cannot be avoided easily because the
+   *       getMIMEInfoFromOS method is not exposed to JavaScript.
+   *
+   * @param type
+   *        MIME type or scheme name of the nsIHandlerInfo to retrieve.
+   *
+   * @return The populated nsIHandlerInfo instance.
+   */
+  getHandlerInfo(type) {
+    if (type.includes("/")) {
+      // We have to use the getFromTypeAndExtension method because we don't have
+      // access to getMIMEInfoFromOS. This means that we have to reset the data
+      // that may have been imported from the default nsIHandlerService instance
+      // and is not overwritten by fillHandlerInfo later.
+      let handlerInfo = lazy.gMIMEService.getFromTypeAndExtension(type, null);
+      if (AppConstants.platform == "android") {
+        // On Android, the first handler application is always the internal one.
+        while (handlerInfo.possibleApplicationHandlers.length > 1) {
+          handlerInfo.possibleApplicationHandlers.removeElementAt(1);
+        }
+      } else {
+        handlerInfo.possibleApplicationHandlers.clear();
+      }
+      handlerInfo.setFileExtensions("");
+      // Populate the object from the handler service instance under testing.
+      if (lazy.gHandlerService.exists(handlerInfo)) {
+        lazy.gHandlerService.fillHandlerInfo(handlerInfo, "");
+      }
+      return handlerInfo;
+    }
+
+    // Populate the protocol information from the handler service instance under
+    // testing, like the nsIExternalProtocolService::GetProtocolHandlerInfo
+    // method does on the default nsIHandlerService instance.
+    let osDefaultHandlerFound = {};
+    let handlerInfo =
+      lazy.gExternalProtocolService.getProtocolHandlerInfoFromOS(
+        type,
+        osDefaultHandlerFound
+      );
+    if (lazy.gHandlerService.exists(handlerInfo)) {
+      lazy.gHandlerService.fillHandlerInfo(handlerInfo, "");
+    } else {
+      lazy.gExternalProtocolService.setProtocolHandlerDefaults(
+        handlerInfo,
+        osDefaultHandlerFound.value
+      );
+    }
+    return handlerInfo;
+  },
+
+  /**
+   * Creates an nsIHandlerInfo for the given MIME type or protocol, initialized
+   * to the default values for the current platform.
+   *
+   * @note For this method to work, the specified MIME type or protocol must not
+   *       be configured in the default handler service instance or the one
+   *       under testing, and must not be registered in the operating system.
+   *
+   * @param type
+   *        MIME type or scheme name of the nsIHandlerInfo to create.
+   *
+   * @return The blank nsIHandlerInfo instance.
+   */
+  getBlankHandlerInfo(type) {
+    let handlerInfo = this.getHandlerInfo(type);
+
+    let preferredAction, preferredApplicationHandler;
+    let alwaysAskBeforeHandling = true;
+
+    if (AppConstants.platform == "android") {
+      // On Android, the default preferredAction for MIME types is useHelperApp.
+      // For protocols, we always behave as if an operating system provided
+      // handler existed, and as such we initialize them to useSystemDefault.
+      // This is because the AndroidBridge is not available in xpcshell tests.
+      preferredAction = type.includes("/")
+        ? Ci.nsIHandlerInfo.useHelperApp
+        : Ci.nsIHandlerInfo.useSystemDefault;
+      // On Android, the default handler application is always the internal one.
+      preferredApplicationHandler = {
+        name: "Android chooser",
+      };
+    } else {
+      // On Desktop, the default preferredAction for MIME types is saveToDisk,
+      // while for protocols it is alwaysAsk. Since Bug 1735843, for new MIME
+      // types we default to not asking before handling unless a pref is set.
+      alwaysAskBeforeHandling = Services.prefs.getBoolPref(
+        "browser.download.always_ask_before_handling_new_types",
+        false
+      );
+
+      if (type.includes("/")) {
+        preferredAction = Ci.nsIHandlerInfo.saveToDisk;
+      } else {
+        preferredAction = Ci.nsIHandlerInfo.alwaysAsk;
+        // we'll always ask before handling protocols
+        alwaysAskBeforeHandling = true;
+      }
+      preferredApplicationHandler = null;
+    }
+
+    this.assertHandlerInfoMatches(handlerInfo, {
+      type,
+      preferredAction,
+      alwaysAskBeforeHandling,
+      preferredApplicationHandler,
+    });
+    return handlerInfo;
+  },
+
+  /**
+   * Checks whether an nsIHandlerInfo instance matches the provided object.
+   */
+  assertHandlerInfoMatches(handlerInfo, expected) {
+    let expectedInterface = expected.type.includes("/")
+      ? Ci.nsIMIMEInfo
+      : Ci.nsIHandlerInfo;
+    Assert.ok(handlerInfo instanceof expectedInterface);
+    Assert.equal(handlerInfo.type, expected.type);
+
+    if (!expected.preferredActionOSDependent) {
+      Assert.equal(handlerInfo.preferredAction, expected.preferredAction);
+      Assert.equal(
+        handlerInfo.alwaysAskBeforeHandling,
+        expected.alwaysAskBeforeHandling
+      );
+    }
+
+    if (expectedInterface == Ci.nsIMIMEInfo) {
+      let fileExtensionsEnumerator = handlerInfo.getFileExtensions();
+      for (let expectedFileExtension of expected.fileExtensions || []) {
+        Assert.equal(fileExtensionsEnumerator.getNext(), expectedFileExtension);
+      }
+      Assert.ok(!fileExtensionsEnumerator.hasMore());
+    }
+
+    if (expected.preferredApplicationHandler) {
+      this.assertHandlerAppMatches(
+        handlerInfo.preferredApplicationHandler,
+        expected.preferredApplicationHandler
+      );
+    } else {
+      Assert.equal(handlerInfo.preferredApplicationHandler, null);
+    }
+
+    let handlerAppsArrayEnumerator =
+      handlerInfo.possibleApplicationHandlers.enumerate();
+    if (AppConstants.platform == "android") {
+      // On Android, the first handler application is always the internal one.
+      this.assertHandlerAppMatches(handlerAppsArrayEnumerator.getNext(), {
+        name: "Android chooser",
+      });
+    }
+    for (let expectedHandlerApp of expected.possibleApplicationHandlers || []) {
+      this.assertHandlerAppMatches(
+        handlerAppsArrayEnumerator.getNext(),
+        expectedHandlerApp
+      );
+    }
+    Assert.ok(!handlerAppsArrayEnumerator.hasMoreElements());
+  },
+
+  /**
+   * Checks whether an nsIHandlerApp instance matches the provided object.
+   */
+  assertHandlerAppMatches(handlerApp, expected) {
+    Assert.ok(handlerApp instanceof Ci.nsIHandlerApp);
+    Assert.equal(handlerApp.name, expected.name);
+    if (expected.executable) {
+      Assert.ok(handlerApp instanceof Ci.nsILocalHandlerApp);
+      Assert.ok(expected.executable.equals(handlerApp.executable));
+    } else if (expected.uriTemplate) {
+      Assert.ok(handlerApp instanceof Ci.nsIWebHandlerApp);
+      Assert.equal(handlerApp.uriTemplate, expected.uriTemplate);
+    } else if (expected.service) {
+      Assert.ok(handlerApp instanceof Ci.nsIDBusHandlerApp);
+      Assert.equal(handlerApp.service, expected.service);
+      Assert.equal(handlerApp.method, expected.method);
+      Assert.equal(handlerApp.dBusInterface, expected.dBusInterface);
+      Assert.equal(handlerApp.objectPath, expected.objectPath);
+    }
+  },
+};
diff --git a/uriloader/exthandler/tests/WriteArgument.cpp b/uriloader/exthandler/tests/WriteArgument.cpp
new file mode 100644
index 0000000000..603965150e
--- /dev/null
+++ b/uriloader/exthandler/tests/WriteArgument.cpp
@@ -0,0 +1,20 @@
+#include <stdio.h>
+#include "prenv.h"
+
+int main(int argc, char* argv[]) {
+  if (argc != 2) return 1;
+
+  const char* value = PR_GetEnv("WRITE_ARGUMENT_FILE");
+
+  if (!value) return 2;
+
+  FILE* outfile = fopen(value, "w");
+  if (!outfile) return 3;
+
+  // We only need to write out the first argument (no newline).
+  fputs(argv[argc - 1], outfile);
+
+  fclose(outfile);
+
+  return 0;
+}
diff --git a/uriloader/exthandler/tests/gtest/ExternalHelperAppServiceTest.cpp b/uriloader/exthandler/tests/gtest/ExternalHelperAppServiceTest.cpp
new file mode 100644
index 0000000000..e63922182c
--- /dev/null
+++ b/uriloader/exthandler/tests/gtest/ExternalHelperAppServiceTest.cpp
@@ -0,0 +1,46 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "nsExternalHelperAppService.h"
+#include "gtest/gtest.h"
+#include "nsNetUtil.h"
+#include "mozilla/StaticPrefs_network.h"
+
+using namespace mozilla;
+
+TEST(ExternalHelperAppService, EscapeURI)
+{
+  if (StaticPrefs::network_url_useDefaultURI()) {
+    return;
+  }
+
+  nsCString input("myproto://hello world");
+  nsCString expected("myproto://hello%20world");
+
+  nsCOMPtr<nsIIOService> ios(do_GetIOService());
+  nsCOMPtr<nsIURI> uri;
+  nsresult rv = ios->NewURI(input, nullptr, nullptr, getter_AddRefs(uri));
+  EXPECT_EQ(rv, NS_OK);
+
+  nsAutoCString spec;
+  rv = uri->GetSpec(spec);
+  EXPECT_EQ(rv, NS_OK);
+
+  // Constructing the URI does not escape the space character.
+  ASSERT_TRUE(spec.Equals(input));
+
+  // Created an escaped version of the URI.
+  nsCOMPtr<nsIURI> escapedURI;
+  rv = nsExternalHelperAppService::EscapeURI(uri, getter_AddRefs(escapedURI));
+  EXPECT_EQ(rv, NS_OK);
+
+  nsAutoCString escapedSpec;
+  rv = escapedURI->GetSpec(escapedSpec);
+  EXPECT_EQ(rv, NS_OK);
+
+  // Escaped URI should have an escaped spec.
+  ASSERT_TRUE(escapedSpec.Equals(expected));
+}
diff --git a/uriloader/exthandler/tests/gtest/moz.build b/uriloader/exthandler/tests/gtest/moz.build
new file mode 100644
index 0000000000..163afe927d
--- /dev/null
+++ b/uriloader/exthandler/tests/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 += [
+    "ExternalHelperAppServiceTest.cpp",
+]
+
+FINAL_LIBRARY = "xul-gtest"
diff --git a/uriloader/exthandler/tests/mochitest/FTPprotocolHandler.html b/uriloader/exthandler/tests/mochitest/FTPprotocolHandler.html
new file mode 100644
index 0000000000..d0e98abace
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/FTPprotocolHandler.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>Protocol handler</title>
+    <meta content="text/html;charset=utf-8" http-equiv="Content-Type">
+    <meta content="utf-8" http-equiv="encoding">
+  </head>
+  <body>
+    <script type="text/javascript">
+      navigator.registerProtocolHandler("ftp",
+          "https://example.com/browser/uriloader/exthandler/tests/mochitest/blank.html?uri=%s",
+          "Test Protocol");
+    </script>
+    <a id="link" href="ftp://user:password@domain.com/path">ftp link</a>
+  </body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/HelperAppLauncherDialog_chromeScript.js b/uriloader/exthandler/tests/mochitest/HelperAppLauncherDialog_chromeScript.js
new file mode 100644
index 0000000000..4f66c99dc6
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/HelperAppLauncherDialog_chromeScript.js
@@ -0,0 +1,104 @@
+/* eslint-env mozilla/chrome-script */
+
+const { ComponentUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/ComponentUtils.sys.mjs"
+);
+
+const { Downloads } = ChromeUtils.importESModule(
+  "resource://gre/modules/Downloads.sys.mjs"
+);
+
+let gMIMEService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+let gHandlerService = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+  Ci.nsIHandlerService
+);
+
+const HELPERAPP_DIALOG_CONTRACT = "@mozilla.org/helperapplauncherdialog;1";
+const HELPERAPP_DIALOG_CID = Components.ID(
+  Cc[HELPERAPP_DIALOG_CONTRACT].number
+);
+
+let tmpDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+tmpDir.append("testsavedir" + Math.floor(Math.random() * 2 ** 32));
+// Create this dir if it doesn't exist (ignores existing dirs)
+try {
+  tmpDir.create(Ci.nsIFile.DIRECTORY_TYPE, 0o777, true);
+} catch (ex) {
+  if (ex.result != Cr.NS_ERROR_FILE_ALREADY_EXISTS) {
+    throw ex;
+  }
+}
+Services.prefs.setIntPref("browser.download.folderList", 2);
+Services.prefs.setCharPref("browser.download.dir", tmpDir.path);
+
+const FAKE_CID = Services.uuid.generateUUID();
+function HelperAppLauncherDialog() {}
+HelperAppLauncherDialog.prototype = {
+  show(aLauncher, aWindowContext, aReason) {
+    if (
+      Services.prefs.getBoolPref(
+        "browser.download.always_ask_before_handling_new_types"
+      )
+    ) {
+      let f = tmpDir.clone();
+      f.append(aLauncher.suggestedFileName);
+      aLauncher.saveDestinationAvailable(f);
+      sendAsyncMessage("suggestedFileName", aLauncher.suggestedFileName);
+    } else {
+      sendAsyncMessage("wrongAPICall", "show");
+    }
+    aLauncher.cancel(Cr.NS_BINDING_ABORTED);
+  },
+  promptForSaveToFileAsync(
+    appLauncher,
+    parent,
+    filename,
+    extension,
+    forceSave
+  ) {
+    if (
+      !Services.prefs.getBoolPref(
+        "browser.download.always_ask_before_handling_new_types"
+      )
+    ) {
+      let f = tmpDir.clone();
+      f.append(filename);
+      appLauncher.saveDestinationAvailable(f);
+      sendAsyncMessage("suggestedFileName", filename);
+    } else {
+      sendAsyncMessage("wrongAPICall", "promptForSaveToFileAsync");
+    }
+    appLauncher.cancel(Cr.NS_BINDING_ABORTED);
+  },
+  QueryInterface: ChromeUtils.generateQI(["nsIHelperAppLauncherDialog"]),
+};
+
+var registrar = Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
+registrar.registerFactory(
+  FAKE_CID,
+  "",
+  HELPERAPP_DIALOG_CONTRACT,
+  ComponentUtils.generateSingletonFactory(HelperAppLauncherDialog)
+);
+
+addMessageListener("unregister", async function () {
+  registrar.registerFactory(
+    HELPERAPP_DIALOG_CID,
+    "",
+    HELPERAPP_DIALOG_CONTRACT,
+    null
+  );
+  let list = await Downloads.getList(Downloads.ALL);
+  let downloads = await list.getAll();
+  for (let dl of downloads) {
+    await dl.refresh();
+    if (dl.target.exists || dl.target.partFileExists) {
+      dump("Finalizing download.\n");
+      await dl.finalize(true).catch(console.error);
+    }
+  }
+  await list.removeFinished();
+  dump("Clearing " + tmpDir.path + "\n");
+  tmpDir.remove(true);
+  sendAsyncMessage("unregistered");
+});
diff --git a/uriloader/exthandler/tests/mochitest/blank.html b/uriloader/exthandler/tests/mochitest/blank.html
new file mode 100644
index 0000000000..5417098d72
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/blank.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title></title>
+</head>
+<body>
+
+</body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/browser.toml b/uriloader/exthandler/tests/mochitest/browser.toml
new file mode 100644
index 0000000000..2eaf6632ac
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser.toml
@@ -0,0 +1,164 @@
+[DEFAULT]
+head = "head.js"
+support-files = ["protocolHandler.html"]
+
+
+["browser_auto_close_window.js"]
+support-files = [
+  "download_page.html",
+  "download.bin",
+  "download.sjs",
+]
+
+["browser_auto_close_window_nodialog.js"]
+support-files = [
+  "download_page.html",
+  "download.bin",
+  "download.sjs",
+]
+
+["browser_bad_download_dir.js"]
+run-if = ["os == 'linux'"]
+support-files = ["download.bin"]
+
+["browser_download_always_ask_preferred_app.js"]
+
+["browser_download_idn_blocklist.js"]
+support-files = ["download.bin"]
+
+["browser_download_open_with_internal_handler.js"]
+support-files = [
+  "file_image_svgxml.svg",
+  "file_image_svgxml.svg^headers^",
+  "file_pdf_application_pdf.pdf",
+  "file_pdf_application_pdf.pdf^headers^",
+  "file_pdf_application_unknown.pdf",
+  "file_pdf_application_unknown.pdf^headers^",
+  "file_pdf_application_octet_stream.pdf",
+  "file_pdf_application_octet_stream.pdf^headers^",
+  "file_pdf_binary_octet_stream.pdf",
+  "file_pdf_binary_octet_stream.pdf^headers^",
+  "file_txt_attachment_test.txt",
+  "file_txt_attachment_test.txt^headers^",
+  "file_xml_attachment_binary_octet_stream.xml",
+  "file_xml_attachment_binary_octet_stream.xml^headers^",
+  "file_xml_attachment_test.xml",
+  "file_xml_attachment_test.xml^headers^",
+  "file_green.webp",
+  "file_green.webp^headers^",
+]
+
+["browser_download_preferred_action.js"]
+support-files = ["mime_type_download.sjs"]
+
+["browser_download_privatebrowsing.js"]
+
+["browser_download_skips_dialog.js"]
+support-files = [
+  "file_green.webp",
+  "file_green.webp^headers^",
+]
+
+["browser_download_spam_permissions.js"]
+support-files = ["test_spammy_page.html"]
+
+["browser_download_urlescape.js"]
+support-files = [
+  "file_with@@funny_name.png",
+  "file_with@@funny_name.png^headers^",
+  "file_with[funny_name.webm",
+  "file_with[funny_name.webm^headers^",
+]
+
+["browser_extension_correction.js"]
+support-files = [
+  "file_as.exe",
+  "file_as.exe^headers^",
+]
+
+["browser_filehandling_loop.js"]
+
+["browser_first_prompt_not_blocked_without_user_interaction.js"]
+support-files = ["file_external_protocol_iframe.html"]
+
+["browser_ftp_protocol_handlers.js"]
+support-files = [
+  "FTPprotocolHandler.html",
+  "blank.html",
+]
+
+["browser_launched_app_save_directory.js"]
+support-files = [
+  "file_pdf_application_pdf.pdf",
+  "file_pdf_application_pdf.pdf^headers^",
+  "file_green.webp",
+  "file_green.webp^headers^",
+]
+
+["browser_local_files_no_save_without_asking.js"]
+support-files = ["file_pdf_binary_octet_stream.pdf"]
+
+["browser_local_files_open_doesnt_duplicate.js"]
+support-files = ["file_pdf_binary_octet_stream.pdf"]
+
+["browser_open_internal_choice_persistence.js"]
+skip-if = ["apple_silicon"] # bug 1752482
+support-files = [
+  "file_pdf_application_pdf.pdf",
+  "file_pdf_application_pdf.pdf^headers^",
+]
+
+["browser_pdf_save_as.js"]
+
+["browser_protocol_ask_dialog.js"]
+support-files = ["file_nested_protocol_request.html"]
+
+["browser_protocol_ask_dialog_external.js"]
+support-files = ["redirect_helper.sjs"]
+
+["browser_protocol_ask_dialog_permission.js"]
+support-files = [
+  "redirect_helper.sjs",
+  "script_redirect.html",
+]
+
+["browser_protocol_custom_sandbox.js"]
+support-files = ["protocol_custom_sandbox_helper.sjs"]
+
+["browser_protocol_custom_sandbox_csp.js"]
+support-files = ["protocol_custom_sandbox_helper.sjs"]
+
+["browser_protocolhandler_loop.js"]
+
+["browser_remember_download_option.js"]
+
+["browser_save_filenames.js"]
+support-files = ["save_filenames.html"]
+
+["browser_shows_where_to_save_dialog.js"]
+support-files = [
+  "file_green.webp",
+  "file_green.webp^headers^",
+  "file_pdf_application_pdf.pdf",
+  "file_pdf_application_pdf.pdf^headers^",
+  "file_txt_attachment_test.txt",
+  "file_txt_attachment_test.txt^headers^",
+]
+
+["browser_standalone_application_chooser_window_fallback.js"]
+support-files = [
+  "FTPprotocolHandler.html",
+  "blank.html",
+]
+
+["browser_txt_download_save_as.js"]
+support-files = [
+  "file_txt_attachment_test.txt",
+  "file_txt_attachment_test.txt^headers^",
+  "!/toolkit/content/tests/browser/common/mockTransfer.js",
+]
+
+["browser_web_handler_app_pinned_tab.js"]
+support-files = ["mailto.html"]
+
+["browser_web_protocol_handlers.js"]
diff --git a/uriloader/exthandler/tests/mochitest/browser_auto_close_window.js b/uriloader/exthandler/tests/mochitest/browser_auto_close_window.js
new file mode 100644
index 0000000000..af60b59c4b
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_auto_close_window.js
@@ -0,0 +1,342 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { ComponentUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/ComponentUtils.sys.mjs"
+);
+
+const ROOT = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+const PAGE_URL = ROOT + "download_page.html";
+const SJS_URL = ROOT + "download.sjs";
+
+const HELPERAPP_DIALOG_CONTRACT_ID = "@mozilla.org/helperapplauncherdialog;1";
+const HELPERAPP_DIALOG_CID = Components.ID(
+  Cc[HELPERAPP_DIALOG_CONTRACT_ID].number
+);
+const MOCK_HELPERAPP_DIALOG_CID = Components.ID(
+  "{2f372b6f-56c9-46d5-af0d-9f09bb69860c}"
+);
+
+let registrar = Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
+let curDialogResolve = null;
+
+function HelperAppLauncherDialog() {}
+
+HelperAppLauncherDialog.prototype = {
+  show(aLauncher, aWindowContext, aReason) {
+    ok(true, "Showing the helper app dialog");
+    curDialogResolve(aWindowContext);
+    executeSoon(() => {
+      aLauncher.cancel(Cr.NS_ERROR_ABORT);
+    });
+  },
+  QueryInterface: ChromeUtils.generateQI(["nsIHelperAppLauncherDialog"]),
+};
+
+function promiseHelperAppDialog() {
+  return new Promise(resolve => {
+    curDialogResolve = resolve;
+  });
+}
+
+let mockHelperAppService;
+
+add_setup(async function () {
+  // Replace the real helper app dialog with our own.
+  mockHelperAppService = ComponentUtils.generateSingletonFactory(
+    HelperAppLauncherDialog
+  );
+  registrar.registerFactory(
+    MOCK_HELPERAPP_DIALOG_CID,
+    "",
+    HELPERAPP_DIALOG_CONTRACT_ID,
+    mockHelperAppService
+  );
+
+  // Ensure we always prompt for these downloads.
+  const HandlerService = Cc[
+    "@mozilla.org/uriloader/handler-service;1"
+  ].getService(Ci.nsIHandlerService);
+
+  const MIMEService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+  const mimeInfo = MIMEService.getFromTypeAndExtension(
+    "application/octet-stream",
+    "bin"
+  );
+  mimeInfo.alwaysAskBeforeHandling = true;
+  HandlerService.store(mimeInfo);
+
+  // On Mac, .bin is application/macbinary
+  let mimeInfoMac;
+  if (AppConstants.platform == "macosx") {
+    mimeInfoMac = MIMEService.getFromTypeAndExtension(
+      "application/macbinary",
+      "bin"
+    );
+    mimeInfoMac.alwaysAskBeforeHandling = true;
+    HandlerService.store(mimeInfoMac);
+  }
+
+  registerCleanupFunction(() => {
+    HandlerService.remove(mimeInfo);
+    if (mimeInfoMac) {
+      HandlerService.remove(mimeInfoMac);
+    }
+  });
+});
+
+add_task(async function simple_navigation() {
+  // Tests that simple navigation gives us the right windowContext (that is,
+  // the window that we're using).
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let dialogAppeared = promiseHelperAppDialog();
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#regular_load",
+        {},
+        browser
+      );
+      let windowContext = await dialogAppeared;
+
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+    }
+  );
+});
+
+add_task(async function accel_navigation() {
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let dialogAppeared = promiseHelperAppDialog();
+      let tabOpened = BrowserTestUtils.waitForEvent(
+        gBrowser.tabContainer,
+        "TabOpen"
+      ).then(event => {
+        return [event.target, BrowserTestUtils.waitForTabClosing(event.target)];
+      });
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#regular_load",
+        { accelKey: true },
+        browser
+      );
+
+      let windowContext = await dialogAppeared;
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+      let [tab, closingPromise] = await tabOpened;
+      await closingPromise;
+      is(tab.linkedBrowser, null, "tab was opened and closed");
+    }
+  );
+});
+
+// Given a browser pointing to download_page.html, clicks on the link that
+// opens with target="_blank" (i.e. a new tab) and ensures that we
+// automatically open and close that tab.
+async function testNewTab(browser) {
+  let dialogAppeared = promiseHelperAppDialog();
+  let tabOpened = BrowserTestUtils.waitForEvent(
+    gBrowser.tabContainer,
+    "TabOpen"
+  ).then(event => {
+    return [event.target, BrowserTestUtils.waitForTabClosing(event.target)];
+  });
+
+  await BrowserTestUtils.synthesizeMouseAtCenter("#target_blank", {}, browser);
+
+  let windowContext = await dialogAppeared;
+  is(windowContext, browser.ownerGlobal, "got the right windowContext");
+  let [tab, closingPromise] = await tabOpened;
+  await closingPromise;
+  is(tab.linkedBrowser, null, "tab was opened and closed");
+}
+
+add_task(async function target_blank() {
+  // Tests that a link with target=_blank opens a new tab and closes it,
+  // returning the window that we're using for navigation.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      await testNewTab(browser);
+    }
+  );
+});
+
+add_task(async function target_blank_no_opener() {
+  // Tests that a link with target=_blank and no opener opens a new tab
+  // and closes it, returning the window that we're using for navigation.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let dialogAppeared = promiseHelperAppDialog();
+      let tabOpened = BrowserTestUtils.waitForEvent(
+        gBrowser.tabContainer,
+        "TabOpen"
+      ).then(event => {
+        return [event.target, BrowserTestUtils.waitForTabClosing(event.target)];
+      });
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#target_blank_no_opener",
+        {},
+        browser
+      );
+
+      let windowContext = await dialogAppeared;
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+      let [tab, closingPromise] = await tabOpened;
+      await closingPromise;
+      is(tab.linkedBrowser, null, "tab was opened and closed");
+    }
+  );
+});
+
+add_task(async function open_in_new_tab_no_opener() {
+  // Tests that a link with target=_blank and no opener opens a new tab
+  // and closes it, returning the window that we're using for navigation.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let dialogAppeared = promiseHelperAppDialog();
+      let tabOpened = BrowserTestUtils.waitForEvent(
+        gBrowser.tabContainer,
+        "TabOpen"
+      ).then(event => {
+        return [event.target, BrowserTestUtils.waitForTabClosing(event.target)];
+      });
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#open_in_new_tab_no_opener",
+        {},
+        browser
+      );
+
+      let windowContext = await dialogAppeared;
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+      let [tab, closingPromise] = await tabOpened;
+      await closingPromise;
+      is(tab.linkedBrowser, null, "tab was opened and closed");
+    }
+  );
+});
+
+add_task(async function new_window() {
+  // Tests that a link that forces us to open a new window (by specifying a
+  // width and a height in window.open) opens a new window for the load,
+  // realizes that we need to close that window and returns the *original*
+  // window as the window context.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let dialogAppeared = promiseHelperAppDialog();
+      let windowOpened = BrowserTestUtils.waitForNewWindow();
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#new_window",
+        {},
+        browser
+      );
+      let win = await windowOpened;
+      // Now allow request to complete:
+      fetch(SJS_URL + "?finish");
+
+      let windowContext = await dialogAppeared;
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+
+      // The window should close on its own. If not, this test will time out.
+      await BrowserTestUtils.domWindowClosed(win);
+      ok(win.closed, "window was opened and closed");
+
+      is(
+        await fetch(SJS_URL + "?reset").then(r => r.text()),
+        "OK",
+        "Test reseted"
+      );
+    }
+  );
+});
+
+add_task(async function new_window_no_opener() {
+  // Tests that a link that forces us to open a new window (by specifying a
+  // width and a height in window.open) opens a new window for the load,
+  // realizes that we need to close that window and returns the *original*
+  // window as the window context.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let dialogAppeared = promiseHelperAppDialog();
+      let windowOpened = BrowserTestUtils.waitForNewWindow();
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#new_window_no_opener",
+        {},
+        browser
+      );
+      let win = await windowOpened;
+      // Now allow request to complete:
+      fetch(SJS_URL + "?finish");
+
+      await dialogAppeared;
+
+      // The window should close on its own. If not, this test will time out.
+      await BrowserTestUtils.domWindowClosed(win);
+      ok(win.closed, "window was opened and closed");
+
+      is(
+        await fetch(SJS_URL + "?reset").then(r => r.text()),
+        "OK",
+        "Test reseted"
+      );
+    }
+  );
+});
+
+add_task(async function nested_window_opens() {
+  // Tests that the window auto-closing feature works if the download is
+  // initiated by a window that, itself, has an opener (see bug 1373109).
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (outerBrowser) {
+      let secondTabPromise = BrowserTestUtils.waitForNewTab(
+        gBrowser,
+        `${PAGE_URL}?newwin`,
+        true
+      );
+      BrowserTestUtils.synthesizeMouseAtCenter(
+        "#open_in_new_tab",
+        {},
+        outerBrowser
+      );
+      let secondTab = await secondTabPromise;
+      let nestedBrowser = secondTab.linkedBrowser;
+
+      await SpecialPowers.spawn(nestedBrowser, [], function () {
+        ok(content.opener, "this window has an opener");
+      });
+
+      await testNewTab(nestedBrowser);
+
+      isnot(
+        secondTab.linkedBrowser,
+        null,
+        "the page that triggered the download is still open"
+      );
+      BrowserTestUtils.removeTab(secondTab);
+    }
+  );
+});
+
+add_task(async function cleanup() {
+  // Unregister our factory from XPCOM and restore the original CID.
+  registrar.unregisterFactory(MOCK_HELPERAPP_DIALOG_CID, mockHelperAppService);
+  registrar.registerFactory(
+    HELPERAPP_DIALOG_CID,
+    "",
+    HELPERAPP_DIALOG_CONTRACT_ID,
+    null
+  );
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_auto_close_window_nodialog.js b/uriloader/exthandler/tests/mochitest/browser_auto_close_window_nodialog.js
new file mode 100644
index 0000000000..4f1e8ab18e
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_auto_close_window_nodialog.js
@@ -0,0 +1,308 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { ComponentUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/ComponentUtils.sys.mjs"
+);
+
+const ROOT = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+const PAGE_URL = ROOT + "download_page.html";
+const SJS_URL = ROOT + "download.sjs";
+
+const HELPERAPP_DIALOG_CONTRACT_ID = "@mozilla.org/helperapplauncherdialog;1";
+const HELPERAPP_DIALOG_CID = Components.ID(
+  Cc[HELPERAPP_DIALOG_CONTRACT_ID].number
+);
+const MOCK_HELPERAPP_DIALOG_CID = Components.ID(
+  "{2f372b6f-56c9-46d5-af0d-9f09bb69860c}"
+);
+
+let registrar = Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
+let curSaveResolve = null;
+
+function HelperAppLauncherDialog() {}
+
+HelperAppLauncherDialog.prototype = {
+  show(aLauncher, aWindowContext, aReason) {
+    ok(false, "Shouldn't be showing the helper app dialog");
+    executeSoon(() => {
+      aLauncher.cancel(Cr.NS_ERROR_ABORT);
+    });
+  },
+  promptForSaveToFileAsync(aLauncher, aWindowContext, aReason) {
+    ok(true, "Shouldn't be showing the helper app dialog");
+    curSaveResolve(aWindowContext);
+    executeSoon(() => {
+      aLauncher.cancel(Cr.NS_ERROR_ABORT);
+    });
+  },
+  QueryInterface: ChromeUtils.generateQI(["nsIHelperAppLauncherDialog"]),
+};
+
+function promiseSave() {
+  return new Promise(resolve => {
+    curSaveResolve = resolve;
+  });
+}
+
+let mockHelperAppService;
+
+add_setup(async function () {
+  // Replace the real helper app dialog with our own.
+  mockHelperAppService = ComponentUtils.generateSingletonFactory(
+    HelperAppLauncherDialog
+  );
+  registrar.registerFactory(
+    MOCK_HELPERAPP_DIALOG_CID,
+    "",
+    HELPERAPP_DIALOG_CONTRACT_ID,
+    mockHelperAppService
+  );
+
+  // Ensure we always prompt for these downloads.
+  const HandlerService = Cc[
+    "@mozilla.org/uriloader/handler-service;1"
+  ].getService(Ci.nsIHandlerService);
+
+  const MIMEService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+  const mimeInfo = MIMEService.getFromTypeAndExtension(
+    "application/octet-stream",
+    "bin"
+  );
+  mimeInfo.alwaysAskBeforeHandling = false;
+  mimeInfo.preferredAction = Ci.nsIHandlerInfo.saveToDisk;
+  HandlerService.store(mimeInfo);
+
+  registerCleanupFunction(() => {
+    HandlerService.remove(mimeInfo);
+  });
+});
+
+add_task(async function simple_navigation() {
+  // Tests that simple navigation gives us the right windowContext (that is,
+  // the window that we're using).
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let saveHappened = promiseSave();
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#regular_load",
+        {},
+        browser
+      );
+      let windowContext = await saveHappened;
+
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+    }
+  );
+});
+
+// Given a browser pointing to download_page.html, clicks on the link that
+// opens with target="_blank" (i.e. a new tab) and ensures that we
+// automatically open and close that tab.
+async function testNewTab(browser) {
+  let saveHappened = promiseSave();
+  let tabOpened = BrowserTestUtils.waitForEvent(
+    gBrowser.tabContainer,
+    "TabOpen"
+  ).then(event => {
+    return [event.target, BrowserTestUtils.waitForTabClosing(event.target)];
+  });
+
+  await BrowserTestUtils.synthesizeMouseAtCenter("#target_blank", {}, browser);
+
+  let windowContext = await saveHappened;
+  is(windowContext, browser.ownerGlobal, "got the right windowContext");
+  let [tab, closingPromise] = await tabOpened;
+  await closingPromise;
+  is(tab.linkedBrowser, null, "tab was opened and closed");
+}
+
+add_task(async function target_blank() {
+  // Tests that a link with target=_blank opens a new tab and closes it,
+  // returning the window that we're using for navigation.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      await testNewTab(browser);
+    }
+  );
+});
+
+add_task(async function target_blank_no_opener() {
+  // Tests that a link with target=_blank and no opener opens a new tab
+  // and closes it, returning the window that we're using for navigation.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let saveHappened = promiseSave();
+      let tabOpened = BrowserTestUtils.waitForEvent(
+        gBrowser.tabContainer,
+        "TabOpen"
+      ).then(event => {
+        return [event.target, BrowserTestUtils.waitForTabClosing(event.target)];
+      });
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#target_blank_no_opener",
+        {},
+        browser
+      );
+
+      let windowContext = await saveHappened;
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+      let [tab, closingPromise] = await tabOpened;
+      await closingPromise;
+      is(tab.linkedBrowser, null, "tab was opened and closed");
+    }
+  );
+});
+
+add_task(async function open_in_new_tab_no_opener() {
+  // Tests that a link with target=_blank and no opener opens a new tab
+  // and closes it, returning the window that we're using for navigation.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let saveHappened = promiseSave();
+      let tabOpened = BrowserTestUtils.waitForEvent(
+        gBrowser.tabContainer,
+        "TabOpen"
+      ).then(event => {
+        return [event.target, BrowserTestUtils.waitForTabClosing(event.target)];
+      });
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#open_in_new_tab_no_opener",
+        {},
+        browser
+      );
+
+      let windowContext = await saveHappened;
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+      let [tab, closingPromise] = await tabOpened;
+      await closingPromise;
+      is(tab.linkedBrowser, null, "tab was opened and closed");
+    }
+  );
+});
+
+add_task(async function new_window() {
+  // Tests that a link that forces us to open a new window (by specifying a
+  // width and a height in window.open) opens a new window for the load,
+  // realizes that we need to close that window and returns the *original*
+  // window as the window context.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let saveHappened = promiseSave();
+      let windowOpened = BrowserTestUtils.waitForNewWindow();
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#new_window",
+        {},
+        browser
+      );
+      let win = await windowOpened;
+      // Now allow request to complete:
+      fetch(SJS_URL + "?finish");
+
+      let windowContext = await saveHappened;
+      is(windowContext, browser.ownerGlobal, "got the right windowContext");
+
+      // The window should close on its own. If not, this test will time out.
+      await BrowserTestUtils.domWindowClosed(win);
+      ok(win.closed, "window was opened and closed");
+
+      is(
+        await fetch(SJS_URL + "?reset").then(r => r.text()),
+        "OK",
+        "Test reseted"
+      );
+    }
+  );
+});
+
+add_task(async function new_window_no_opener() {
+  // Tests that a link that forces us to open a new window (by specifying a
+  // width and a height in window.open) opens a new window for the load,
+  // realizes that we need to close that window and returns the *original*
+  // window as the window context.
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (browser) {
+      let saveHappened = promiseSave();
+      let windowOpened = BrowserTestUtils.waitForNewWindow();
+
+      await BrowserTestUtils.synthesizeMouseAtCenter(
+        "#new_window_no_opener",
+        {},
+        browser
+      );
+      let win = await windowOpened;
+      // Now allow request to complete:
+      fetch(SJS_URL + "?finish");
+
+      await saveHappened;
+
+      // The window should close on its own. If not, this test will time out.
+      await BrowserTestUtils.domWindowClosed(win);
+      ok(win.closed, "window was opened and closed");
+
+      is(
+        await fetch(SJS_URL + "?reset").then(r => r.text()),
+        "OK",
+        "Test reseted"
+      );
+    }
+  );
+});
+
+add_task(async function nested_window_opens() {
+  // Tests that the window auto-closing feature works if the download is
+  // initiated by a window that, itself, has an opener (see bug 1373109).
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: PAGE_URL },
+    async function (outerBrowser) {
+      let secondTabPromise = BrowserTestUtils.waitForNewTab(
+        gBrowser,
+        `${PAGE_URL}?newwin`,
+        true
+      );
+      BrowserTestUtils.synthesizeMouseAtCenter(
+        "#open_in_new_tab",
+        {},
+        outerBrowser
+      );
+      let secondTab = await secondTabPromise;
+      let nestedBrowser = secondTab.linkedBrowser;
+
+      await SpecialPowers.spawn(nestedBrowser, [], function () {
+        ok(content.opener, "this window has an opener");
+      });
+
+      await testNewTab(nestedBrowser);
+
+      isnot(
+        secondTab.linkedBrowser,
+        null,
+        "the page that triggered the download is still open"
+      );
+      BrowserTestUtils.removeTab(secondTab);
+    }
+  );
+});
+
+add_task(async function cleanup() {
+  // Unregister our factory from XPCOM and restore the original CID.
+  registrar.unregisterFactory(MOCK_HELPERAPP_DIALOG_CID, mockHelperAppService);
+  registrar.registerFactory(
+    HELPERAPP_DIALOG_CID,
+    "",
+    HELPERAPP_DIALOG_CONTRACT_ID,
+    null
+  );
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_bad_download_dir.js b/uriloader/exthandler/tests/mochitest/browser_bad_download_dir.js
new file mode 100644
index 0000000000..7fe22e0661
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_bad_download_dir.js
@@ -0,0 +1,90 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+let { FileTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/FileTestUtils.sys.mjs"
+);
+
+const ROOT = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+const TEST_FILE = ROOT + "download.bin";
+
+add_task(async function test_check_download_dir() {
+  // Force XDG dir to somewhere that has no config files, causing lookups of the
+  // system download dir to fail:
+  let newXDGRoot = FileTestUtils.getTempFile("xdgstuff");
+  newXDGRoot.create(Ci.nsIFile.DIRECTORY_TYPE, 0o755);
+  let oldXDG = Services.env.exists("XDG_CONFIG_HOME")
+    ? Services.env.get("XDG_CONFIG_HOME")
+    : "";
+  registerCleanupFunction(() => Services.env.set("XDG_CONFIG_HOME", oldXDG));
+  Services.env.set("XDG_CONFIG_HOME", newXDGRoot.path + "/");
+
+  let propBundle = Services.strings.createBundle(
+    "chrome://mozapps/locale/downloads/downloads.properties"
+  );
+  let dlRoot = PathUtils.join(
+    Services.dirsvc.get("Home", Ci.nsIFile).path,
+    propBundle.GetStringFromName("downloadsFolder")
+  );
+
+  // Check lookups fail:
+  Assert.throws(
+    () => Services.dirsvc.get("DfltDwnld", Ci.nsIFile),
+    /NS_ERROR_FAILURE/,
+    "Should throw when asking for downloads dir."
+  );
+
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      // Avoid opening dialogs
+      ["browser.download.always_ask_before_handling_new_types", false],
+      // Switch back to default OS downloads dir (changed in head.js):
+      ["browser.download.folderList", 1],
+    ],
+  });
+
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  let downloadFinished = promiseDownloadFinished(publicList);
+  let tab = BrowserTestUtils.addTab(gBrowser, TEST_FILE);
+  let dl = await downloadFinished;
+  ok(dl.succeeded, "Download should succeed.");
+  Assert.stringContains(
+    dl.target.path,
+    dlRoot,
+    "Should store download under DL folder root."
+  );
+  let dlKids = await IOUtils.getChildren(dlRoot);
+  ok(
+    dlKids.includes(dl.target.path),
+    "Download should be a direct child of the DL folder."
+  );
+  await IOUtils.remove(dl.target.path);
+
+  BrowserTestUtils.removeTab(tab);
+
+  // Download a second file to make sure we're not continuously adding filenames
+  // onto the download folder path.
+  downloadFinished = promiseDownloadFinished(publicList);
+  tab = BrowserTestUtils.addTab(gBrowser, TEST_FILE);
+  dl = await downloadFinished;
+  Assert.stringContains(
+    dl.target.path,
+    dlRoot,
+    "Second download should store download under new DL folder root."
+  );
+  dlKids = await IOUtils.getChildren(dlRoot);
+  ok(
+    dlKids.includes(dl.target.path),
+    "Second download should be a direct child of the new DL folder."
+  );
+  BrowserTestUtils.removeTab(tab);
+  await IOUtils.remove(dl.target.path);
+
+  await publicList.removeFinished();
+  await IOUtils.remove(newXDGRoot.path);
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_always_ask_preferred_app.js b/uriloader/exthandler/tests/mochitest/browser_download_always_ask_preferred_app.js
new file mode 100644
index 0000000000..b4bfe5e51a
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_always_ask_preferred_app.js
@@ -0,0 +1,25 @@
+add_task(async function () {
+  // Create mocked objects for test
+  let launcher = createMockedObjects(false);
+  // Open helper app dialog with mocked launcher
+  let dlg = await openHelperAppDialog(launcher);
+  let doc = dlg.document;
+  let location = doc.getElementById("source");
+  let expectedValue = launcher.source.prePath;
+  if (location.value != expectedValue) {
+    info("Waiting for dialog to be populated.");
+    await BrowserTestUtils.waitForAttribute("value", location, expectedValue);
+  }
+  is(
+    doc.getElementById("mode").selectedItem.id,
+    "open",
+    "Should be opening the file."
+  );
+  ok(
+    !dlg.document.getElementById("openHandler").selectedItem.hidden,
+    "Should not have selected a hidden item."
+  );
+  let helperAppDialogHiddenPromise = BrowserTestUtils.windowClosed(dlg);
+  doc.getElementById("unknownContentType").cancelDialog();
+  await helperAppDialogHiddenPromise;
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_idn_blocklist.js b/uriloader/exthandler/tests/mochitest/browser_download_idn_blocklist.js
new file mode 100644
index 0000000000..0d49a898a0
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_idn_blocklist.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_HOST = "example.org";
+const TEST_FILE = "\u3002.bin";
+const TEST_URL = `http://${TEST_HOST}/${TEST_FILE}`;
+
+const { XPCShellContentUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/XPCShellContentUtils.sys.mjs"
+);
+XPCShellContentUtils.initMochitest(this);
+const server = XPCShellContentUtils.createHttpServer({
+  hosts: [TEST_HOST],
+});
+let file = getChromeDir(getResolvedURI(gTestPath));
+file.append("download.bin");
+server.registerFile(`/${encodeURIComponent(TEST_FILE)}`, file);
+
+/**
+ * Check that IDN blocklisted characters are not escaped in
+ * download file names.
+ */
+add_task(async function test_idn_blocklisted_char_not_escaped() {
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.download.always_ask_before_handling_new_types", false]],
+  });
+
+  info("Testing with " + TEST_URL);
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  registerCleanupFunction(async () => {
+    await publicList.removeFinished();
+  });
+  let downloadFinished = promiseDownloadFinished(publicList);
+  var tab = BrowserTestUtils.addTab(gBrowser, TEST_URL);
+  let dl = await downloadFinished;
+  ok(dl.succeeded, "Download should succeed.");
+  Assert.equal(
+    PathUtils.filename(dl.target.path),
+    TEST_FILE,
+    "Should not escape a download file name."
+  );
+  await IOUtils.remove(dl.target.path);
+
+  BrowserTestUtils.removeTab(tab);
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_open_with_internal_handler.js b/uriloader/exthandler/tests/mochitest/browser_download_open_with_internal_handler.js
new file mode 100644
index 0000000000..c73538fe19
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_open_with_internal_handler.js
@@ -0,0 +1,814 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+requestLongerTimeout(2);
+
+const { Downloads } = ChromeUtils.importESModule(
+  "resource://gre/modules/Downloads.sys.mjs"
+);
+const { DownloadIntegration } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+const MimeSvc = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+const HandlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+  Ci.nsIHandlerService
+);
+
+let MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+
+function waitForAcceptButtonToGetEnabled(doc) {
+  let dialog = doc.querySelector("#unknownContentType");
+  let button = dialog.getButton("accept");
+  return TestUtils.waitForCondition(
+    () => !button.disabled,
+    "Wait for Accept button to get enabled"
+  );
+}
+
+async function waitForPdfJS(browser, url) {
+  // Runs tests after all "load" event handlers have fired off
+  let loadPromise = BrowserTestUtils.waitForContentEvent(
+    browser,
+    "documentloaded",
+    false,
+    null,
+    true
+  );
+  BrowserTestUtils.startLoadingURIString(browser, url);
+  return loadPromise;
+}
+
+/**
+ * This test covers which choices are presented for downloaded files and how
+ * those choices are handled. Unless a pref is enabled
+ * (browser.download.always_ask_before_handling_new_types) the unknown content
+ * dialog will be skipped altogether by default when downloading.
+ * To retain coverage for the non-default scenario, each task sets `alwaysAskBeforeHandling`
+ * to true for the relevant mime-type and extensions.
+ */
+function alwaysAskForHandlingTypes(typeExtensions, ask = true) {
+  let mimeInfos = [];
+  for (let [type, ext] of Object.entries(typeExtensions)) {
+    const mimeInfo = MimeSvc.getFromTypeAndExtension(type, ext);
+    mimeInfo.alwaysAskBeforeHandling = ask;
+    if (!ask) {
+      mimeInfo.preferredAction = mimeInfo.handleInternally;
+    }
+    HandlerSvc.store(mimeInfo);
+    mimeInfos.push(mimeInfo);
+  }
+  return mimeInfos;
+}
+
+add_setup(async function () {
+  // Remove the security delay for the dialog during the test.
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["security.dialog_enable_delay", 0],
+      ["browser.helperApps.showOpenOptionForPdfJS", true],
+      ["browser.helperApps.showOpenOptionForViewableInternally", true],
+    ],
+  });
+
+  // Restore handlers after the whole test has run
+  const registerRestoreHandler = function (type, ext) {
+    const mimeInfo = MimeSvc.getFromTypeAndExtension(type, ext);
+    const existed = HandlerSvc.exists(mimeInfo);
+
+    registerCleanupFunction(() => {
+      if (existed) {
+        HandlerSvc.store(mimeInfo);
+      } else {
+        HandlerSvc.remove(mimeInfo);
+      }
+    });
+  };
+  registerRestoreHandler("application/pdf", "pdf");
+  registerRestoreHandler("binary/octet-stream", "pdf");
+  registerRestoreHandler("application/unknown", "pdf");
+  registerRestoreHandler("image/webp", "webp");
+});
+
+/**
+ * Check that loading a PDF file with content-disposition: attachment
+ * shows an option to open with the internal handler, and that the
+ * internal option handler is not present when the download button
+ * is clicked from pdf.js.
+ */
+add_task(async function test_check_open_with_internal_handler() {
+  const mimeInfosToRestore = alwaysAskForHandlingTypes({
+    "application/pdf": "pdf",
+    "binary/octet-stream": "pdf",
+  });
+
+  for (let file of [
+    "file_pdf_application_pdf.pdf",
+    "file_pdf_binary_octet_stream.pdf",
+  ]) {
+    info("Testing with " + file);
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    registerCleanupFunction(async () => {
+      await publicList.removeFinished();
+    });
+    let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: TEST_PATH + file,
+      waitForLoad: false,
+      waitForStateStop: true,
+    });
+    // Add an extra tab after the loading tab so we can test that
+    // pdf.js is opened in the adjacent tab and not at the end of
+    // the tab strip.
+    let extraTab = await BrowserTestUtils.addTab(gBrowser, "about:blank");
+    let dialogWindow = await dialogWindowPromise;
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should have seen the unknown content dialogWindow."
+    );
+    let doc = dialogWindow.document;
+    let internalHandlerRadio = doc.querySelector("#handleInternally");
+
+    await waitForAcceptButtonToGetEnabled(doc);
+
+    ok(!internalHandlerRadio.hidden, "The option should be visible for PDF");
+    ok(internalHandlerRadio.selected, "The option should be selected");
+
+    let downloadFinishedPromise = promiseDownloadFinished(publicList);
+    let newTabPromise = BrowserTestUtils.waitForNewTab(gBrowser);
+    let dialog = doc.querySelector("#unknownContentType");
+    let button = dialog.getButton("accept");
+    button.disabled = false;
+    dialog.acceptDialog();
+    info("waiting for new tab to open");
+    let newTab = await newTabPromise;
+
+    is(
+      newTab._tPos - 1,
+      loadingTab._tPos,
+      "pdf.js should be opened in an adjacent tab"
+    );
+
+    await ContentTask.spawn(newTab.linkedBrowser, null, async () => {
+      await ContentTaskUtils.waitForCondition(
+        () => content.document.readyState == "complete"
+      );
+    });
+
+    let publicDownloads = await publicList.getAll();
+    is(
+      publicDownloads.length,
+      1,
+      "download should appear in publicDownloads list"
+    );
+
+    let download = await downloadFinishedPromise;
+
+    let subdialogPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    // Current tab has file: URI and TEST_PATH is http uri, so uri will be different
+    BrowserTestUtils.startLoadingURIString(
+      newTab.linkedBrowser,
+      TEST_PATH + file
+    );
+    let subDialogWindow = await subdialogPromise;
+    let subDoc = subDialogWindow.document;
+
+    // Prevent racing with initialization of the dialog and make sure that
+    // the final state of the dialog has the correct visibility of the internal-handler option.
+    await waitForAcceptButtonToGetEnabled(subDoc);
+    let subInternalHandlerRadio = subDoc.querySelector("#handleInternally");
+    ok(
+      !subInternalHandlerRadio.hidden,
+      "This option should be shown when the dialog is shown for another PDF"
+    );
+    // Cancel dialog
+    subDoc.querySelector("#unknownContentType").cancelDialog();
+
+    let filepickerPromise = new Promise(resolve => {
+      MockFilePicker.showCallback = function (fp) {
+        setTimeout(() => {
+          resolve(fp.defaultString);
+        }, 0);
+        return Ci.nsIFilePicker.returnCancel;
+      };
+    });
+
+    subdialogPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    await SpecialPowers.spawn(newTab.linkedBrowser, [], async () => {
+      let downloadButton;
+      await ContentTaskUtils.waitForCondition(() => {
+        downloadButton = content.document.querySelector("#download");
+        return !!downloadButton;
+      });
+      ok(downloadButton, "Download button should be present in pdf.js");
+      downloadButton.click();
+    });
+    info(
+      "Waiting for unknown content type dialog to appear from pdf.js download button click"
+    );
+    let filename = await filepickerPromise;
+    is(filename, file, "filename was set in filepicker");
+
+    // Remove the first file (can't do this sooner or the second load fails):
+    if (download?.target.exists) {
+      try {
+        info("removing " + download.target.path);
+        await IOUtils.remove(download.target.path);
+      } catch (ex) {
+        /* ignore */
+      }
+    }
+
+    BrowserTestUtils.removeTab(loadingTab);
+    BrowserTestUtils.removeTab(newTab);
+    BrowserTestUtils.removeTab(extraTab);
+
+    await publicList.removeFinished();
+  }
+  for (let mimeInfo of mimeInfosToRestore) {
+    HandlerSvc.remove(mimeInfo);
+  }
+});
+
+/**
+ * Test that choosing to open in an external application doesn't
+ * open the PDF into pdf.js
+ */
+add_task(async function test_check_open_with_external_application() {
+  const mimeInfosToRestore = alwaysAskForHandlingTypes({
+    "application/pdf": "pdf",
+    "binary/octet-stream": "pdf",
+  });
+
+  for (let file of [
+    "file_pdf_application_pdf.pdf",
+    "file_pdf_binary_octet_stream.pdf",
+  ]) {
+    info("Testing with " + file);
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    registerCleanupFunction(async () => {
+      await publicList.removeFinished();
+    });
+    let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: TEST_PATH + file,
+      waitForLoad: false,
+      waitForStateStop: true,
+    });
+    let dialogWindow = await dialogWindowPromise;
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should have seen the unknown content dialogWindow."
+    );
+
+    let oldLaunchFile = DownloadIntegration.launchFile;
+    let waitForLaunchFileCalled = new Promise(resolve => {
+      DownloadIntegration.launchFile = () => {
+        ok(true, "The file should be launched with an external application");
+        resolve();
+      };
+    });
+
+    let doc = dialogWindow.document;
+    await waitForAcceptButtonToGetEnabled(doc);
+    let dialog = doc.querySelector("#unknownContentType");
+    doc.querySelector("#open").click();
+    let button = dialog.getButton("accept");
+    button.disabled = false;
+    info("Accepting the dialog");
+    dialog.acceptDialog();
+    info("Waiting until DownloadIntegration.launchFile is called");
+    await waitForLaunchFileCalled;
+    DownloadIntegration.launchFile = oldLaunchFile;
+
+    let publicDownloads = await publicList.getAll();
+    is(
+      publicDownloads.length,
+      1,
+      "download should appear in publicDownloads list"
+    );
+    let download = publicDownloads[0];
+    ok(
+      !download.launchWhenSucceeded,
+      "launchWhenSucceeded should be false after launchFile is called"
+    );
+
+    BrowserTestUtils.removeTab(loadingTab);
+    if (download?.target.exists) {
+      try {
+        info("removing " + download.target.path);
+        await IOUtils.remove(download.target.path);
+      } catch (ex) {
+        /* ignore */
+      }
+    }
+    await publicList.removeFinished();
+  }
+  for (let mimeInfo of mimeInfosToRestore) {
+    HandlerSvc.remove(mimeInfo);
+  }
+});
+
+/**
+ * Test that choosing to open a PDF with an external application works and
+ * then downloading the same file again and choosing Open with Firefox opens
+ * the download in Firefox.
+ */
+add_task(async function test_check_open_with_external_then_internal() {
+  // This test only runs on Windows because appPicker.xhtml is only used on Windows.
+  if (AppConstants.platform != "win") {
+    return;
+  }
+
+  // This test covers a bug that only occurs when the mimeInfo is set to Always Ask
+  const mimeInfo = MimeSvc.getFromTypeAndExtension("application/pdf", "pdf");
+  console.log(
+    "mimeInfo.preferredAction is currently:",
+    mimeInfo.preferredAction
+  );
+  mimeInfo.preferredAction = mimeInfo.alwaysAsk;
+  mimeInfo.alwaysAskBeforeHandling = true;
+  HandlerSvc.store(mimeInfo);
+
+  for (let [file, mimeType] of [
+    ["file_pdf_application_pdf.pdf", "application/pdf"],
+    ["file_pdf_binary_octet_stream.pdf", "binary/octet-stream"],
+    ["file_pdf_application_unknown.pdf", "application/unknown"],
+  ]) {
+    info("Testing with " + file);
+    let originalMimeInfo = MimeSvc.getFromTypeAndExtension(mimeType, "pdf");
+
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    registerCleanupFunction(async () => {
+      await publicList.removeFinished();
+    });
+    let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    // Open a new tab to the PDF file which will trigger the Unknown Content Type dialog
+    // and choose to open the PDF with an external application.
+    let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: TEST_PATH + file,
+      waitForLoad: false,
+      waitForStateStop: true,
+    });
+    let dialogWindow = await dialogWindowPromise;
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should have seen the unknown content dialogWindow."
+    );
+
+    let oldLaunchFile = DownloadIntegration.launchFile;
+    let waitForLaunchFileCalled = new Promise(resolve => {
+      DownloadIntegration.launchFile = () => {
+        ok(true, "The file should be launched with an external application");
+        resolve();
+      };
+    });
+
+    let doc = dialogWindow.document;
+    await waitForAcceptButtonToGetEnabled(doc);
+    let dialog = doc.querySelector("#unknownContentType");
+    let openHandlerMenulist = doc.querySelector("#openHandler");
+    let originalDefaultHandler = openHandlerMenulist.label;
+    doc.querySelector("#open").click();
+    doc.querySelector("#openHandlerPopup").click();
+    let oldOpenDialog = dialogWindow.openDialog;
+    dialogWindow.openDialog = (location, unused2, unused3, params) => {
+      is(location, "chrome://global/content/appPicker.xhtml", "app picker");
+      let handlerApp = params.mimeInfo.possibleLocalHandlers.queryElementAt(
+        0,
+        Ci.nsILocalHandlerApp
+      );
+      ok(handlerApp.executable, "handlerApp should be executable");
+      ok(handlerApp.executable.isFile(), "handlerApp should be a file");
+      params.handlerApp = handlerApp;
+    };
+    doc.querySelector("#choose").click();
+    dialogWindow.openDialog = oldOpenDialog;
+    await TestUtils.waitForCondition(
+      () => originalDefaultHandler != openHandlerMenulist.label,
+      "waiting for openHandler to get updated"
+    );
+    let newDefaultHandler = openHandlerMenulist.label;
+    info(`was ${originalDefaultHandler}, now ${newDefaultHandler}`);
+    let button = dialog.getButton("accept");
+    button.disabled = false;
+    info("Accepting the dialog");
+    dialog.acceptDialog();
+    info("Waiting until DownloadIntegration.launchFile is called");
+    await waitForLaunchFileCalled;
+    BrowserTestUtils.removeTab(loadingTab);
+
+    // Now, open a new tab to the PDF file which will trigger the Unknown Content Type dialog
+    // and choose to open the PDF internally. The previously used external application should be shown as
+    // the external option.
+    dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    loadingTab = await BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: TEST_PATH + file,
+      waitForLoad: false,
+      waitForStateStop: true,
+    });
+    dialogWindow = await dialogWindowPromise;
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should have seen the unknown content dialogWindow."
+    );
+
+    DownloadIntegration.launchFile = () => {
+      ok(false, "The file should not be launched with an external application");
+    };
+
+    doc = dialogWindow.document;
+    await waitForAcceptButtonToGetEnabled(doc);
+    openHandlerMenulist = doc.querySelector("#openHandler");
+    is(openHandlerMenulist.label, newDefaultHandler, "'new' handler");
+    dialog = doc.querySelector("#unknownContentType");
+    doc.querySelector("#handleInternally").click();
+    info("Accepting the dialog");
+    button = dialog.getButton("accept");
+    button.disabled = false;
+    let newTabPromise = BrowserTestUtils.waitForNewTab(gBrowser);
+    dialog.acceptDialog();
+
+    info("waiting for new tab to open");
+    let newTab = await newTabPromise;
+
+    await ContentTask.spawn(newTab.linkedBrowser, null, async () => {
+      await ContentTaskUtils.waitForCondition(
+        () => content.document.readyState == "complete"
+      );
+    });
+
+    is(
+      newTab.linkedBrowser.contentPrincipal.origin,
+      "resource://pdf.js",
+      "PDF should be opened with pdf.js"
+    );
+
+    BrowserTestUtils.removeTab(loadingTab);
+    BrowserTestUtils.removeTab(newTab);
+
+    // Now trigger the dialog again and select the system
+    // default option to reset the state for the next iteration of the test.
+    // Reset the state for the next iteration of the test.
+    HandlerSvc.store(originalMimeInfo);
+    DownloadIntegration.launchFile = oldLaunchFile;
+    let [download] = await publicList.getAll();
+    if (download?.target.exists) {
+      try {
+        info("removing " + download.target.path);
+        await IOUtils.remove(download.target.path);
+      } catch (ex) {
+        /* ignore */
+      }
+    }
+    await publicList.removeFinished();
+  }
+});
+
+/**
+ * Check that the "Open with internal handler" option is presented
+ * for other viewable internally types.
+ */
+add_task(
+  async function test_internal_handler_hidden_with_viewable_internally_type() {
+    const mimeInfosToRestore = alwaysAskForHandlingTypes({
+      "binary/octet-stream": "xml",
+      "image/webp": "webp",
+    });
+
+    for (let [file, checkDefault] of [
+      // The default for binary/octet-stream is changed by the PDF tests above,
+      // this may change given bug 1659008, so I'm just ignoring the default for now.
+      ["file_xml_attachment_binary_octet_stream.xml", false],
+      ["file_green.webp", true],
+    ]) {
+      let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+      let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+        gBrowser,
+        opening: TEST_PATH + file,
+        waitForLoad: false,
+        waitForStateStop: true,
+      });
+      let dialogWindow = await dialogWindowPromise;
+      is(
+        dialogWindow.location.href,
+        "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+        "Should have seen the unknown content dialogWindow."
+      );
+      let doc = dialogWindow.document;
+      let internalHandlerRadio = doc.querySelector("#handleInternally");
+
+      // Prevent racing with initialization of the dialog and make sure that
+      // the final state of the dialog has the correct visibility of the internal-handler option.
+      await waitForAcceptButtonToGetEnabled(doc);
+
+      let fileDesc = file.substring(file.lastIndexOf(".") + 1);
+
+      ok(
+        !internalHandlerRadio.hidden,
+        `The option should be visible for ${fileDesc}`
+      );
+      if (checkDefault) {
+        ok(
+          internalHandlerRadio.selected,
+          `The option should be selected for ${fileDesc}`
+        );
+      }
+
+      let dialog = doc.querySelector("#unknownContentType");
+      dialog.cancelDialog();
+      BrowserTestUtils.removeTab(loadingTab);
+    }
+    for (let mimeInfo of mimeInfosToRestore) {
+      HandlerSvc.remove(mimeInfo);
+    }
+  }
+);
+
+/**
+ * Check that the "Open with internal handler" option is not presented
+ * for non-PDF, non-viewable-internally types.
+ */
+add_task(async function test_internal_handler_hidden_with_other_type() {
+  const mimeInfosToRestore = alwaysAskForHandlingTypes({
+    "text/plain": "txt",
+  });
+
+  let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+  let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_txt_attachment_test.txt",
+    waitForLoad: false,
+    waitForStateStop: true,
+  });
+  let dialogWindow = await dialogWindowPromise;
+  is(
+    dialogWindow.location.href,
+    "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+    "Should have seen the unknown content dialogWindow."
+  );
+  let doc = dialogWindow.document;
+
+  // Prevent racing with initialization of the dialog and make sure that
+  // the final state of the dialog has the correct visibility of the internal-handler option.
+  await waitForAcceptButtonToGetEnabled(doc);
+
+  let internalHandlerRadio = doc.querySelector("#handleInternally");
+  ok(
+    internalHandlerRadio.hidden,
+    "The option should be hidden for unknown file type"
+  );
+
+  let dialog = doc.querySelector("#unknownContentType");
+  dialog.cancelDialog();
+  BrowserTestUtils.removeTab(loadingTab);
+  for (let mimeInfo of mimeInfosToRestore) {
+    HandlerSvc.remove(mimeInfo);
+  }
+});
+
+/**
+ * Check that the "Open with internal handler" option is not presented
+ * when the feature is disabled for PDFs.
+ */
+add_task(async function test_internal_handler_hidden_with_pdf_pref_disabled() {
+  const mimeInfosToRestore = alwaysAskForHandlingTypes({
+    "application/pdf": "pdf",
+    "binary/octet-stream": "pdf",
+  });
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.helperApps.showOpenOptionForPdfJS", false]],
+  });
+  for (let file of [
+    "file_pdf_application_pdf.pdf",
+    "file_pdf_binary_octet_stream.pdf",
+  ]) {
+    let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: TEST_PATH + file,
+      waitForLoad: false,
+      waitForStateStop: true,
+    });
+    let dialogWindow = await dialogWindowPromise;
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should have seen the unknown content dialogWindow."
+    );
+    let doc = dialogWindow.document;
+
+    await waitForAcceptButtonToGetEnabled(doc);
+
+    let internalHandlerRadio = doc.querySelector("#handleInternally");
+    ok(
+      internalHandlerRadio.hidden,
+      "The option should be hidden for PDF when the pref is false"
+    );
+
+    let dialog = doc.querySelector("#unknownContentType");
+    dialog.cancelDialog();
+    BrowserTestUtils.removeTab(loadingTab);
+  }
+  for (let mimeInfo of mimeInfosToRestore) {
+    HandlerSvc.remove(mimeInfo);
+  }
+});
+
+/**
+ * Check that the "Open with internal handler" option is not presented
+ * for other viewable internally types when disabled.
+ */
+add_task(
+  async function test_internal_handler_hidden_with_viewable_internally_pref_disabled() {
+    const mimeInfosToRestore = alwaysAskForHandlingTypes({
+      "text/xml": "xml",
+    });
+    await SpecialPowers.pushPrefEnv({
+      set: [["browser.helperApps.showOpenOptionForViewableInternally", false]],
+    });
+    let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: TEST_PATH + "file_xml_attachment_test.xml",
+      waitForLoad: false,
+      waitForStateStop: true,
+    });
+    let dialogWindow = await dialogWindowPromise;
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should have seen the unknown content dialogWindow."
+    );
+    let doc = dialogWindow.document;
+
+    await waitForAcceptButtonToGetEnabled(doc);
+
+    let internalHandlerRadio = doc.querySelector("#handleInternally");
+    ok(
+      internalHandlerRadio.hidden,
+      "The option should be hidden for XML when the pref is false"
+    );
+
+    let dialog = doc.querySelector("#unknownContentType");
+    dialog.cancelDialog();
+    BrowserTestUtils.removeTab(loadingTab);
+    for (let mimeInfo of mimeInfosToRestore) {
+      HandlerSvc.remove(mimeInfo);
+    }
+  }
+);
+
+/*
+ * This test sets the action to internal. The files should open directly without asking.
+ */
+add_task(async function test_check_open_with_internal_handler_noask() {
+  const mimeInfosToRestore = alwaysAskForHandlingTypes(
+    {
+      "application/pdf": "pdf",
+      "binary/octet-stream": "pdf",
+      "application/octet-stream": "pdf",
+    },
+    false
+  );
+
+  // Build the matrix of tests to perform.
+  let matrix = {
+    alwaysOpenPDFInline: [false, true],
+    file: [
+      "file_pdf_application_pdf.pdf",
+      "file_pdf_binary_octet_stream.pdf",
+      "file_pdf_application_octet_stream.pdf",
+    ],
+    where: ["top", "popup", "frame"],
+  };
+  let tests = [{}];
+  for (let [key, values] of Object.entries(matrix)) {
+    tests = tests.flatMap(test =>
+      values.map(value => ({ [key]: value, ...test }))
+    );
+  }
+
+  for (let test of tests) {
+    info(`test case: ${JSON.stringify(test)}`);
+    let { alwaysOpenPDFInline, file, where } = test;
+
+    // These are the cases that can be opened inline. binary/octet-stream
+    // isn't handled by pdfjs.
+    let canHandleInline =
+      file == "file_pdf_application_pdf.pdf" ||
+      (file == "file_pdf_application_octet_stream.pdf" && where != "frame");
+
+    await SpecialPowers.pushPrefEnv({
+      set: [
+        ["browser.helperApps.showOpenOptionForPdfJS", true],
+        ["browser.helperApps.showOpenOptionForViewableInternally", true],
+        ["browser.download.open_pdf_attachments_inline", alwaysOpenPDFInline],
+      ],
+    });
+
+    async function doNavigate(browser) {
+      await SpecialPowers.spawn(
+        browser,
+        [TEST_PATH + file, where],
+        async (contentUrl, where_) => {
+          switch (where_) {
+            case "top":
+              content.location = contentUrl;
+              break;
+            case "popup":
+              content.open(contentUrl);
+              break;
+            case "frame":
+              let frame = content.document.createElement("iframe");
+              frame.setAttribute("src", contentUrl);
+              content.document.body.appendChild(frame);
+              break;
+            default:
+              ok(false, "Unknown where value");
+              break;
+          }
+        }
+      );
+    }
+
+    // If this is true, the pdf is opened directly without downloading it.
+    // Otherwise, it must first be downloaded and optionally displayed in
+    // a tab with a file url.
+    let openPDFDirectly = alwaysOpenPDFInline && canHandleInline;
+
+    await BrowserTestUtils.withNewTab(
+      { gBrowser, url: TEST_PATH + "blank.html" },
+      async browser => {
+        let readyPromise = BrowserTestUtils.waitForNewTab(
+          gBrowser,
+          null,
+          false,
+          !openPDFDirectly
+        );
+
+        await doNavigate(browser);
+
+        await readyPromise;
+
+        is(
+          gBrowser.selectedBrowser.currentURI.scheme,
+          openPDFDirectly ? "https" : "file",
+          "Loaded PDF uri has the correct scheme"
+        );
+
+        // intentionally don't bother checking session history without ship to
+        // keep complexity down.
+        if (Services.appinfo.sessionHistoryInParent) {
+          let shistory = browser.browsingContext.sessionHistory;
+          is(shistory.count, 1, "should a single shentry");
+          is(shistory.index, 0, "should be on the first entry");
+          let shentry = shistory.getEntryAtIndex(shistory.index);
+          is(shentry.URI.spec, TEST_PATH + "blank.html");
+        }
+
+        await SpecialPowers.spawn(
+          browser,
+          [TEST_PATH + "blank.html"],
+          async blankUrl => {
+            ok(
+              !docShell.isAttemptingToNavigate,
+              "should not still be attempting to navigate"
+            );
+            is(
+              content.location.href,
+              blankUrl,
+              "original browser hasn't navigated"
+            );
+          }
+        );
+
+        await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+      }
+    );
+  }
+
+  for (let mimeInfo of mimeInfosToRestore) {
+    HandlerSvc.remove(mimeInfo);
+  }
+});
+
+add_task(async () => {
+  MockFilePicker.cleanup();
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_preferred_action.js b/uriloader/exthandler/tests/mochitest/browser_download_preferred_action.js
new file mode 100644
index 0000000000..b66266ec9e
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_preferred_action.js
@@ -0,0 +1,299 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { DownloadIntegration } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+const { FileTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/FileTestUtils.sys.mjs"
+);
+const gHandlerService = Cc[
+  "@mozilla.org/uriloader/handler-service;1"
+].getService(Ci.nsIHandlerService);
+const gMIMEService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+const localHandlerAppFactory = Cc["@mozilla.org/uriloader/local-handler-app;1"];
+
+const ROOT_URL = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+const FILE_TYPES_MIME_SETTINGS = {};
+
+// File types to test
+const FILE_TYPES_TO_TEST = [
+  // application/ms-word files cannot render in the browser so
+  // handleInternally does not work for it
+  {
+    extension: "doc",
+    mimeType: "application/ms-word",
+    blockHandleInternally: true,
+  },
+  {
+    extension: "pdf",
+    mimeType: "application/pdf",
+  },
+  {
+    extension: "pdf",
+    mimeType: "application/unknown",
+  },
+  {
+    extension: "pdf",
+    mimeType: "binary/octet-stream",
+  },
+  // text/plain files automatically render in the browser unless
+  // the CD header explicitly tells the browser to download it
+  {
+    extension: "txt",
+    mimeType: "text/plain",
+    requireContentDispositionHeader: true,
+  },
+  {
+    extension: "xml",
+    mimeType: "binary/octet-stream",
+  },
+].map(file => {
+  return {
+    ...file,
+    url: `${ROOT_URL}mime_type_download.sjs?contentType=${file.mimeType}&extension=${file.extension}`,
+  };
+});
+
+// Preferred action types to apply to each downloaded file
+const PREFERRED_ACTIONS = [
+  "saveToDisk",
+  "alwaysAsk",
+  "useHelperApp",
+  "handleInternally",
+  "useSystemDefault",
+].map(property => {
+  let label = property.replace(/([A-Z])/g, " $1");
+  label = label.charAt(0).toUpperCase() + label.slice(1);
+  return {
+    id: Ci.nsIHandlerInfo[property],
+    label,
+  };
+});
+
+async function createDownloadTest(
+  downloadList,
+  localHandlerApp,
+  file,
+  action,
+  useContentDispositionHeader
+) {
+  // Skip handleInternally case for files that cannot be handled internally
+  if (
+    action.id === Ci.nsIHandlerInfo.handleInternally &&
+    file.blockHandleInternally
+  ) {
+    return;
+  }
+  let skipDownload =
+    action.id === Ci.nsIHandlerInfo.handleInternally &&
+    file.mimeType === "application/pdf";
+  // Types that require the CD header only display as handleInternally
+  // when the CD header is missing
+  if (file.requireContentDispositionHeader && !useContentDispositionHeader) {
+    if (action.id === Ci.nsIHandlerInfo.handleInternally) {
+      skipDownload = true;
+    } else {
+      return;
+    }
+  }
+  info(
+    `Testing download with mime-type ${file.mimeType} and extension ${
+      file.extension
+    }, preferred action "${action.label}," and ${
+      useContentDispositionHeader
+        ? "Content-Disposition: attachment"
+        : "no Content-Disposition"
+    } header.`
+  );
+  info("Preparing for download...");
+  // apply preferredAction settings
+  let mimeSettings = gMIMEService.getFromTypeAndExtension(
+    file.mimeType,
+    file.extension
+  );
+  mimeSettings.preferredAction = action.id;
+  mimeSettings.alwaysAskBeforeHandling =
+    action.id === Ci.nsIHandlerInfo.alwaysAsk;
+  if (action.id === Ci.nsIHandlerInfo.useHelperApp) {
+    mimeSettings.preferredApplicationHandler = localHandlerApp;
+  }
+  gHandlerService.store(mimeSettings);
+  // delayed check for files opened in a new tab, except for skipped downloads
+  let expectViewInBrowserTab =
+    action.id === Ci.nsIHandlerInfo.handleInternally && !skipDownload;
+  let viewInBrowserTabOpened = null;
+  if (expectViewInBrowserTab) {
+    viewInBrowserTabOpened = BrowserTestUtils.waitForNewTab(
+      gBrowser,
+      uri => uri.includes("file://"),
+      true
+    );
+  }
+  // delayed check for launched files
+  let expectLaunch =
+    action.id === Ci.nsIHandlerInfo.useSystemDefault ||
+    action.id === Ci.nsIHandlerInfo.useHelperApp;
+  let oldLaunchFile = DownloadIntegration.launchFile;
+  let fileLaunched = null;
+  if (expectLaunch) {
+    fileLaunched = Promise.withResolvers();
+    DownloadIntegration.launchFile = () => {
+      ok(
+        expectLaunch,
+        `The file ${file.mimeType} should be launched with an external application.`
+      );
+      fileLaunched.resolve();
+    };
+  }
+  info(`Start download of ${file.url}`);
+  let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+  let downloadFinishedPromise = skipDownload
+    ? null
+    : promiseDownloadFinished(downloadList);
+  BrowserTestUtils.startLoadingURIString(gBrowser.selectedBrowser, file.url);
+  if (action.id === Ci.nsIHandlerInfo.alwaysAsk) {
+    info("Check Always Ask dialog.");
+    let dialogWindow = await dialogWindowPromise;
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should show unknownContentType dialog for Always Ask preferred actions."
+    );
+    let doc = dialogWindow.document;
+    let dialog = doc.querySelector("#unknownContentType");
+    let acceptButton = dialog.getButton("accept");
+    acceptButton.disabled = false;
+    let saveItem = doc.querySelector("#save");
+    saveItem.disabled = false;
+    saveItem.click();
+    dialog.acceptDialog();
+  }
+  let download = null;
+  let downloadPath = null;
+  if (!skipDownload) {
+    info("Wait for download to finish...");
+    download = await downloadFinishedPromise;
+    downloadPath = download.target.path;
+  }
+  // check delayed assertions
+  if (expectLaunch) {
+    info("Wait for file to be launched in external application...");
+    await fileLaunched.promise;
+  }
+  if (expectViewInBrowserTab) {
+    info("Wait for file to be opened in new tab...");
+    let viewInBrowserTab = await viewInBrowserTabOpened;
+    ok(
+      viewInBrowserTab,
+      `The file ${file.mimeType} should be opened in a new tab.`
+    );
+    BrowserTestUtils.removeTab(viewInBrowserTab);
+  }
+  info("Checking for saved file...");
+  let saveFound = downloadPath && (await IOUtils.exists(downloadPath));
+  info("Cleaning up...");
+  if (saveFound) {
+    try {
+      info(`Deleting file ${downloadPath}...`);
+      await IOUtils.remove(downloadPath);
+    } catch (ex) {
+      info(`Error: ${ex}`);
+    }
+  }
+  info("Removing download from list...");
+  await downloadList.removeFinished();
+  info("Clearing settings...");
+  DownloadIntegration.launchFile = oldLaunchFile;
+  info("Asserting results...");
+  if (download) {
+    ok(download.succeeded, "Download should complete successfully");
+    ok(
+      !download._launchedFromPanel,
+      "Download should never be launched from panel"
+    );
+  }
+  if (skipDownload) {
+    ok(!saveFound, "Download should not be saved to disk");
+  } else {
+    ok(saveFound, "Download should be saved to disk");
+  }
+}
+
+add_task(async function test_download_preferred_action() {
+  // Prepare tests
+  for (const index in FILE_TYPES_TO_TEST) {
+    let file = FILE_TYPES_TO_TEST[index];
+    let originalMimeSettings = gMIMEService.getFromTypeAndExtension(
+      file.mimeType,
+      file.extension
+    );
+    if (gHandlerService.exists(originalMimeSettings)) {
+      FILE_TYPES_MIME_SETTINGS[index] = originalMimeSettings;
+    }
+  }
+  let downloadList = await Downloads.getList(Downloads.PUBLIC);
+  let oldLaunchFile = DownloadIntegration.launchFile;
+  registerCleanupFunction(async function () {
+    await removeAllDownloads();
+    DownloadIntegration.launchFile = oldLaunchFile;
+    Services.prefs.clearUserPref(
+      "browser.download.always_ask_before_handling_new_types"
+    );
+    BrowserTestUtils.startLoadingURIString(
+      gBrowser.selectedBrowser,
+      "about:home"
+    );
+    for (const index in FILE_TYPES_TO_TEST) {
+      let file = FILE_TYPES_TO_TEST[index];
+      let mimeSettings = gMIMEService.getFromTypeAndExtension(
+        file.mimeType,
+        file.extension
+      );
+      if (FILE_TYPES_MIME_SETTINGS[index]) {
+        gHandlerService.store(FILE_TYPES_MIME_SETTINGS[index]);
+      } else {
+        gHandlerService.remove(mimeSettings);
+      }
+    }
+  });
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.download.always_ask_before_handling_new_types", false]],
+  });
+  let launcherPath = FileTestUtils.getTempFile("app-launcher").path;
+  let localHandlerApp = localHandlerAppFactory.createInstance(
+    Ci.nsILocalHandlerApp
+  );
+  localHandlerApp.executable = new FileUtils.File(launcherPath);
+  localHandlerApp.executable.create(Ci.nsIFile.NORMAL_FILE_TYPE, 0o755);
+  // run tests
+  for (const file of FILE_TYPES_TO_TEST) {
+    // The CD header specifies the download file extension on download
+    let fileNoHeader = file;
+    let fileWithHeader = structuredClone(file);
+    fileWithHeader.url += "&withHeader";
+    for (const action of PREFERRED_ACTIONS) {
+      // Clone file objects to prevent side-effects between iterations
+      await createDownloadTest(
+        downloadList,
+        localHandlerApp,
+        structuredClone(fileWithHeader),
+        action,
+        true
+      );
+      await createDownloadTest(
+        downloadList,
+        localHandlerApp,
+        structuredClone(fileNoHeader),
+        action,
+        false
+      );
+    }
+  }
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_privatebrowsing.js b/uriloader/exthandler/tests/mochitest/browser_download_privatebrowsing.js
new file mode 100644
index 0000000000..8ded5e6401
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_privatebrowsing.js
@@ -0,0 +1,78 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests that downloads started from a private window by clicking on a link end
+ * up in the global list of private downloads (see bug 1367581).
+ */
+
+"use strict";
+
+const { Downloads } = ChromeUtils.importESModule(
+  "resource://gre/modules/Downloads.sys.mjs"
+);
+const { DownloadPaths } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadPaths.sys.mjs"
+);
+const { FileTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/FileTestUtils.sys.mjs"
+);
+const { MockRegistrar } = ChromeUtils.importESModule(
+  "resource://testing-common/MockRegistrar.sys.mjs"
+);
+
+add_task(async function test_setup() {
+  // Save downloads to disk without showing the dialog.
+  let cid = MockRegistrar.register("@mozilla.org/helperapplauncherdialog;1", {
+    QueryInterface: ChromeUtils.generateQI(["nsIHelperAppLauncherDialog"]),
+    show(launcher) {
+      launcher.promptForSaveDestination();
+    },
+    promptForSaveToFileAsync(launcher) {
+      // The dialog should create the empty placeholder file.
+      let file = FileTestUtils.getTempFile();
+      file.create(Ci.nsIFile.NORMAL_FILE_TYPE, FileUtils.PERMS_FILE);
+      launcher.saveDestinationAvailable(file);
+    },
+  });
+  registerCleanupFunction(() => {
+    MockRegistrar.unregister(cid);
+  });
+});
+
+add_task(async function test_download_privatebrowsing() {
+  let privateList = await Downloads.getList(Downloads.PRIVATE);
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+
+  let win = await BrowserTestUtils.openNewBrowserWindow({ private: true });
+  try {
+    let tab = await BrowserTestUtils.openNewForegroundTab(
+      win.gBrowser,
+      `data:text/html,<a download href="data:text/plain,">download</a>`
+    );
+
+    let promiseNextPrivateDownload = new Promise(resolve => {
+      privateList.addView({
+        onDownloadAdded(download) {
+          privateList.removeView(this);
+          resolve(download);
+        },
+      });
+    });
+
+    await SpecialPowers.spawn(tab.linkedBrowser, [], async function () {
+      content.document.querySelector("a").click();
+    });
+
+    // Wait for the download to finish so the file can be safely deleted later.
+    let download = await promiseNextPrivateDownload;
+    await download.whenSucceeded();
+
+    // Clean up after checking that there are no new public downloads either.
+    let publicDownloads = await publicList.getAll();
+    Assert.equal(publicDownloads.length, 0);
+    await privateList.removeFinished();
+  } finally {
+    await BrowserTestUtils.closeWindow(win);
+  }
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_skips_dialog.js b/uriloader/exthandler/tests/mochitest/browser_download_skips_dialog.js
new file mode 100644
index 0000000000..0b8c816d2e
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_skips_dialog.js
@@ -0,0 +1,59 @@
+const { DownloadIntegration } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+// New file is being downloaded and no dialogs are shown in the way.
+add_task(async function skipDialogAndDownloadFile() {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["browser.download.always_ask_before_handling_new_types", false],
+      ["browser.download.useDownloadDir", true],
+    ],
+  });
+
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  registerCleanupFunction(async () => {
+    await publicList.removeFinished();
+  });
+  let downloadFinishedPromise = promiseDownloadFinished(publicList);
+
+  let initialTabsCount = gBrowser.tabs.length;
+
+  let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_green.webp",
+    waitForLoad: false,
+    waitForStateStop: true,
+  });
+
+  // We just open the file to be downloaded... and wait for it to be downloaded!
+  // We see no dialogs to be accepted in the process.
+  let download = await downloadFinishedPromise;
+  await BrowserTestUtils.waitForCondition(
+    () => gBrowser.tabs.length == initialTabsCount + 2
+  );
+
+  gBrowser.removeCurrentTab();
+  BrowserTestUtils.removeTab(loadingTab);
+
+  Assert.ok(
+    await IOUtils.exists(download.target.path),
+    "The file should have been downloaded."
+  );
+
+  try {
+    info("removing " + download.target.path);
+    if (Services.appinfo.OS === "WINNT") {
+      // We need to make the file writable to delete it on Windows.
+      await IOUtils.setPermissions(download.target.path, 0o600);
+    }
+    await IOUtils.remove(download.target.path);
+  } catch (ex) {
+    info("The file " + download.target.path + " is not removed, " + ex);
+  }
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_spam_permissions.js b/uriloader/exthandler/tests/mochitest/browser_download_spam_permissions.js
new file mode 100644
index 0000000000..0f80af5ac5
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_spam_permissions.js
@@ -0,0 +1,158 @@
+/* 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/. */
+
+"use strict";
+
+const { PermissionTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/PermissionTestUtils.sys.mjs"
+);
+
+const TEST_URI = "https://example.com";
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  TEST_URI
+);
+
+const AUTOMATIC_DOWNLOAD_TOPIC = "blocked-automatic-download";
+
+var MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+registerCleanupFunction(() => MockFilePicker.cleanup());
+
+let gTempDownloadDir;
+
+add_setup(async function () {
+  // Create temp directory
+  let time = new Date().getTime();
+  let tempDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  tempDir.append(time);
+  gTempDownloadDir = tempDir;
+  Services.prefs.setIntPref("browser.download.folderList", 2);
+  Services.prefs.setComplexValue("browser.download.dir", Ci.nsIFile, tempDir);
+
+  PermissionTestUtils.add(
+    TEST_URI,
+    "automatic-download",
+    Services.perms.UNKNOWN_ACTION
+  );
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      // We disable browser.download.always_ask_before_handling_new_types here
+      // since the test expects the download to be saved directly to disk and
+      // not prompted by the UnknownContentType window.
+      ["browser.download.always_ask_before_handling_new_types", false],
+      ["browser.download.enable_spam_prevention", true],
+    ],
+  });
+
+  registerCleanupFunction(async () => {
+    Services.prefs.clearUserPref("browser.download.folderList");
+    Services.prefs.clearUserPref("browser.download.dir");
+    await IOUtils.remove(tempDir.path, { recursive: true });
+  });
+});
+
+add_task(async function check_download_spam_permissions() {
+  const INITIAL_TABS_COUNT = gBrowser.tabs.length;
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  let downloadFinishedPromise = promiseDownloadFinished(
+    publicList,
+    true /* stop the download from openning */
+  );
+  let blockedDownloadsCount = 0;
+  let blockedDownloadsURI = "";
+  let automaticDownloadObserver = {
+    observe: function automatic_download_observe(aSubject, aTopic, aData) {
+      if (aTopic === AUTOMATIC_DOWNLOAD_TOPIC) {
+        blockedDownloadsCount++;
+        blockedDownloadsURI = aData;
+      }
+    },
+  };
+  Services.obs.addObserver(automaticDownloadObserver, AUTOMATIC_DOWNLOAD_TOPIC);
+
+  let newTab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_PATH + "test_spammy_page.html"
+  );
+  registerCleanupFunction(async () => {
+    DownloadIntegration.downloadSpamProtection.removeDownloadSpamForWindow(
+      TEST_URI,
+      window
+    );
+    DownloadsPanel.hidePanel();
+    await publicList.removeFinished();
+    BrowserTestUtils.removeTab(newTab);
+    Services.obs.removeObserver(
+      automaticDownloadObserver,
+      AUTOMATIC_DOWNLOAD_TOPIC
+    );
+  });
+
+  let download = await downloadFinishedPromise;
+  await TestUtils.waitForCondition(
+    () => gBrowser.tabs.length == INITIAL_TABS_COUNT + 1
+  );
+  is(
+    PermissionTestUtils.testPermission(TEST_URI, "automatic-download"),
+    Services.perms.PROMPT_ACTION,
+    "The permission to prompt the user should be stored."
+  );
+
+  ok(
+    await IOUtils.exists(download.target.path),
+    "One file should be downloaded"
+  );
+
+  let aCopyFilePath = download.target.path.replace(".pdf", "(1).pdf");
+  is(
+    await IOUtils.exists(aCopyFilePath),
+    false,
+    "An other file should be blocked"
+  );
+
+  info("Will wait for blockedDownloadsCount to be >= 99");
+  await TestUtils.waitForCondition(() => blockedDownloadsCount >= 99);
+  is(blockedDownloadsCount, 99, "Browser should block 99 downloads");
+  is(
+    blockedDownloadsURI,
+    TEST_URI,
+    "The test URI should have blocked automatic downloads"
+  );
+
+  await savelink();
+});
+
+// Check to ensure that a link saved manually is not blocked.
+async function savelink() {
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  let menu = document.getElementById("contentAreaContextMenu");
+  let popupShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
+  BrowserTestUtils.synthesizeMouse(
+    "#image",
+    5,
+    5,
+    { type: "contextmenu", button: 2 },
+    gBrowser.selectedBrowser
+  );
+  await popupShown;
+
+  await new Promise(resolve => {
+    MockFilePicker.showCallback = function (fp) {
+      resolve();
+      let file = gTempDownloadDir.clone();
+      file.append("file_with__funny_name.png");
+      MockFilePicker.setFiles([file]);
+      return Ci.nsIFilePicker.returnOK;
+    };
+    let menuitem = menu.querySelector("#context-savelink");
+    menu.activateItem(menuitem);
+  });
+
+  await promiseDownloadFinished(
+    publicList,
+    true // stop the download from openning
+  );
+}
diff --git a/uriloader/exthandler/tests/mochitest/browser_download_urlescape.js b/uriloader/exthandler/tests/mochitest/browser_download_urlescape.js
new file mode 100644
index 0000000000..ababcc095c
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_download_urlescape.js
@@ -0,0 +1,75 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+var MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+registerCleanupFunction(() => MockFilePicker.cleanup());
+
+/**
+ * Check downloading files URL-escapes content-disposition
+ * information when necessary.
+ */
+add_task(async function test_check_filename_urlescape() {
+  let pendingPromise;
+  let pendingTest = "";
+  let expectedFileName = "";
+  MockFilePicker.showCallback = function (fp) {
+    info(`${pendingTest} - Filepicker shown, checking filename`);
+    is(
+      fp.defaultString,
+      expectedFileName,
+      `${pendingTest} - Should have escaped filename`
+    );
+    ok(
+      pendingPromise,
+      `${pendingTest} - Should have expected this picker open.`
+    );
+    if (pendingPromise) {
+      pendingPromise.resolve();
+    }
+    return Ci.nsIFilePicker.returnCancel;
+  };
+  function runTestFor(fileName, selector) {
+    return BrowserTestUtils.withNewTab(TEST_PATH + fileName, async browser => {
+      expectedFileName = fileName;
+      let tabLabel = gBrowser.getTabForBrowser(browser).getAttribute("label");
+      ok(
+        tabLabel.startsWith(fileName),
+        `"${tabLabel}" should have been escaped.`
+      );
+
+      pendingTest = "save browser";
+      pendingPromise = Promise.withResolvers();
+      // First try to save the browser
+      saveBrowser(browser);
+      await pendingPromise.promise;
+
+      // Next, try the context menu:
+      pendingTest = "save from context menu";
+      pendingPromise = Promise.withResolvers();
+      let menu = document.getElementById("contentAreaContextMenu");
+      let menuShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
+      BrowserTestUtils.synthesizeMouse(
+        selector,
+        5,
+        5,
+        { type: "contextmenu", button: 2 },
+        browser
+      );
+      await menuShown;
+      gContextMenu.saveMedia();
+      menu.hidePopup();
+      await pendingPromise.promise;
+      pendingPromise = null;
+    });
+  }
+  await runTestFor("file_with@@funny_name.png", "img");
+  await runTestFor("file_with[funny_name.webm", "video");
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_extension_correction.js b/uriloader/exthandler/tests/mochitest/browser_extension_correction.js
new file mode 100644
index 0000000000..9d67bf7213
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_extension_correction.js
@@ -0,0 +1,222 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+let gPathsToRemove = [];
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.download.useDownloadDir", true]],
+  });
+  registerCleanupFunction(async () => {
+    for (let path of gPathsToRemove) {
+      // IOUtils.remove ignores non-existing files out of the box.
+      await IOUtils.remove(path);
+    }
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    await publicList.removeFinished();
+  });
+});
+
+async function testLinkWithoutExtension(type, shouldHaveExtension) {
+  info("Checking " + type);
+
+  let task = function () {
+    return SpecialPowers.spawn(gBrowser.selectedBrowser, [type], mimetype => {
+      let link = content.document.createElement("a");
+      link.textContent = "Click me";
+      link.href = "data:" + mimetype + ",hello";
+      link.download = "somefile";
+      content.document.body.appendChild(link);
+      link.click();
+    });
+  };
+
+  await checkDownloadWithExtensionState(task, {
+    type,
+    shouldHaveExtension,
+    alwaysViewPDFInline: false,
+  });
+
+  if (type == "application/pdf") {
+    // For PDF, try again with the always open inline preference set
+    await SpecialPowers.pushPrefEnv({
+      set: [["browser.download.open_pdf_attachments_inline", true]],
+    });
+
+    await checkDownloadWithExtensionState(task, {
+      type,
+      shouldHaveExtension,
+      alwaysViewPDFInline: true,
+    });
+
+    await SpecialPowers.popPrefEnv();
+  }
+}
+
+async function checkDownloadWithExtensionState(
+  task,
+  { type, shouldHaveExtension, expectedName = null, alwaysViewPDFInline }
+) {
+  const shouldExpectDialog = Services.prefs.getBoolPref(
+    "browser.download.always_ask_before_handling_new_types",
+    false
+  );
+
+  let winPromise;
+  if (shouldExpectDialog) {
+    winPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+  }
+
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  let shouldCheckFilename = shouldHaveExtension || !shouldExpectDialog;
+
+  let downloadFinishedPromise = shouldCheckFilename
+    ? promiseDownloadFinished(publicList)
+    : null;
+
+  // PDF should load using the internal viewer without downloading it.
+  let waitForLoad;
+  if (
+    (!shouldExpectDialog || alwaysViewPDFInline) &&
+    type == "application/pdf"
+  ) {
+    waitForLoad = BrowserTestUtils.waitForNewTab(gBrowser);
+  }
+
+  await task();
+  await waitForLoad;
+
+  let win;
+  if (shouldExpectDialog) {
+    info("Waiting for dialog.");
+    win = await winPromise;
+  }
+
+  expectedName ??= shouldHaveExtension
+    ? "somefile." + getMIMEInfoForType(type).primaryExtension
+    : "somefile";
+
+  let closedPromise = true;
+  if (shouldExpectDialog) {
+    let actualName = win.document.getElementById("location").value;
+    closedPromise = BrowserTestUtils.windowClosed(win);
+
+    if (shouldHaveExtension) {
+      is(actualName, expectedName, `${type} should get an extension`);
+    } else {
+      is(actualName, expectedName, `${type} should not get an extension`);
+    }
+  }
+
+  if (shouldExpectDialog && shouldHaveExtension) {
+    // Then pick "save" in the dialog, if we have a dialog.
+    let dialog = win.document.getElementById("unknownContentType");
+    win.document.getElementById("save").click();
+    let button = dialog.getButton("accept");
+    button.disabled = false;
+    dialog.acceptDialog();
+  }
+
+  if (!shouldExpectDialog && type == "application/pdf") {
+    if (alwaysViewPDFInline) {
+      is(
+        gURLBar.inputField.value,
+        "data:application/pdf,hello",
+        "url is correct for " + type
+      );
+    } else {
+      ok(
+        gURLBar.inputField.value.startsWith("file://") &&
+          gURLBar.inputField.value.endsWith("somefile.pdf"),
+        "url is correct for " + type
+      );
+    }
+
+    BrowserTestUtils.removeTab(gBrowser.selectedTab);
+  }
+
+  if (shouldExpectDialog || !alwaysViewPDFInline || type != "application/pdf") {
+    // Wait for the download if it exists (may produce null).
+    let download = await downloadFinishedPromise;
+    if (download) {
+      // Check the download's extension is correct.
+      is(
+        PathUtils.filename(download.target.path),
+        expectedName,
+        `Downloaded file should match ${expectedName}`
+      );
+      gPathsToRemove.push(download.target.path);
+      let pathToRemove = download.target.path;
+      // Avoid one file interfering with subsequent files.
+      await publicList.removeFinished();
+      await IOUtils.remove(pathToRemove);
+    } else if (win) {
+      // We just cancel out for files that would end up without a path, as we'd
+      // prompt for a filename.
+      win.close();
+    }
+  }
+
+  return closedPromise;
+}
+
+/**
+ * Check that for document types, images, videos and audio files,
+ * we enforce a useful extension.
+ */
+add_task(async function test_enforce_useful_extension() {
+  await BrowserTestUtils.withNewTab("data:text/html,", async browser => {
+    await testLinkWithoutExtension("image/png", true);
+    await testLinkWithoutExtension("audio/ogg", true);
+    await testLinkWithoutExtension("video/webm", true);
+    await testLinkWithoutExtension("application/pdf", true);
+
+    await testLinkWithoutExtension("application/x-nonsense", false);
+    await testLinkWithoutExtension("application/octet-stream", false);
+    await testLinkWithoutExtension("binary/octet-stream", false);
+    await testLinkWithoutExtension("application/x-msdownload", false);
+  });
+});
+
+/**
+ * Check that we still use URL extension info when we don't have anything else,
+ * despite bogus local info.
+ */
+add_task(async function test_broken_saved_handlerinfo_and_useless_mimetypes() {
+  let bogusType = getMIMEInfoForType("binary/octet-stream");
+  registerCleanupFunction(() => {
+    handlerSvc.remove(bogusType);
+  });
+  bogusType.setFileExtensions(["jpg"]);
+  let handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+  handlerSvc.store(bogusType);
+  let tabToClean = null;
+  let task = function () {
+    return BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: TEST_PATH + "file_as.exe?foo=bar",
+      waitForLoad: false,
+      waitForStateStop: true,
+    }).then(tab => {
+      return (tabToClean = tab);
+    });
+  };
+  await checkDownloadWithExtensionState(task, {
+    type: "binary/octet-stream",
+    shouldHaveExtension: true,
+    expectedName: "file_as.exe",
+  });
+  // Downloads should really close their tabs...
+  if (tabToClean?.isConnected) {
+    BrowserTestUtils.removeTab(tabToClean);
+  }
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_filehandling_loop.js b/uriloader/exthandler/tests/mochitest/browser_filehandling_loop.js
new file mode 100644
index 0000000000..0aabb222d9
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_filehandling_loop.js
@@ -0,0 +1,93 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * If the user has set Firefox itself as a helper app,
+ * we should force prompting what to do, rather than ending up
+ * in an infinite loop.
+ * In an ideal world, we'd also test the case where we are the OS
+ * default handler app, but that would require test infrastructure
+ * to make ourselves the OS default (or at least fool ourselves into
+ * believing we are) which we don't have...
+ */
+add_task(async function test_helperapp() {
+  // Set up the test infrastructure:
+  const mimeSvc = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+  const handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+  let handlerInfo = mimeSvc.getFromTypeAndExtension("application/x-foo", "foo");
+  registerCleanupFunction(() => {
+    handlerSvc.remove(handlerInfo);
+  });
+  // Say we want to use a specific app:
+  handlerInfo.preferredAction = Ci.nsIHandlerInfo.useHelperApp;
+  handlerInfo.alwaysAskBeforeHandling = false;
+
+  // Say it's us:
+  let selfFile = Services.dirsvc.get("XREExeF", Ci.nsIFile);
+  // Make sure it's the .app
+  if (AppConstants.platform == "macosx") {
+    while (
+      !selfFile.leafName.endsWith(".app") &&
+      !selfFile.leafName.endsWith(".app/")
+    ) {
+      selfFile = selfFile.parent;
+    }
+  }
+  let selfHandlerApp = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  selfHandlerApp.executable = selfFile;
+  handlerInfo.possibleApplicationHandlers.appendElement(selfHandlerApp);
+  handlerInfo.preferredApplicationHandler = selfHandlerApp;
+  handlerSvc.store(handlerInfo);
+
+  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+    // Now, do some safety stubbing. If we do end up recursing we spawn
+    // infinite tabs. We definitely don't want that. Avoid it by stubbing
+    // our external URL handling bits:
+    let oldAddTab = gBrowser.addTab;
+    registerCleanupFunction(() => (gBrowser.addTab = oldAddTab));
+    let wrongThingHappenedPromise = new Promise(resolve => {
+      gBrowser.addTab = function (aURI) {
+        ok(false, "Tried to open unexpected URL in a tab: " + aURI);
+        resolve(null);
+        // Pass a dummy object to avoid upsetting BrowserContentHandler -
+        // if it thinks opening the tab failed, it tries to open a window instead,
+        // which we can't prevent as easily, and at which point we still end up
+        // with runaway tabs.
+        return {};
+      };
+    });
+
+    let askedUserPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+
+    info("Clicking a link that should open the unknown content type dialog");
+    await SpecialPowers.spawn(browser, [], () => {
+      let link = content.document.createElement("a");
+      link.download = "foo.foo";
+      link.textContent = "Foo file";
+      link.href = "data:application/x-foo,hello";
+      content.document.body.append(link);
+      link.click();
+    });
+    let dialog = await Promise.race([
+      wrongThingHappenedPromise,
+      askedUserPromise,
+    ]);
+    ok(dialog, "Should have gotten a dialog");
+    Assert.stringContains(
+      dialog.document.location.href,
+      "unknownContentType",
+      "Should have opened correct dialog."
+    );
+
+    let closePromise = BrowserTestUtils.windowClosed(dialog);
+    dialog.close();
+    await closePromise;
+    askedUserPromise = null;
+  });
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_first_prompt_not_blocked_without_user_interaction.js b/uriloader/exthandler/tests/mochitest/browser_first_prompt_not_blocked_without_user_interaction.js
new file mode 100644
index 0000000000..df446f4c52
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_first_prompt_not_blocked_without_user_interaction.js
@@ -0,0 +1,70 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+add_task(setupMailHandler);
+
+add_task(async function test_open_without_user_interaction() {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["dom.disable_open_during_load", true],
+      ["dom.block_external_protocol_in_iframes", true],
+      ["dom.delay.block_external_protocol_in_iframes.enabled", false],
+    ],
+  });
+
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
+
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+
+  BrowserTestUtils.startLoadingURIString(
+    tab.linkedBrowser,
+    TEST_PATH + "file_external_protocol_iframe.html"
+  );
+
+  let dialog = await dialogWindowPromise;
+  ok(dialog, "Should show the dialog even without user interaction");
+
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+
+  // Adding another iframe without user interaction should be blocked.
+  let blockedWarning = new Promise(resolve => {
+    Services.console.registerListener(function onMessage(msg) {
+      let { message, logLevel } = msg;
+      if (logLevel != Ci.nsIConsoleMessage.warn) {
+        return;
+      }
+      if (!message.includes("Iframe with external protocol was blocked")) {
+        return;
+      }
+      Services.console.unregisterListener(onMessage);
+      resolve();
+    });
+  });
+
+  info("Adding another frame without user interaction");
+
+  await SpecialPowers.spawn(tab.linkedBrowser, [], async function () {
+    let frame = content.document.createElement("iframe");
+    frame.src = "mailto:foo@baz.com";
+    content.document.body.appendChild(frame);
+  });
+
+  await blockedWarning;
+
+  info("Removing tab to close the dialog.");
+  gBrowser.removeTab(tab);
+  await dialogClosedPromise;
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_ftp_protocol_handlers.js b/uriloader/exthandler/tests/mochitest/browser_ftp_protocol_handlers.js
new file mode 100644
index 0000000000..6b7c70cc01
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_ftp_protocol_handlers.js
@@ -0,0 +1,111 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+ChromeUtils.defineESModuleGetters(this, {
+  UrlbarTestUtils: "resource://testing-common/UrlbarTestUtils.sys.mjs",
+});
+
+let testURL =
+  "https://example.com/browser/" +
+  "uriloader/exthandler/tests/mochitest/FTPprotocolHandler.html";
+
+add_task(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["security.external_protocol_requires_permission", false]],
+  });
+
+  // Load a page registering a protocol handler.
+  let browser = gBrowser.selectedBrowser;
+  BrowserTestUtils.startLoadingURIString(browser, testURL);
+  await BrowserTestUtils.browserLoaded(browser, false, testURL);
+
+  // Register the protocol handler by clicking the notificationbar button.
+  let notificationValue = "Protocol Registration: ftp";
+  let getNotification = () =>
+    gBrowser.getNotificationBox().getNotificationWithValue(notificationValue);
+  await BrowserTestUtils.waitForCondition(getNotification);
+  let notification = getNotification();
+  let button = notification.buttonContainer.querySelector("button");
+  ok(button, "got registration button");
+  button.click();
+
+  // Set the new handler as default.
+  const protoSvc = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  let protoInfo = protoSvc.getProtocolHandlerInfo("ftp");
+  ok(!protoInfo.preferredApplicationHandler, "no preferred handler is set");
+  let handlers = protoInfo.possibleApplicationHandlers;
+  is(1, handlers.length, "only one handler registered for ftp");
+  let handler = handlers.queryElementAt(0, Ci.nsIHandlerApp);
+  ok(handler instanceof Ci.nsIWebHandlerApp, "the handler is a web handler");
+  is(
+    handler.uriTemplate,
+    "https://example.com/browser/uriloader/exthandler/tests/mochitest/blank.html?uri=%s",
+    "correct url template"
+  );
+  protoInfo.preferredAction = protoInfo.useHelperApp;
+  protoInfo.preferredApplicationHandler = handler;
+  protoInfo.alwaysAskBeforeHandling = false;
+  const handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+  handlerSvc.store(protoInfo);
+
+  const expectedURL =
+    "https://example.com/browser/uriloader/exthandler/tests/mochitest/blank.html?uri=ftp%3A%2F%2Fdomain.com%2Fpath";
+
+  // Middle-click a testprotocol link and check the new tab is correct
+  let link = "#link";
+
+  let promiseTabOpened = BrowserTestUtils.waitForNewTab(gBrowser, expectedURL);
+  await BrowserTestUtils.synthesizeMouseAtCenter(link, { button: 1 }, browser);
+  let tab = await promiseTabOpened;
+  gBrowser.selectedTab = tab;
+  is(
+    gURLBar.value,
+    UrlbarTestUtils.trimURL(expectedURL),
+    "the expected URL is displayed in the location bar"
+  );
+  BrowserTestUtils.removeTab(tab);
+
+  // Shift-click the testprotocol link and check the new window.
+  let newWindowPromise = BrowserTestUtils.waitForNewWindow({
+    url: expectedURL,
+  });
+  await BrowserTestUtils.synthesizeMouseAtCenter(
+    link,
+    { shiftKey: true },
+    browser
+  );
+  let win = await newWindowPromise;
+  await BrowserTestUtils.waitForCondition(
+    () => win.gBrowser.currentURI.spec == expectedURL
+  );
+  is(
+    win.gURLBar.value,
+    UrlbarTestUtils.trimURL(expectedURL),
+    "the expected URL is displayed in the location bar"
+  );
+  await BrowserTestUtils.closeWindow(win);
+
+  // Click the testprotocol link and check the url in the current tab.
+  let loadPromise = BrowserTestUtils.browserLoaded(browser);
+  await BrowserTestUtils.synthesizeMouseAtCenter(link, {}, browser);
+  await loadPromise;
+  await BrowserTestUtils.waitForCondition(
+    () => gURLBar.value != UrlbarTestUtils.trimURL(testURL)
+  );
+  is(
+    gURLBar.value,
+    UrlbarTestUtils.trimURL(expectedURL),
+    "the expected URL is displayed in the location bar"
+  );
+
+  // Cleanup.
+  protoInfo.preferredApplicationHandler = null;
+  handlers.removeElementAt(0);
+  handlerSvc.store(protoInfo);
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_launched_app_save_directory.js b/uriloader/exthandler/tests/mochitest/browser_launched_app_save_directory.js
new file mode 100644
index 0000000000..d978410263
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_launched_app_save_directory.js
@@ -0,0 +1,111 @@
+const { DownloadIntegration } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+const { TestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/TestUtils.sys.mjs"
+);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.download.always_ask_before_handling_new_types", false]],
+  });
+  const allowDirectoriesVal = DownloadIntegration.allowDirectories;
+  DownloadIntegration.allowDirectories = true;
+  registerCleanupFunction(() => {
+    DownloadIntegration.allowDirectories = allowDirectoriesVal;
+    Services.prefs.clearUserPref("browser.download.dir");
+    Services.prefs.clearUserPref("browser.download.folderList");
+  });
+});
+
+async function aDownloadLaunchedWithAppIsSavedInFolder(downloadDir) {
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  registerCleanupFunction(async () => {
+    await publicList.removeFinished();
+  });
+
+  let downloadFinishedPromise = promiseDownloadFinished(publicList);
+  let initialTabsCount = gBrowser.tabs.length;
+
+  let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_green.webp",
+    waitForLoad: false,
+    waitForStateStop: true,
+  });
+
+  let download = await downloadFinishedPromise;
+  await BrowserTestUtils.waitForCondition(
+    () => gBrowser.tabs.length == initialTabsCount + 2
+  );
+
+  gBrowser.removeCurrentTab();
+  BrowserTestUtils.removeTab(loadingTab);
+
+  ok(
+    download.target.path.startsWith(downloadDir),
+    "Download should be placed in default download directory: " +
+      downloadDir +
+      ", and it's located in " +
+      download.target.path
+  );
+
+  Assert.ok(
+    await IOUtils.exists(download.target.path),
+    "The file should not have been deleted."
+  );
+
+  try {
+    info("removing " + download.target.path);
+    if (Services.appinfo.OS === "WINNT") {
+      // We need to make the file writable to delete it on Windows.
+      await IOUtils.setPermissions(download.target.path, 0o600);
+    }
+    await IOUtils.remove(download.target.path);
+  } catch (ex) {
+    info("The file " + download.target.path + " is not removed, " + ex);
+  }
+}
+
+add_task(async function aDownloadLaunchedWithAppIsSavedInCustomDir() {
+  //Test the temp dir.
+  let time = new Date().getTime();
+  let tempDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  tempDir.append(time);
+  Services.prefs.setComplexValue("browser.download.dir", Ci.nsIFile, tempDir);
+  let downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, tempDir.path);
+  Assert.ok(await IOUtils.exists(downloadDir));
+  registerCleanupFunction(async () => {
+    await IOUtils.remove(tempDir.path, { recursive: true });
+  });
+  await aDownloadLaunchedWithAppIsSavedInFolder(downloadDir);
+});
+
+add_task(async function aDownloadLaunchedWithAppIsSavedInDownloadsDir() {
+  // Test the system downloads directory.
+  Services.prefs.setIntPref("browser.download.folderList", 1);
+  let systemDir = await DownloadIntegration.getSystemDownloadsDirectory();
+  let downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, systemDir);
+
+  await aDownloadLaunchedWithAppIsSavedInFolder(downloadDir);
+});
+
+add_task(async function aDownloadLaunchedWithAppIsSavedInDesktopDir() {
+  // Test the desktop directory.
+  Services.prefs.setIntPref("browser.download.folderList", 0);
+  let downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, Services.dirsvc.get("Desk", Ci.nsIFile).path);
+
+  await aDownloadLaunchedWithAppIsSavedInFolder(downloadDir);
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_local_files_no_save_without_asking.js b/uriloader/exthandler/tests/mochitest/browser_local_files_no_save_without_asking.js
new file mode 100644
index 0000000000..cfa4788a43
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_local_files_no_save_without_asking.js
@@ -0,0 +1,61 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check that loading a local PDF file
+ * prompts the user when pdfjs.disabled is set to true,
+ * and alwaysAsk is false;
+ */
+add_task(
+  async function test_check_browser_local_files_no_save_without_asking() {
+    // Get a ref to the pdf we want to open.
+    let file = getChromeDir(getResolvedURI(gTestPath));
+    file.append("file_pdf_binary_octet_stream.pdf");
+
+    await SpecialPowers.pushPrefEnv({ set: [["pdfjs.disabled", true]] });
+
+    const mimeSvc = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+    const handlerSvc = Cc[
+      "@mozilla.org/uriloader/handler-service;1"
+    ].getService(Ci.nsIHandlerService);
+    const mimeInfo = mimeSvc.getFromTypeAndExtension("application/pdf", "pdf");
+    // This test covers a bug that only occurs when the mimeInfo is set to Always Ask = false
+    // Here we check if we ask the user what to do for local files, if the file is set to save to disk automatically;
+    // that is, we check that we prompt the user despite the user's preference.
+    mimeInfo.preferredAction = mimeInfo.saveToDisk;
+    mimeInfo.alwaysAskBeforeHandling = false;
+    handlerSvc.store(mimeInfo);
+
+    info("Testing with " + file.path);
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    registerCleanupFunction(async () => {
+      await publicList.removeFinished();
+    });
+
+    let publicDownloads = await publicList.getAll();
+    is(
+      publicDownloads.length,
+      0,
+      "download should not appear in publicDownloads list"
+    );
+
+    let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+    var loadingTab = BrowserTestUtils.addTab(gBrowser, file.path);
+
+    let dialogWindow = await dialogWindowPromise;
+
+    is(
+      dialogWindow.location.href,
+      "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+      "Should have seen the unknown content dialogWindow."
+    );
+
+    let doc = dialogWindow.document;
+
+    let dialog = doc.querySelector("#unknownContentType");
+    dialog.cancelDialog();
+    BrowserTestUtils.removeTab(loadingTab);
+  }
+);
diff --git a/uriloader/exthandler/tests/mochitest/browser_local_files_open_doesnt_duplicate.js b/uriloader/exthandler/tests/mochitest/browser_local_files_open_doesnt_duplicate.js
new file mode 100644
index 0000000000..d8e7c87c10
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_local_files_open_doesnt_duplicate.js
@@ -0,0 +1,121 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const mimeSvc = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+const handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+  Ci.nsIHandlerService
+);
+let mimeInfo = mimeSvc.getFromTypeAndExtension("application/pdf", "pdf");
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({ set: [["pdfjs.disabled", true]] });
+
+  let oldAsk = mimeInfo.alwaysAskBeforeHandling;
+  let oldPreferredAction = mimeInfo.preferredAction;
+  let oldPreferredApp = mimeInfo.preferredApplicationHandler;
+  registerCleanupFunction(() => {
+    mimeInfo.preferredApplicationHandler = oldPreferredApp;
+    mimeInfo.preferredAction = oldPreferredAction;
+    mimeInfo.alwaysAskBeforeHandling = oldAsk;
+    handlerSvc.store(mimeInfo);
+  });
+
+  if (!mimeInfo.preferredApplicationHandler) {
+    let handlerApp = Cc[
+      "@mozilla.org/uriloader/local-handler-app;1"
+    ].createInstance(Ci.nsILocalHandlerApp);
+    handlerApp.executable = Services.dirsvc.get("TmpD", Ci.nsIFile);
+    handlerApp.executable.append("foopydoo.exe");
+    mimeInfo.possibleApplicationHandlers.appendElement(handlerApp);
+    mimeInfo.preferredApplicationHandler = handlerApp;
+  }
+});
+
+add_task(async function open_from_dialog() {
+  // Force PDFs to prompt:
+  mimeInfo.preferredAction = mimeInfo.useHelperApp;
+  mimeInfo.alwaysAskBeforeHandling = true;
+  handlerSvc.store(mimeInfo);
+
+  let openingPromise = TestUtils.topicObserved(
+    "test-only-opening-downloaded-file",
+    (subject, data) => {
+      subject.QueryInterface(Ci.nsISupportsPRBool);
+      // Block opening the file:
+      subject.data = false;
+      return true;
+    }
+  );
+
+  let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+  let openedFile = getChromeDir(getResolvedURI(gTestPath));
+  openedFile.append("file_pdf_binary_octet_stream.pdf");
+  let expectedPath = openedFile.isSymlink()
+    ? openedFile.target
+    : openedFile.path;
+  let loadingTab = BrowserTestUtils.addTab(gBrowser, expectedPath);
+
+  let dialogWindow = await dialogWindowPromise;
+  is(
+    dialogWindow.location.href,
+    "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+    "Should have seen the unknown content dialogWindow."
+  );
+
+  let doc = dialogWindow.document;
+
+  // Select the 'open' entry.
+  doc.querySelector("#open").click();
+  let dialog = doc.querySelector("#unknownContentType");
+  dialog.getButton("accept").removeAttribute("disabled");
+  dialog.acceptDialog();
+  let [, openedPath] = await openingPromise;
+  is(
+    openedPath,
+    expectedPath,
+    "Should have opened file directly (not created a copy)."
+  );
+  if (openedPath != expectedPath) {
+    await IOUtils.setPermissions(openedPath, 0o666);
+    await IOUtils.remove(openedPath);
+  }
+  BrowserTestUtils.removeTab(loadingTab);
+});
+
+add_task(async function open_directly() {
+  // Force PDFs to open immediately:
+  mimeInfo.preferredAction = mimeInfo.useHelperApp;
+  mimeInfo.alwaysAskBeforeHandling = false;
+  handlerSvc.store(mimeInfo);
+
+  let openingPromise = TestUtils.topicObserved(
+    "test-only-opening-downloaded-file",
+    (subject, data) => {
+      subject.QueryInterface(Ci.nsISupportsPRBool);
+      // Block opening the file:
+      subject.data = false;
+      return true;
+    }
+  );
+
+  let openedFile = getChromeDir(getResolvedURI(gTestPath));
+  openedFile.append("file_pdf_binary_octet_stream.pdf");
+  let expectedPath = openedFile.isSymlink()
+    ? openedFile.target
+    : openedFile.path;
+  let loadingTab = BrowserTestUtils.addTab(gBrowser, expectedPath);
+
+  let [, openedPath] = await openingPromise;
+  is(
+    openedPath,
+    expectedPath,
+    "Should have opened file directly (not created a copy)."
+  );
+  if (openedPath != expectedPath) {
+    await IOUtils.setPermissions(openedPath, 0o666);
+    await IOUtils.remove(openedPath);
+  }
+  BrowserTestUtils.removeTab(loadingTab);
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_open_internal_choice_persistence.js b/uriloader/exthandler/tests/mochitest/browser_open_internal_choice_persistence.js
new file mode 100644
index 0000000000..424fd424f9
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_open_internal_choice_persistence.js
@@ -0,0 +1,310 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { Downloads } = ChromeUtils.importESModule(
+  "resource://gre/modules/Downloads.sys.mjs"
+);
+const { DownloadIntegration } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+const {
+  handleInternally,
+  saveToDisk,
+  useSystemDefault,
+  alwaysAsk,
+  useHelperApp,
+} = Ci.nsIHandlerInfo;
+
+function waitForAcceptButtonToGetEnabled(doc) {
+  let dialog = doc.querySelector("#unknownContentType");
+  let button = dialog.getButton("accept");
+  return TestUtils.waitForCondition(
+    () => !button.disabled,
+    "Wait for Accept button to get enabled"
+  );
+}
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      // Remove the security delay for the dialog during the test.
+      ["security.dialog_enable_delay", 0],
+      ["browser.helperApps.showOpenOptionForViewableInternally", true],
+      // Make sure we don't open a file picker dialog somehow.
+      ["browser.download.useDownloadDir", true],
+    ],
+  });
+
+  // Restore handlers after the whole test has run
+  const registerRestoreHandler = function (type, ext) {
+    const mimeInfo = gMimeSvc.getFromTypeAndExtension(type, ext);
+    const existed = gHandlerSvc.exists(mimeInfo);
+    registerCleanupFunction(() => {
+      if (existed) {
+        gHandlerSvc.store(mimeInfo);
+      } else {
+        gHandlerSvc.remove(mimeInfo);
+      }
+    });
+  };
+  registerRestoreHandler("image/svg+xml", "svg");
+});
+
+function ensureMIMEState({ preferredAction, alwaysAskBeforeHandling }) {
+  const mimeInfo = gMimeSvc.getFromTypeAndExtension("image/svg+xml", "svg");
+  mimeInfo.preferredAction = preferredAction;
+  mimeInfo.alwaysAskBeforeHandling = alwaysAskBeforeHandling;
+  gHandlerSvc.store(mimeInfo);
+}
+
+function waitDelay(delay) {
+  return new Promise((resolve, reject) => {
+    /* eslint-disable mozilla/no-arbitrary-setTimeout */
+    window.setTimeout(resolve, delay);
+  });
+}
+
+function promisePanelOpened() {
+  if (DownloadsPanel.panel && DownloadsPanel.panel.state == "open") {
+    return Promise.resolve();
+  }
+  return BrowserTestUtils.waitForEvent(DownloadsPanel.panel, "popupshown");
+}
+
+const kTestCasesPrefEnabled = [
+  {
+    description:
+      "Pref enabled - internal handling as default should not change prefs",
+    preDialogState: {
+      preferredAction: handleInternally,
+      alwaysAskBeforeHandling: false,
+    },
+    expectTab: true,
+    expectLaunch: false,
+    expectedPreferredAction: handleInternally,
+    expectedAlwaysAskBeforeHandling: false,
+    expectUCT: false,
+  },
+  {
+    description:
+      "Pref enabled - external handling as default should not change prefs",
+    preDialogState: {
+      preferredAction: useSystemDefault,
+      alwaysAskBeforeHandling: false,
+    },
+    expectTab: false,
+    expectLaunch: true,
+    expectedPreferredAction: useSystemDefault,
+    expectedAlwaysAskBeforeHandling: false,
+    expectUCT: false,
+  },
+  {
+    description: "Pref enabled - saveToDisk as default should not change prefs",
+    preDialogState: {
+      preferredAction: saveToDisk,
+      alwaysAskBeforeHandling: false,
+    },
+    expectTab: false,
+    expectLaunch: false,
+    expectedPreferredAction: saveToDisk,
+    expectedAlwaysAskBeforeHandling: false,
+    expectUCT: false,
+  },
+  {
+    description:
+      "Pref enabled - choose internal + alwaysAsk default + checkbox should update persisted default",
+    preDialogState: {
+      preferredAction: alwaysAsk,
+      alwaysAskBeforeHandling: false,
+    },
+    dialogActions(doc) {
+      let handleItem = doc.querySelector("#handleInternally");
+      handleItem.click();
+      ok(handleItem.selected, "The 'open' option should now be selected");
+      let checkbox = doc.querySelector("#rememberChoice");
+      checkbox.checked = true;
+      checkbox.doCommand();
+    },
+    // new tab will not launch in test environment when alwaysAsk is preferredAction
+    expectTab: false,
+    expectLaunch: false,
+    expectedPreferredAction: handleInternally,
+    expectedAlwaysAskBeforeHandling: false,
+    expectUCT: true,
+  },
+  {
+    description:
+      "Pref enabled - saveToDisk with alwaysAsk default should update persisted default",
+    preDialogState: {
+      preferredAction: alwaysAsk,
+      alwaysAskBeforeHandling: false,
+    },
+    dialogActions(doc) {
+      let saveItem = doc.querySelector("#save");
+      saveItem.click();
+      ok(saveItem.selected, "The 'save' option should now be selected");
+    },
+    expectTab: false,
+    expectLaunch: false,
+    expectedPreferredAction: saveToDisk,
+    expectedAlwaysAskBeforeHandling: false,
+    expectUCT: true,
+  },
+];
+
+add_task(
+  async function test_check_saving_handler_choices_with_always_ask_before_handling_new_types_pref_enabled() {
+    SpecialPowers.pushPrefEnv({
+      set: [["browser.download.always_ask_before_handling_new_types", false]],
+    });
+
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    registerCleanupFunction(async () => {
+      await publicList.removeFinished();
+    });
+    let file = "file_image_svgxml.svg";
+
+    for (let testCase of kTestCasesPrefEnabled) {
+      info("Testing with " + file + "; " + testCase.description);
+      ensureMIMEState(testCase.preDialogState);
+      const { expectTab, expectLaunch, description, expectUCT } = testCase;
+
+      let oldLaunchFile = DownloadIntegration.launchFile;
+      let fileLaunched = Promise.withResolvers();
+      DownloadIntegration.launchFile = () => {
+        ok(
+          expectLaunch,
+          `The file should ${
+            expectLaunch ? "" : "not "
+          }be launched with an external application - ${description}`
+        );
+        fileLaunched.resolve();
+      };
+
+      info("Load window and tabs");
+      let dialogWindowPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+      let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+        gBrowser,
+        opening: TEST_PATH + file,
+        waitForLoad: false,
+        waitForStateStop: true,
+      });
+
+      // See if UCT window appears in loaded tab.
+      let dialogWindow = await Promise.race([
+        waitDelay(1000),
+        dialogWindowPromise,
+      ]);
+
+      is(
+        !!dialogWindow,
+        expectUCT,
+        `UCT window should${expectUCT ? "" : " not"} have appeared`
+      );
+
+      let download;
+
+      if (dialogWindow) {
+        is(
+          dialogWindow.location.href,
+          "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+          "Unknown content dialogWindow should be loaded correctly."
+        );
+        let doc = dialogWindow.document;
+        let internalHandlerRadio = doc.querySelector("#handleInternally");
+
+        info("Waiting for accept button to get enabled");
+        await waitForAcceptButtonToGetEnabled(doc);
+
+        ok(
+          !internalHandlerRadio.hidden,
+          "The option should be visible for SVG"
+        );
+
+        info("Running UCT dialog options before downloading file");
+        await testCase.dialogActions(doc);
+
+        let dialog = doc.querySelector("#unknownContentType");
+        dialog.acceptDialog();
+
+        info("Waiting for downloads to finish");
+        let downloadFinishedPromise = promiseDownloadFinished(publicList);
+        download = await downloadFinishedPromise;
+      } else {
+        let downloadPanelPromise = promisePanelOpened();
+        await downloadPanelPromise;
+        is(
+          DownloadsPanel.isPanelShowing,
+          true,
+          "DownloadsPanel should be open"
+        );
+
+        info("Skipping UCT dialog options");
+        info("Waiting for downloads to finish");
+        // Unlike when the UCT window opens, the download immediately starts.
+        let downloadList = await publicList;
+        [download] = downloadList._downloads;
+      }
+
+      if (expectLaunch) {
+        info("Waiting for launch to finish");
+        await fileLaunched.promise;
+      }
+      DownloadIntegration.launchFile = oldLaunchFile;
+
+      is(
+        download.contentType,
+        "image/svg+xml",
+        "File contentType should be correct"
+      );
+      is(
+        download.source.url,
+        `${TEST_PATH + file}`,
+        "File name should be correct."
+      );
+      is(
+        (await publicList.getAll()).length,
+        1,
+        "download should appear in public list"
+      );
+
+      // Check mime info:
+      const mimeInfo = gMimeSvc.getFromTypeAndExtension("image/svg+xml", "svg");
+      gHandlerSvc.fillHandlerInfo(mimeInfo, "");
+      is(
+        mimeInfo.preferredAction,
+        testCase.expectedPreferredAction,
+        "preferredAction - " + description
+      );
+      is(
+        mimeInfo.alwaysAskBeforeHandling,
+        testCase.expectedAlwaysAskBeforeHandling,
+        "alwaysAskBeforeHandling - " + description
+      );
+
+      info("Cleaning up");
+      BrowserTestUtils.removeTab(loadingTab);
+      // By default, if internal is default with pref enabled, we view the svg file in
+      // in a new tab. Close this tab in order for the test case to pass.
+      if (expectTab && testCase.preferredAction !== alwaysAsk) {
+        BrowserTestUtils.removeTab(gBrowser.selectedTab);
+      }
+      await publicList.removeFinished();
+      if (download?.target.exists) {
+        try {
+          await IOUtils.remove(download.target.path);
+        } catch (ex) {
+          /* ignore */
+        }
+      }
+    }
+  }
+);
diff --git a/uriloader/exthandler/tests/mochitest/browser_pdf_save_as.js b/uriloader/exthandler/tests/mochitest/browser_pdf_save_as.js
new file mode 100644
index 0000000000..a08fe342cc
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_pdf_save_as.js
@@ -0,0 +1,121 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const HandlerService = Cc[
+  "@mozilla.org/uriloader/handler-service;1"
+].getService(Ci.nsIHandlerService);
+const MIMEService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+const { saveToDisk, alwaysAsk, handleInternally, useSystemDefault } =
+  Ci.nsIHandlerInfo;
+const MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+
+async function testPdfFilePicker(mimeInfo) {
+  await BrowserTestUtils.withNewTab(
+    `data:text/html,<a id="test-link" href="${TEST_PATH}/file_pdf_application_pdf.pdf">Test PDF Link</a>`,
+    async browser => {
+      let menu = document.getElementById("contentAreaContextMenu");
+      ok(menu, "Context menu exists on the page");
+
+      let popupShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
+      BrowserTestUtils.synthesizeMouseAtCenter(
+        "a#test-link",
+        { type: "contextmenu", button: 2 },
+        browser
+      );
+      await popupShown;
+      info("Context menu popup was successfully displayed");
+
+      let filePickerPromise = new Promise(resolve => {
+        MockFilePicker.showCallback = fp => {
+          ok(true, "filepicker should be visible");
+          ok(
+            fp.defaultExtension === "pdf",
+            "Default extension in filepicker should be pdf"
+          );
+          ok(
+            fp.defaultString === "file_pdf_application_pdf.pdf",
+            "Default string name in filepicker should have the correct pdf file name"
+          );
+          setTimeout(resolve, 0);
+          return Ci.nsIFilePicker.returnCancel;
+        };
+      });
+
+      let menuitem = menu.querySelector("#context-savelink");
+      menu.activateItem(menuitem);
+      await filePickerPromise;
+    }
+  );
+}
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["browser.download.always_ask_before_handling_new_types", false],
+      ["browser.download.useDownloadDir", false],
+    ],
+  });
+
+  registerCleanupFunction(async () => {
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    await publicList.removeFinished();
+
+    if (DownloadsPanel.isVisible) {
+      info("Closing downloads panel");
+      let hiddenPromise = BrowserTestUtils.waitForEvent(
+        DownloadsPanel.panel,
+        "popuphidden"
+      );
+      DownloadsPanel.hidePanel();
+      await hiddenPromise;
+    }
+
+    let mimeInfo = MIMEService.getFromTypeAndExtension(
+      "application/pdf",
+      "pdf"
+    );
+    let existed = HandlerService.exists(mimeInfo);
+    if (existed) {
+      HandlerService.store(mimeInfo);
+    } else {
+      HandlerService.remove(mimeInfo);
+    }
+
+    // We only want to run MockFilerPicker.cleanup after the entire test is run.
+    // Otherwise, we cannot use MockFilePicker for each preferredAction.
+    MockFilePicker.cleanup();
+  });
+});
+
+/**
+ * Tests that selecting the context menu item `Save Link As…` on a PDF link
+ * opens the file picker when always_ask_before_handling_new_types is disabled,
+ * regardless of preferredAction.
+ */
+add_task(async function test_pdf_save_as_link() {
+  let mimeInfo;
+
+  for (let preferredAction of [
+    saveToDisk,
+    alwaysAsk,
+    handleInternally,
+    useSystemDefault,
+  ]) {
+    mimeInfo = MIMEService.getFromTypeAndExtension("application/pdf", "pdf");
+    mimeInfo.alwaysAskBeforeHandling = preferredAction === alwaysAsk;
+    mimeInfo.preferredAction = preferredAction;
+    HandlerService.store(mimeInfo);
+
+    info(`Testing filepicker for preferredAction ${preferredAction}`);
+    await testPdfFilePicker(mimeInfo);
+  }
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog.js b/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog.js
new file mode 100644
index 0000000000..7244aec503
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog.js
@@ -0,0 +1,467 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+requestLongerTimeout(2);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+const CONTENT_HANDLING_URL =
+  "chrome://mozapps/content/handling/appChooser.xhtml";
+
+add_task(setupMailHandler);
+
+/**
+ * Check that if we open the protocol handler dialog from a subframe, we close
+ * it when closing the tab.
+ */
+add_task(async function test_closed_by_tab_closure() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_PATH + "file_nested_protocol_request.html"
+  );
+
+  // Wait for the window and then click the link.
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+
+  BrowserTestUtils.synthesizeMouseAtCenter(
+    "a:link",
+    {},
+    tab.linkedBrowser.browsingContext.children[0]
+  );
+
+  let dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Dialog URL is as expected"
+  );
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+
+  info("Removing tab to close the dialog.");
+  gBrowser.removeTab(tab);
+  await dialogClosedPromise;
+  ok(!dialog._frame.contentWindow, "The dialog should have been closed.");
+});
+
+/**
+ * Check that if we open the protocol handler dialog from a subframe, we close
+ * it when navigating the tab to a non-same-origin URL.
+ */
+add_task(async function test_closed_by_tab_navigation() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_PATH + "file_nested_protocol_request.html"
+  );
+
+  // Wait for the window and then click the link.
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+
+  BrowserTestUtils.synthesizeMouseAtCenter(
+    "a:link",
+    {},
+    tab.linkedBrowser.browsingContext.children[0]
+  );
+  let dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Dialog URL is as expected"
+  );
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+  info(
+    "Set up unload handler to ensure we don't break when the window global gets cleared"
+  );
+  await SpecialPowers.spawn(tab.linkedBrowser, [], async function () {
+    content.addEventListener("unload", function () {});
+  });
+
+  info("Navigating tab to a different but same origin page.");
+  BrowserTestUtils.startLoadingURIString(tab.linkedBrowser, TEST_PATH);
+  await BrowserTestUtils.browserLoaded(tab.linkedBrowser, false, TEST_PATH);
+  ok(dialog._frame.contentWindow, "Dialog should stay open.");
+
+  // The use of weak references in various parts of the code means that we're
+  // susceptible to dropping crucial bits of our implementation on the floor,
+  // if they get GC'd, and then the test hangs.
+  // Do a bunch of GC/CC runs so that if we ever break, it's deterministic.
+  let numCycles = 3;
+  for (let i = 0; i < numCycles; i++) {
+    Cu.forceGC();
+    Cu.forceCC();
+  }
+
+  info("Now navigate to a cross-origin page.");
+  const CROSS_ORIGIN_TEST_PATH = TEST_PATH.replace(".com", ".org");
+  BrowserTestUtils.startLoadingURIString(
+    tab.linkedBrowser,
+    CROSS_ORIGIN_TEST_PATH
+  );
+  let loadPromise = BrowserTestUtils.browserLoaded(
+    tab.linkedBrowser,
+    false,
+    CROSS_ORIGIN_TEST_PATH
+  );
+  await dialogClosedPromise;
+  ok(!dialog._frame.contentWindow, "The dialog should have been closed.");
+
+  // Avoid errors from aborted loads by waiting for it to finish.
+  await loadPromise;
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Check that we cannot open more than one of these dialogs.
+ */
+add_task(async function test_multiple_dialogs() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_PATH + "file_nested_protocol_request.html"
+  );
+
+  // Wait for the window and then click the link.
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+  BrowserTestUtils.synthesizeMouseAtCenter(
+    "a:link",
+    {},
+    tab.linkedBrowser.browsingContext.children[0]
+  );
+  let dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Dialog URL is as expected"
+  );
+
+  // Navigate the parent frame:
+  await ContentTask.spawn(tab.linkedBrowser, [], () =>
+    content.eval("location.href = 'mailto:help@example.com'")
+  );
+
+  // Wait for a few ticks:
+  // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
+  await new Promise(r => setTimeout(r, 100));
+  // Check we only have one dialog
+
+  let tabDialogBox = gBrowser.getTabDialogBox(tab.linkedBrowser);
+  let dialogs = tabDialogBox
+    .getTabDialogManager()
+    ._dialogs.filter(d => d._openedURL == CONTENT_HANDLING_URL);
+
+  is(dialogs.length, 1, "Should only have 1 dialog open");
+
+  // Close the dialog:
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+  dialog.close();
+  dialog = await dialogClosedPromise;
+
+  ok(!dialog._openedURL, "The dialog should have been closed.");
+
+  // Then reopen the dialog again, to make sure we don't keep blocking:
+  dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+  BrowserTestUtils.synthesizeMouseAtCenter(
+    "a:link",
+    {},
+    tab.linkedBrowser.browsingContext.children[0]
+  );
+  dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Second dialog URL is as expected"
+  );
+
+  dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+  info("Removing tab to close the dialog.");
+  gBrowser.removeTab(tab);
+  await dialogClosedPromise;
+  ok(!dialog._frame.contentWindow, "The dialog should have been closed again.");
+});
+
+/**
+ * Check that navigating invisible frames to external-proto URLs
+ * is handled correctly.
+ */
+add_task(async function invisible_iframes() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "https://example.com/"
+  );
+
+  // Ensure we notice the dialog opening:
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+  await SpecialPowers.spawn(tab.linkedBrowser, [], function () {
+    let frame = content.document.createElement("iframe");
+    frame.style.display = "none";
+    frame.src = "mailto:help@example.com";
+    content.document.body.append(frame);
+  });
+  let dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Dialog opens as expected for invisible iframe"
+  );
+  // Close the dialog:
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+  dialog.close();
+  await dialogClosedPromise;
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Check that nested iframes are handled correctly.
+ */
+add_task(async function nested_iframes() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "https://example.com/"
+  );
+
+  // Ensure we notice the dialog opening:
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+  let innerLoaded = BrowserTestUtils.browserLoaded(
+    tab.linkedBrowser,
+    true,
+    "https://example.org/"
+  );
+  info("Constructing top frame");
+  await SpecialPowers.spawn(tab.linkedBrowser, [], function () {
+    let frame = content.document.createElement("iframe");
+    frame.src = "https://example.org/"; // cross-origin frame.
+    content.document.body.prepend(frame);
+
+    content.eval(
+      `window.addEventListener("message", e => e.source.location = "mailto:help@example.com");`
+    );
+  });
+
+  await innerLoaded;
+  let parentBC = tab.linkedBrowser.browsingContext;
+
+  info("Creating innermost frame");
+  await SpecialPowers.spawn(parentBC.children[0], [], async function () {
+    let innerFrame = content.document.createElement("iframe");
+    let frameLoaded = ContentTaskUtils.waitForEvent(innerFrame, "load", true);
+    content.document.body.prepend(innerFrame);
+    await frameLoaded;
+  });
+
+  info("Posting event from innermost frame");
+  await SpecialPowers.spawn(
+    parentBC.children[0].children[0],
+    [],
+    async function () {
+      // Top browsing context needs reference to the innermost, which is cross origin.
+      content.eval("top.postMessage('hello', '*')");
+    }
+  );
+
+  let dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Dialog opens as expected for deeply nested cross-origin iframe"
+  );
+  // Close the dialog:
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+  dialog.close();
+  await dialogClosedPromise;
+  gBrowser.removeTab(tab);
+});
+
+add_task(async function test_oop_iframe() {
+  const URI = `data:text/html,<div id="root"><iframe src="http://example.com/document-builder.sjs?html=<a href='mailto:help@example.com'>Mail it</a>"></iframe></div>`;
+
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, URI);
+
+  // Wait for the window and then click the link.
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+
+  BrowserTestUtils.synthesizeMouseAtCenter(
+    "a:link",
+    {},
+    tab.linkedBrowser.browsingContext.children[0]
+  );
+
+  let dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Dialog URL is as expected"
+  );
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+
+  info("Removing tab to close the dialog.");
+  gBrowser.removeTab(tab);
+  await dialogClosedPromise;
+  ok(!dialog._frame.contentWindow, "The dialog should have been closed.");
+});
+
+/**
+ * Check that a cross-origin iframe can navigate the top frame
+ * to an external protocol.
+ */
+add_task(async function xorigin_iframe_can_navigate_top() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "https://example.com/"
+  );
+
+  // Ensure we notice the dialog opening:
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    true
+  );
+  let innerLoaded = BrowserTestUtils.browserLoaded(
+    tab.linkedBrowser,
+    true,
+    "https://example.org/"
+  );
+  info("Constructing frame");
+  await SpecialPowers.spawn(tab.linkedBrowser, [], function () {
+    let frame = content.document.createElement("iframe");
+    frame.src = "https://example.org/"; // cross-origin frame.
+    content.document.body.prepend(frame);
+  });
+  await innerLoaded;
+
+  info("Navigating top bc from frame");
+  let parentBC = tab.linkedBrowser.browsingContext;
+  await SpecialPowers.spawn(parentBC.children[0], [], async function () {
+    content.eval("window.top.location.href = 'mailto:example@example.com';");
+  });
+
+  let dialog = await dialogWindowPromise;
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Dialog opens as expected for navigating the top frame from an x-origin frame."
+  );
+  // Close the dialog:
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    tab.linkedBrowser,
+    false
+  );
+  dialog.close();
+  await dialogClosedPromise;
+  gBrowser.removeTab(tab);
+});
+
+/**
+ * Check that when navigating to an external protocol from an iframe in a
+ * background tab, we show the dialog in the correct tab.
+ */
+add_task(async function iframe_background_tab() {
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "https://example.com/"
+  );
+
+  let innerLoaded = BrowserTestUtils.browserLoaded(
+    tab.linkedBrowser,
+    true,
+    "https://example.org/"
+  );
+  info("Constructing frame");
+  await SpecialPowers.spawn(tab.linkedBrowser, [], function () {
+    let frame = content.document.createElement("iframe");
+    frame.src = "https://example.org/";
+    content.document.body.prepend(frame);
+  });
+  await innerLoaded;
+
+  info("Switching to new tab");
+  let newTab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "https://example.net/"
+  );
+
+  // Wait for the chooser dialog to open in the background tab. It should not
+  // open in the foreground tab which is unrelated to the external protocol
+  // navigation.
+  let dialogWindowPromise = waitForProtocolAppChooserDialog(gBrowser, true);
+
+  info("Navigating to external proto from frame in background tab");
+  let parentBC = tab.linkedBrowser.browsingContext;
+  await SpecialPowers.spawn(parentBC.children[0], [], async function () {
+    content.eval("location.href = 'mailto:example@example.com';");
+  });
+
+  // Wait for dialog to open in one of the tabs.
+  let dialog = await dialogWindowPromise;
+
+  is(
+    gBrowser.getTabDialogBox(tab.linkedBrowser)._tabDialogManager._topDialog,
+    dialog,
+    "Dialog opened in the background tab"
+  );
+
+  is(
+    dialog._frame.contentDocument.location.href,
+    CONTENT_HANDLING_URL,
+    "Opened dialog is appChooser dialog."
+  );
+
+  // Close the dialog:
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(gBrowser, false);
+  dialog.close();
+  await dialogClosedPromise;
+
+  gBrowser.removeTab(tab);
+  gBrowser.removeTab(newTab);
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog_external.js b/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog_external.js
new file mode 100644
index 0000000000..591f1afbc5
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog_external.js
@@ -0,0 +1,199 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+let gHandlerService = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+  Ci.nsIHandlerService
+);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+/**
+ * Creates dummy protocol handler
+ */
+function initTestHandlers() {
+  let handlerInfoThatAsks =
+    HandlerServiceTestUtils.getBlankHandlerInfo("local-app-test");
+
+  let appHandler = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  // This is a dir and not executable, but that's enough for here.
+  appHandler.executable = Services.dirsvc.get("XCurProcD", Ci.nsIFile);
+  handlerInfoThatAsks.possibleApplicationHandlers.appendElement(appHandler);
+  handlerInfoThatAsks.preferredApplicationHandler = appHandler;
+  handlerInfoThatAsks.preferredAction = handlerInfoThatAsks.useHelperApp;
+  handlerInfoThatAsks.alwaysAskBeforeHandling = false;
+  gHandlerService.store(handlerInfoThatAsks);
+
+  let webHandlerInfo =
+    HandlerServiceTestUtils.getBlankHandlerInfo("web+somesite");
+  let webHandler = Cc[
+    "@mozilla.org/uriloader/web-handler-app;1"
+  ].createInstance(Ci.nsIWebHandlerApp);
+  webHandler.name = "Somesite";
+  webHandler.uriTemplate = "https://example.com/handle_url?u=%s";
+  webHandlerInfo.possibleApplicationHandlers.appendElement(webHandler);
+  webHandlerInfo.preferredApplicationHandler = webHandler;
+  webHandlerInfo.preferredAction = webHandlerInfo.useHelperApp;
+  webHandlerInfo.alwaysAskBeforeHandling = false;
+  gHandlerService.store(webHandlerInfo);
+
+  registerCleanupFunction(() => {
+    gHandlerService.remove(webHandlerInfo);
+    gHandlerService.remove(handlerInfoThatAsks);
+  });
+}
+
+function makeCmdLineHelper(url) {
+  return Cu.createCommandLine(
+    ["-url", url],
+    null,
+    Ci.nsICommandLine.STATE_REMOTE_EXPLICIT
+  );
+}
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["network.protocol-handler.prompt-from-external", true]],
+  });
+  initTestHandlers();
+});
+
+/**
+ * Check that if we get a direct request from another app / the OS to open a
+ * link, we always prompt, even if we think we know what the correct answer
+ * is. This is to avoid infinite loops in such situations where the OS and
+ * Firefox have conflicting ideas about the default handler, or where our
+ * checks with the OS don't work (Linux and/or Snap, at time of this comment).
+ */
+add_task(async function test_external_asks_anyway() {
+  let cmdLineHandler = Cc["@mozilla.org/browser/final-clh;1"].getService(
+    Ci.nsICommandLineHandler
+  );
+  let chooserDialogOpenPromise = waitForProtocolAppChooserDialog(
+    gBrowser,
+    true
+  );
+  let fakeCmdLine = makeCmdLineHelper("local-app-test:dummy");
+  cmdLineHandler.handle(fakeCmdLine);
+  let dialog = await chooserDialogOpenPromise;
+  ok(dialog, "Should have prompted.");
+
+  let dialogClosedPromise = waitForProtocolAppChooserDialog(
+    gBrowser.selectedBrowser,
+    false
+  );
+  let dialogEl = dialog._frame.contentDocument.querySelector("dialog");
+  dialogEl.cancelDialog();
+  await dialogClosedPromise;
+  // We will have opened a tab; close it.
+  BrowserTestUtils.removeTab(gBrowser.selectedTab);
+});
+
+/**
+ * Like the previous test, but avoid asking for web and extension handlers,
+ * as we can open those ourselves without looping.
+ */
+add_task(async function test_web_app_doesnt_ask() {
+  // Listen for a dialog open and fail the test if it does:
+  let dialogOpenListener = () => ok(false, "Shouldn't have opened a dialog!");
+  document.documentElement.addEventListener("dialogopen", dialogOpenListener);
+  registerCleanupFunction(() =>
+    document.documentElement.removeEventListener(
+      "dialogopen",
+      dialogOpenListener
+    )
+  );
+
+  // Set up a promise for a tab to open with the right URL:
+  const kURL = "web+somesite:dummy";
+  const kLoadedURL =
+    "https://example.com/handle_url?u=" + encodeURIComponent(kURL);
+  let tabPromise = BrowserTestUtils.waitForNewTab(gBrowser, kLoadedURL);
+
+  // Load the URL:
+  let cmdLineHandler = Cc["@mozilla.org/browser/final-clh;1"].getService(
+    Ci.nsICommandLineHandler
+  );
+  let fakeCmdLine = makeCmdLineHelper(kURL);
+  cmdLineHandler.handle(fakeCmdLine);
+
+  // Check that the tab loaded. If instead the dialog opened, the dialogopen handler
+  // will fail the test.
+  let tab = await tabPromise;
+  is(
+    tab.linkedBrowser.currentURI.spec,
+    kLoadedURL,
+    "Should have opened the right URL."
+  );
+  BrowserTestUtils.removeTab(tab);
+
+  // We do this both here and in cleanup so it's easy to add tasks to this test,
+  // and so we clean up correctly if the test aborts before we get here.
+  document.documentElement.removeEventListener(
+    "dialogopen",
+    dialogOpenListener
+  );
+});
+
+add_task(async function external_https_redirect_doesnt_ask() {
+  Services.perms.addFromPrincipal(
+    Services.scriptSecurityManager.createContentPrincipalFromOrigin(
+      "https://example.com"
+    ),
+    "open-protocol-handler^local-app-test",
+    Services.perms.ALLOW_ACTION
+  );
+  // Listen for a dialog open and fail the test if it does:
+  let dialogOpenListener = () => ok(false, "Shouldn't have opened a dialog!");
+  document.documentElement.addEventListener("dialogopen", dialogOpenListener);
+  registerCleanupFunction(() => {
+    document.documentElement.removeEventListener(
+      "dialogopen",
+      dialogOpenListener
+    );
+    Services.perms.removeAll();
+  });
+
+  let initialTab = gBrowser.selectedTab;
+
+  gHandlerService.wrappedJSObject.mockProtocolHandler("local-app-test");
+  registerCleanupFunction(() =>
+    gHandlerService.wrappedJSObject.mockProtocolHandler()
+  );
+
+  // Set up a promise for an app to have launched with the right URI:
+  let loadPromise = TestUtils.topicObserved("mocked-protocol-handler");
+
+  // Load the URL:
+  const kURL = "local-app-test:redirect";
+  let cmdLineHandler = Cc["@mozilla.org/browser/final-clh;1"].getService(
+    Ci.nsICommandLineHandler
+  );
+  let fakeCmdLine = makeCmdLineHelper(
+    TEST_PATH + "redirect_helper.sjs?uri=" + encodeURIComponent(kURL)
+  );
+  cmdLineHandler.handle(fakeCmdLine);
+
+  // Check that the mock app was launched. If the dialog showed instead,
+  // the test will fail.
+  let [uri] = await loadPromise;
+  is(uri.spec, "local-app-test:redirect", "Should have seen correct URI.");
+  // We might have opened a blank tab, see bug 1718104 and friends.
+  if (gBrowser.selectedTab != initialTab) {
+    BrowserTestUtils.removeTab(gBrowser.selectedTab);
+  }
+
+  // We do this both here and in cleanup so it's easy to add tasks to this test,
+  // and so we clean up correctly if the test aborts before we get here.
+  document.documentElement.removeEventListener(
+    "dialogopen",
+    dialogOpenListener
+  );
+  gHandlerService.wrappedJSObject.mockProtocolHandler();
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog_permission.js b/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog_permission.js
new file mode 100644
index 0000000000..c567722b6d
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_protocol_ask_dialog_permission.js
@@ -0,0 +1,1383 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+var { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+const { ExtensionPermissions } = ChromeUtils.importESModule(
+  "resource://gre/modules/ExtensionPermissions.sys.mjs"
+);
+
+let gHandlerService = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+  Ci.nsIHandlerService
+);
+
+const ROOT_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content/",
+  ""
+);
+
+// Testing multiple protocol / origin combinations takes long on debug.
+requestLongerTimeout(7);
+
+const DIALOG_URL_APP_CHOOSER =
+  "chrome://mozapps/content/handling/appChooser.xhtml";
+const DIALOG_URL_PERMISSION =
+  "chrome://mozapps/content/handling/permissionDialog.xhtml";
+
+const PROTOCOL_HANDLER_OPEN_PERM_KEY = "open-protocol-handler";
+const PERMISSION_KEY_DELIMITER = "^";
+
+const TEST_PROTOS = ["foo", "bar"];
+
+let testDir = getChromeDir(getResolvedURI(gTestPath));
+
+const ORIGIN1 = "https://example.com";
+const ORIGIN2 = "https://example.org";
+const ORIGIN3 = Services.io.newFileURI(testDir).spec;
+const PRINCIPAL1 =
+  Services.scriptSecurityManager.createContentPrincipalFromOrigin(ORIGIN1);
+const PRINCIPAL2 =
+  Services.scriptSecurityManager.createContentPrincipalFromOrigin(ORIGIN2);
+const PRINCIPAL3 =
+  Services.scriptSecurityManager.createContentPrincipalFromOrigin(ORIGIN3);
+
+const NULL_PRINCIPAL_SCHEME = Services.scriptSecurityManager
+  .createNullPrincipal({})
+  .scheme.toLowerCase();
+
+/**
+ * Get the open protocol handler permission key for a given protocol scheme.
+ * @param {string} aProtocolScheme - Scheme of protocol to construct permission
+ * key with.
+ */
+function getSkipProtoDialogPermissionKey(aProtocolScheme) {
+  return (
+    PROTOCOL_HANDLER_OPEN_PERM_KEY + PERMISSION_KEY_DELIMITER + aProtocolScheme
+  );
+}
+
+function getSystemProtocol() {
+  // TODO add a scheme for Windows 10 or greater once support is added (see bug 1764599).
+  if (AppConstants.platform == "macosx") {
+    return "itunes";
+  }
+
+  info(
+    "Skipping this test since there isn't a suitable default protocol on this platform"
+  );
+  return null;
+}
+
+/**
+ * Creates dummy web protocol handlers used for testing.
+ */
+function initTestHandlers() {
+  TEST_PROTOS.forEach(scheme => {
+    let webHandler = Cc[
+      "@mozilla.org/uriloader/web-handler-app;1"
+    ].createInstance(Ci.nsIWebHandlerApp);
+    webHandler.name = scheme + "Handler";
+    webHandler.uriTemplate = ORIGIN1 + "/?url=%s";
+
+    let handlerInfo = HandlerServiceTestUtils.getBlankHandlerInfo(scheme);
+    handlerInfo.possibleApplicationHandlers.appendElement(webHandler);
+    handlerInfo.preferredApplicationHandler = webHandler;
+    gHandlerService.store(handlerInfo);
+  });
+}
+
+/**
+ * Update whether the protocol handler dialog is shown for our test protocol +
+ * handler.
+ * @param {string} scheme - Scheme of the protocol to change the ask state for.
+ * @param {boolean} ask - true => show dialog, false => skip dialog.
+ */
+function updateAlwaysAsk(scheme, ask) {
+  let handlerInfo = HandlerServiceTestUtils.getHandlerInfo(scheme);
+  handlerInfo.alwaysAskBeforeHandling = ask;
+  gHandlerService.store(handlerInfo);
+}
+
+/**
+ * Test whether the protocol handler dialog is set to show for our
+ * test protocol + handler.
+ * @param {string} scheme - Scheme of the protocol to test the ask state for.
+ * @param {boolean} ask - true => show dialog, false => skip dialog.
+ */
+function testAlwaysAsk(scheme, ask) {
+  is(
+    HandlerServiceTestUtils.getHandlerInfo(scheme).alwaysAskBeforeHandling,
+    ask,
+    "Should have correct alwaysAsk state."
+  );
+}
+
+/**
+ * Triggers the load via a server redirect.
+ * @param {string} serverRedirect - The redirect type.
+ */
+function useServerRedirect(serverRedirect) {
+  return async (browser, scheme) => {
+    let uri = `${scheme}://test`;
+
+    let innerParams = new URLSearchParams();
+    innerParams.set("uri", uri);
+    innerParams.set("redirectType", serverRedirect);
+    let params = new URLSearchParams();
+    params.set(
+      "uri",
+      "https://example.com/" +
+        ROOT_PATH +
+        "redirect_helper.sjs?" +
+        innerParams.toString()
+    );
+    uri =
+      "https://example.org/" +
+      ROOT_PATH +
+      "redirect_helper.sjs?" +
+      params.toString();
+    BrowserTestUtils.startLoadingURIString(browser, uri);
+  };
+}
+
+/**
+ * Triggers the load with a specific principal or the browser's current
+ * principal.
+ * @param {nsIPrincipal} [principal] - Principal to use to trigger the load.
+ */
+function useTriggeringPrincipal(principal = undefined) {
+  return async (browser, scheme) => {
+    let uri = `${scheme}://test`;
+    let triggeringPrincipal = principal ?? browser.contentPrincipal;
+
+    info("Loading uri: " + uri);
+    browser.loadURI(Services.io.newURI(uri), { triggeringPrincipal });
+  };
+}
+
+/**
+ * Navigates to a test URL with the given protocol scheme and waits for the
+ * result.
+ * @param {MozBrowser} browser - Browser to navigate.
+ * @param {string} scheme - Scheme of the test url. e.g. irc
+ * @param {Object} [options] - Test options.
+ * @param {Object} [options.permDialogOptions] - Test options for the permission
+ * dialog. If defined, we expect this dialog to be shown.
+ * @param {Object} [options.chooserDialogOptions] - Test options for the chooser
+ * dialog. If defined, we expect this dialog to be shown.
+ * @param {Function} [options.triggerLoad] - An async callback function to
+ * trigger the load. Will be passed the browser and scheme to use.
+ * @param {nsIPrincipal} [options.triggeringPrincipal] - Principal to trigger
+ * the load with. Defaults to the browsers content principal.
+ * @returns {Promise} - A promise which resolves once the test is complete.
+ */
+async function testOpenProto(
+  browser,
+  scheme,
+  {
+    permDialogOptions,
+    chooserDialogOptions,
+    triggerLoad = useTriggeringPrincipal(),
+  } = {}
+) {
+  let permDialogOpenPromise;
+  let chooserDialogOpenPromise;
+
+  if (permDialogOptions) {
+    info("Should see permission dialog");
+    permDialogOpenPromise = waitForProtocolPermissionDialog(browser, true);
+  }
+
+  if (chooserDialogOptions) {
+    info("Should see chooser dialog");
+    chooserDialogOpenPromise = waitForProtocolAppChooserDialog(browser, true);
+  }
+  await triggerLoad(browser, scheme);
+  let webHandlerLoadedPromise;
+
+  let webHandlerShouldOpen =
+    (!permDialogOptions && !chooserDialogOptions) ||
+    ((permDialogOptions?.actionConfirm || permDialogOptions?.actionChangeApp) &&
+      chooserDialogOptions?.actionConfirm);
+
+  // Register web handler load listener if we expect to trigger it.
+  if (webHandlerShouldOpen) {
+    webHandlerLoadedPromise = waitForHandlerURL(browser, scheme);
+  }
+
+  if (permDialogOpenPromise) {
+    let dialog = await permDialogOpenPromise;
+    let dialogEl = getDialogElementFromSubDialog(dialog);
+    let dialogType = getDialogType(dialog);
+
+    let {
+      hasCheckbox,
+      checkboxOrigin,
+      hasChangeApp,
+      chooserIsNext,
+      actionCheckbox,
+      actionConfirm,
+      actionChangeApp,
+      checkContents,
+    } = permDialogOptions;
+
+    if (actionChangeApp) {
+      actionConfirm = false;
+    }
+
+    let descriptionEl = dialogEl.querySelector("#description");
+    ok(
+      descriptionEl && BrowserTestUtils.isVisible(descriptionEl),
+      "Has a visible description element."
+    );
+
+    ok(
+      !descriptionEl.innerHTML.toLowerCase().includes(NULL_PRINCIPAL_SCHEME),
+      "Description does not include NullPrincipal scheme."
+    );
+
+    await testCheckbox(dialogEl, dialogType, {
+      hasCheckbox,
+      actionCheckbox,
+      checkboxOrigin,
+    });
+
+    // Check the button label depending on whether we would show the chooser
+    // dialog next or directly open the handler.
+    let acceptBtnLabel = dialogEl.getButton("accept")?.label;
+
+    if (chooserIsNext) {
+      is(
+        acceptBtnLabel,
+        "Choose Application",
+        "Accept button has choose app label"
+      );
+    } else {
+      is(acceptBtnLabel, "Open Link", "Accept button has open link label");
+    }
+
+    let changeAppLink = dialogEl.ownerDocument.getElementById("change-app");
+    if (typeof hasChangeApp == "boolean") {
+      ok(changeAppLink, "Permission dialog should have changeApp link label");
+      is(
+        !changeAppLink.hidden,
+        hasChangeApp,
+        "Permission dialog change app link label"
+      );
+    }
+
+    if (checkContents) {
+      checkContents(dialogEl);
+    }
+
+    if (actionChangeApp) {
+      let dialogClosedPromise = waitForProtocolPermissionDialog(browser, false);
+      changeAppLink.click();
+      await dialogClosedPromise;
+    } else {
+      await closeDialog(browser, dialog, actionConfirm, scheme);
+    }
+  }
+
+  if (chooserDialogOpenPromise) {
+    let dialog = await chooserDialogOpenPromise;
+    let dialogEl = getDialogElementFromSubDialog(dialog);
+    let dialogType = getDialogType(dialog);
+
+    let { hasCheckbox, actionCheckbox, actionConfirm } = chooserDialogOptions;
+
+    await testCheckbox(dialogEl, dialogType, {
+      hasCheckbox,
+      actionCheckbox,
+    });
+
+    await closeDialog(browser, dialog, actionConfirm, scheme);
+  }
+
+  if (webHandlerShouldOpen) {
+    info("Waiting for web handler to open");
+    await webHandlerLoadedPromise;
+  } else {
+    info("Web handler open canceled");
+  }
+}
+
+/**
+ * Inspects the checkbox state and interacts with it.
+ * @param {dialog} dialogEl
+ * @param {string} dialogType - String identifier of dialog type.
+ * Either "permission" or "chooser".
+ * @param {Object} options - Test Options.
+ * @param {boolean} [options.hasCheckbox] - Whether the dialog is expected to
+ * have a visible checkbox.
+ * @param {boolean} [options.hasCheckboxState] - The check state of the checkbox
+ * to test for. true = checked, false = unchecked.
+ * @param {boolean} [options.actionCheckbox] - The state to set on the checkbox.
+ * true = checked, false = unchecked.
+ */
+async function testCheckbox(
+  dialogEl,
+  dialogType,
+  { hasCheckbox, hasCheckboxState = false, actionCheckbox, checkboxOrigin }
+) {
+  let checkbox = dialogEl.ownerDocument.getElementById("remember");
+  if (typeof hasCheckbox == "boolean") {
+    is(
+      checkbox && BrowserTestUtils.isVisible(checkbox),
+      hasCheckbox,
+      "Dialog checkbox has correct visibility."
+    );
+
+    let checkboxLabel = dialogEl.ownerDocument.getElementById("remember-label");
+    is(
+      checkbox && BrowserTestUtils.isVisible(checkboxLabel),
+      hasCheckbox,
+      "Dialog checkbox label has correct visibility."
+    );
+    if (hasCheckbox) {
+      ok(
+        !checkboxLabel.innerHTML.toLowerCase().includes(NULL_PRINCIPAL_SCHEME),
+        "Dialog checkbox label does not include NullPrincipal scheme."
+      );
+    }
+  }
+
+  if (typeof hasCheckboxState == "boolean") {
+    is(checkbox.checked, hasCheckboxState, "Dialog checkbox has correct state");
+  }
+
+  if (checkboxOrigin) {
+    let doc = dialogEl.ownerDocument;
+    let hostFromLabel = doc.l10n.getAttributes(
+      doc.getElementById("remember-label")
+    ).args.host;
+    is(hostFromLabel, checkboxOrigin, "Checkbox should be for correct domain.");
+  }
+
+  if (typeof actionCheckbox == "boolean") {
+    checkbox.click();
+  }
+}
+
+/**
+ * Get the dialog element which is a child of the SubDialogs browser frame.
+ * @param {SubDialog} subDialog - Dialog to get the dialog element for.
+ */
+function getDialogElementFromSubDialog(subDialog) {
+  let dialogEl = subDialog._frame.contentDocument.querySelector("dialog");
+  ok(dialogEl, "SubDialog should have dialog element");
+  return dialogEl;
+}
+
+/**
+ * Wait for the test handler to be opened.
+ * @param {MozBrowser} browser - The browser the load should occur in.
+ * @param {string} scheme - Scheme which triggered the handler to open.
+ */
+function waitForHandlerURL(browser, scheme) {
+  return BrowserTestUtils.browserLoaded(
+    browser,
+    false,
+    url => url == `${ORIGIN1}/?url=${scheme}%3A%2F%2Ftest`
+  );
+}
+
+/**
+ * Test for open-protocol-handler permission.
+ * @param {nsIPrincipal} principal - The principal to test the permission on.
+ * @param {string} scheme - Scheme to generate permission key.
+ * @param {boolean} hasPerm - Whether we expect the princial to set the
+ * permission (true), or not (false).
+ */
+function testPermission(principal, scheme, hasPerm) {
+  let permKey = getSkipProtoDialogPermissionKey(scheme);
+  let result = Services.perms.testPermissionFromPrincipal(principal, permKey);
+  let message = `${permKey} ${hasPerm ? "is" : "is not"} set for ${
+    principal.origin
+  }.`;
+  is(result == Services.perms.ALLOW_ACTION, hasPerm, message);
+}
+
+/**
+ * Get the checkbox element of the dialog used to remember the handler choice or
+ * store the permission.
+ * @param {SubDialog} dialog - Protocol handler dialog embedded in a SubDialog.
+ * @param {string} dialogType - Type of the dialog which holds the checkbox.
+ * @returns {HTMLInputElement} - Checkbox of the dialog.
+ */
+function getDialogCheckbox(dialog, dialogType) {
+  let id;
+  if (dialogType == "permission") {
+    id = "remember-permission";
+  } else {
+    id = "remember";
+  }
+  return dialog._frame.contentDocument.getElementById(id);
+}
+
+function getDialogType(dialog) {
+  let url = dialog._frame.currentURI.spec;
+
+  if (url === DIALOG_URL_PERMISSION) {
+    return "permission";
+  }
+  if (url === DIALOG_URL_APP_CHOOSER) {
+    return "chooser";
+  }
+  throw new Error("Dialog with unexpected url");
+}
+
+/**
+ * Exit a protocol handler SubDialog and wait for it to be fully closed.
+ * @param {MozBrowser} browser - Browser element of the tab where the dialog is
+ * shown.
+ * @param {SubDialog} dialog - SubDialog object which holds the protocol handler
+ * @param {boolean} confirm - Whether to confirm (true) or cancel (false) the
+ * dialog.
+ * @param {string} scheme - The scheme of the protocol the dialog is opened for.
+ * dialog.
+ */
+async function closeDialog(browser, dialog, confirm, scheme) {
+  let dialogClosedPromise = waitForSubDialog(browser, dialog._openedURL, false);
+  let dialogEl = getDialogElementFromSubDialog(dialog);
+
+  if (confirm) {
+    if (getDialogType(dialog) == "chooser") {
+      // Select our test protocol handler
+      let listItem = dialogEl.ownerDocument.querySelector(
+        `richlistitem[name="${scheme}Handler"]`
+      );
+      listItem.click();
+    }
+
+    dialogEl.setAttribute("buttondisabledaccept", false);
+    dialogEl.acceptDialog();
+  } else {
+    dialogEl.cancelDialog();
+  }
+
+  return dialogClosedPromise;
+}
+
+registerCleanupFunction(function () {
+  // Clean up test handlers
+  TEST_PROTOS.forEach(scheme => {
+    let handlerInfo = HandlerServiceTestUtils.getHandlerInfo(scheme);
+    gHandlerService.remove(handlerInfo);
+  });
+
+  // Clear permissions
+  Services.perms.removeAll();
+});
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["security.external_protocol_requires_permission", true]],
+  });
+  initTestHandlers();
+});
+
+/**
+ * Tests that when "remember" is unchecked, we only allow the protocol to be
+ * opened once and don't store any permission.
+ */
+add_task(async function test_permission_allow_once() {
+  for (let scheme of TEST_PROTOS) {
+    await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+      await testOpenProto(browser, scheme, {
+        permDialogOptions: {
+          hasCheckbox: true,
+          hasChangeApp: false,
+          chooserIsNext: true,
+          actionConfirm: true,
+        },
+        chooserDialogOptions: { hasCheckbox: true, actionConfirm: true },
+      });
+    });
+
+    // No permission should be set
+    testPermission(PRINCIPAL1, scheme, false);
+    testPermission(PRINCIPAL2, scheme, false);
+
+    // No preferred app should be set
+    testAlwaysAsk(scheme, true);
+
+    // If we open again we should see the permission dialog
+    await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+      await testOpenProto(browser, scheme, {
+        permDialogOptions: {
+          hasCheckbox: true,
+          hasChangeApp: false,
+          chooserIsNext: true,
+          actionConfirm: false,
+        },
+      });
+    });
+  }
+});
+
+/**
+ * Tests that when checking the "remember" checkbox, the protocol permission
+ * is set correctly and allows the caller to skip the permission dialog in
+ * subsequent calls.
+ */
+add_task(async function test_permission_allow_persist() {
+  for (let [origin, principal] of [
+    [ORIGIN1, PRINCIPAL1],
+    [ORIGIN3, PRINCIPAL3],
+  ]) {
+    for (let scheme of TEST_PROTOS) {
+      info("Testing with origin " + origin);
+      info("testing with principal of origin " + principal.origin);
+      info("testing with protocol " + scheme);
+
+      // Set a permission for an unrelated protocol.
+      // We should still see the permission dialog.
+      Services.perms.addFromPrincipal(
+        principal,
+        getSkipProtoDialogPermissionKey("foobar"),
+        Services.perms.ALLOW_ACTION
+      );
+
+      await BrowserTestUtils.withNewTab(origin, async browser => {
+        await testOpenProto(browser, scheme, {
+          permDialogOptions: {
+            hasCheckbox: true,
+            hasChangeApp: false,
+            chooserIsNext: true,
+            actionCheckbox: true,
+            actionConfirm: true,
+          },
+          chooserDialogOptions: { hasCheckbox: true, actionConfirm: true },
+        });
+      });
+
+      // Permission should be set
+      testPermission(principal, scheme, true);
+      testPermission(PRINCIPAL2, scheme, false);
+
+      // No preferred app should be set
+      testAlwaysAsk(scheme, true);
+
+      // If we open again with the origin where we granted permission, we should
+      // directly get the chooser dialog.
+      await BrowserTestUtils.withNewTab(origin, async browser => {
+        await testOpenProto(browser, scheme, {
+          chooserDialogOptions: {
+            hasCheckbox: true,
+            actionConfirm: false,
+          },
+        });
+      });
+
+      // If we open with the other origin, we should see the permission dialog
+      await BrowserTestUtils.withNewTab(ORIGIN2, async browser => {
+        await testOpenProto(browser, scheme, {
+          permDialogOptions: {
+            hasCheckbox: true,
+            hasChangeApp: false,
+            chooserIsNext: true,
+            actionConfirm: false,
+          },
+        });
+      });
+
+      // Cleanup permissions
+      Services.perms.removeAll();
+    }
+  }
+});
+
+/**
+ * Tests that if a preferred protocol handler is set, the permission dialog
+ * shows the application name and a link which leads to the app chooser.
+ */
+add_task(async function test_permission_application_set() {
+  let scheme = TEST_PROTOS[0];
+  updateAlwaysAsk(scheme, false);
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      permDialogOptions: {
+        hasCheckbox: true,
+        hasChangeApp: true,
+        chooserIsNext: false,
+        actionChangeApp: true,
+      },
+      chooserDialogOptions: { hasCheckbox: true, actionConfirm: true },
+    });
+  });
+
+  // Cleanup
+  updateAlwaysAsk(scheme, true);
+});
+
+/**
+ * Tests that we correctly handle system principals. They should always
+ * show the permission dialog, but give the option to choose another
+ * app if there isn't a default handler.
+ */
+add_task(async function test_permission_system_principal() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      permDialogOptions: {
+        hasCheckbox: false,
+        hasChangeApp: false,
+        chooserIsNext: true,
+        actionChangeApp: false,
+      },
+      triggerLoad: useTriggeringPrincipal(
+        Services.scriptSecurityManager.getSystemPrincipal()
+      ),
+    });
+  });
+});
+
+/**
+ * Tests that we correctly handle system principals and show
+ * a simplified permission dialog if there is a default handler.
+ */
+add_task(async function test_permission_system_principal() {
+  let scheme = getSystemProtocol();
+  if (!scheme) {
+    return;
+  }
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      permDialogOptions: {
+        hasCheckbox: false,
+        hasChangeApp: false,
+        chooserIsNext: false,
+        actionChangeApp: false,
+      },
+      triggerLoad: useTriggeringPrincipal(
+        Services.scriptSecurityManager.getSystemPrincipal()
+      ),
+    });
+  });
+});
+
+/**
+ * Tests that we don't show the permission dialog if the permission is disabled
+ * by pref.
+ */
+add_task(async function test_permission_disabled() {
+  let scheme = TEST_PROTOS[0];
+
+  await SpecialPowers.pushPrefEnv({
+    set: [["security.external_protocol_requires_permission", false]],
+  });
+
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      chooserDialogOptions: { hasCheckbox: true, actionConfirm: true },
+    });
+  });
+
+  await SpecialPowers.popPrefEnv();
+});
+
+/**
+ * Tests that we directly open the handler if permission and handler are set.
+ */
+add_task(async function test_app_and_permission_set() {
+  let scheme = TEST_PROTOS[1];
+
+  updateAlwaysAsk(scheme, false);
+  Services.perms.addFromPrincipal(
+    PRINCIPAL2,
+    getSkipProtoDialogPermissionKey(scheme),
+    Services.perms.ALLOW_ACTION
+  );
+
+  await BrowserTestUtils.withNewTab(ORIGIN2, async browser => {
+    await testOpenProto(browser, scheme);
+  });
+
+  // Cleanup
+  Services.perms.removeAll();
+  updateAlwaysAsk(scheme, true);
+});
+
+/**
+ * Tests that the alwaysAsk state is not updated if the user cancels the dialog
+ */
+add_task(async function test_change_app_checkbox_cancel() {
+  let scheme = TEST_PROTOS[0];
+
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      permDialogOptions: {
+        hasCheckbox: true,
+        chooserIsNext: true,
+        hasChangeApp: false,
+        actionConfirm: true,
+      },
+      chooserDialogOptions: {
+        hasCheckbox: true,
+        actionCheckbox: true, // Activate checkbox
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+
+  // Should not have applied value from checkbox
+  testAlwaysAsk(scheme, true);
+});
+
+/**
+ * Tests that the external protocol dialogs behave correctly when a null
+ * principal is passed.
+ */
+add_task(async function test_null_principal() {
+  let scheme = TEST_PROTOS[0];
+
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      triggerLoad: () => {
+        let uri = `${scheme}://test`;
+        ContentTask.spawn(browser, { uri }, args => {
+          let frame = content.document.createElement("iframe");
+          frame.src = `data:text/html,<script>location.href="${args.uri}"</script>`;
+          content.document.body.appendChild(frame);
+        });
+      },
+      permDialogOptions: {
+        hasCheckbox: false,
+        chooserIsNext: true,
+        hasChangeApp: false,
+        actionConfirm: true,
+      },
+      chooserDialogOptions: {
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+});
+
+/**
+ * Tests that the external protocol dialogs behave correctly when no principal
+ * is passed.
+ */
+add_task(async function test_no_principal() {
+  let scheme = TEST_PROTOS[1];
+
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      triggerLoad: () => {
+        let uri = `${scheme}://test`;
+
+        let contentDispatchChooser = Cc[
+          "@mozilla.org/content-dispatch-chooser;1"
+        ].createInstance(Ci.nsIContentDispatchChooser);
+
+        let handler = HandlerServiceTestUtils.getHandlerInfo(scheme);
+
+        contentDispatchChooser.handleURI(
+          handler,
+          Services.io.newURI(uri),
+          null,
+          browser.browsingContext
+        );
+      },
+      permDialogOptions: {
+        hasCheckbox: false,
+        chooserIsNext: true,
+        hasChangeApp: false,
+        actionConfirm: true,
+      },
+      chooserDialogOptions: {
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+});
+
+/**
+ * Tests that if a URI scheme has a non-standard protocol, an OS default exists,
+ * and the user hasn't selected an alternative only the permission dialog is shown.
+ */
+add_task(async function test_non_standard_protocol() {
+  let scheme = getSystemProtocol();
+  if (!scheme) {
+    return;
+  }
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    await testOpenProto(browser, scheme, {
+      permDialogOptions: {
+        hasCheckbox: true,
+        hasChangeApp: true,
+        chooserIsNext: false,
+        actionChangeApp: false,
+      },
+    });
+  });
+});
+
+/**
+ * Tests that we show the permission dialog for extension content scripts.
+ */
+add_task(async function test_extension_content_script_permission() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    let testExtension;
+
+    await testOpenProto(browser, scheme, {
+      triggerLoad: async () => {
+        let uri = `${scheme}://test`;
+
+        const EXTENSION_DATA = {
+          manifest: {
+            content_scripts: [
+              {
+                matches: [browser.currentURI.spec],
+                js: ["navigate.js"],
+              },
+            ],
+            browser_specific_settings: {
+              gecko: { id: "allowed@mochi.test" },
+            },
+          },
+          files: {
+            "navigate.js": `window.location.href = "${uri}";`,
+          },
+          useAddonManager: "permanent",
+        };
+
+        testExtension = ExtensionTestUtils.loadExtension(EXTENSION_DATA);
+        await testExtension.startup();
+      },
+      permDialogOptions: {
+        hasCheckbox: true,
+        chooserIsNext: true,
+        hasChangeApp: false,
+        actionCheckbox: true,
+        actionConfirm: true,
+        checkContents: dialogEl => {
+          let description = dialogEl.querySelector("#description");
+          let { id, args } =
+            description.ownerDocument.l10n.getAttributes(description);
+          is(
+            id,
+            "permission-dialog-description-extension",
+            "Should be using the correct string."
+          );
+          is(
+            args.extension,
+            "Generated extension",
+            "Should have the correct extension name."
+          );
+        },
+      },
+      chooserDialogOptions: {
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+
+    let extensionPrincipal =
+      Services.scriptSecurityManager.createContentPrincipal(
+        Services.io.newURI(`moz-extension://${testExtension.uuid}/`),
+        {}
+      );
+    let extensionPrivatePrincipal =
+      Services.scriptSecurityManager.createContentPrincipal(
+        Services.io.newURI(`moz-extension://${testExtension.uuid}/`),
+        { privateBrowsingId: 1 }
+      );
+
+    let key = getSkipProtoDialogPermissionKey(scheme);
+    is(
+      Services.perms.testPermissionFromPrincipal(extensionPrincipal, key),
+      Services.perms.ALLOW_ACTION,
+      "Should have permanently allowed the extension"
+    );
+    is(
+      Services.perms.testPermissionFromPrincipal(
+        extensionPrivatePrincipal,
+        key
+      ),
+      Services.perms.UNKNOWN_ACTION,
+      "Should not have changed the private principal permission"
+    );
+    is(
+      Services.perms.testPermissionFromPrincipal(PRINCIPAL1, key),
+      Services.perms.UNKNOWN_ACTION,
+      "Should not have allowed the page"
+    );
+
+    await testExtension.unload();
+
+    is(
+      Services.perms.testPermissionFromPrincipal(extensionPrincipal, key),
+      Services.perms.UNKNOWN_ACTION,
+      "Should have cleared the extension's normal principal permission"
+    );
+    is(
+      Services.perms.testPermissionFromPrincipal(
+        extensionPrivatePrincipal,
+        key
+      ),
+      Services.perms.UNKNOWN_ACTION,
+      "Should have cleared the private browsing principal"
+    );
+  });
+});
+
+/**
+ * Tests that we show the permission dialog for extension content scripts.
+ */
+add_task(async function test_extension_private_content_script_permission() {
+  let scheme = TEST_PROTOS[0];
+  let win = await BrowserTestUtils.openNewBrowserWindow({ private: true });
+
+  await BrowserTestUtils.withNewTab(
+    { gBrowser: win.gBrowser, url: ORIGIN1 },
+    async browser => {
+      let testExtension;
+
+      await testOpenProto(browser, scheme, {
+        triggerLoad: async () => {
+          let uri = `${scheme}://test`;
+
+          const EXTENSION_DATA = {
+            manifest: {
+              content_scripts: [
+                {
+                  matches: [browser.currentURI.spec],
+                  js: ["navigate.js"],
+                },
+              ],
+              browser_specific_settings: {
+                gecko: { id: "allowed@mochi.test" },
+              },
+            },
+            files: {
+              "navigate.js": `window.location.href = "${uri}";`,
+            },
+            useAddonManager: "permanent",
+          };
+
+          testExtension = ExtensionTestUtils.loadExtension(EXTENSION_DATA);
+          await testExtension.startup();
+          let perms = {
+            permissions: ["internal:privateBrowsingAllowed"],
+            origins: [],
+          };
+          await ExtensionPermissions.add("allowed@mochi.test", perms);
+          let addon = await AddonManager.getAddonByID("allowed@mochi.test");
+          await addon.reload();
+        },
+        permDialogOptions: {
+          hasCheckbox: true,
+          chooserIsNext: true,
+          hasChangeApp: false,
+          actionCheckbox: true,
+          actionConfirm: true,
+          checkContents: dialogEl => {
+            let description = dialogEl.querySelector("#description");
+            let { id, args } =
+              description.ownerDocument.l10n.getAttributes(description);
+            is(
+              id,
+              "permission-dialog-description-extension",
+              "Should be using the correct string."
+            );
+            is(
+              args.extension,
+              "Generated extension",
+              "Should have the correct extension name."
+            );
+          },
+        },
+        chooserDialogOptions: {
+          hasCheckbox: true,
+          actionConfirm: false, // Cancel dialog
+        },
+      });
+
+      let extensionPrincipal =
+        Services.scriptSecurityManager.createContentPrincipal(
+          Services.io.newURI(`moz-extension://${testExtension.uuid}/`),
+          {}
+        );
+      let extensionPrivatePrincipal =
+        Services.scriptSecurityManager.createContentPrincipal(
+          Services.io.newURI(`moz-extension://${testExtension.uuid}/`),
+          { privateBrowsingId: 1 }
+        );
+
+      let key = getSkipProtoDialogPermissionKey(scheme);
+      is(
+        Services.perms.testPermissionFromPrincipal(extensionPrincipal, key),
+        Services.perms.UNKNOWN_ACTION,
+        "Should not have changed the extension's normal principal permission"
+      );
+      is(
+        Services.perms.testPermissionFromPrincipal(
+          extensionPrivatePrincipal,
+          key
+        ),
+        Services.perms.ALLOW_ACTION,
+        "Should have allowed the private browsing principal"
+      );
+      is(
+        Services.perms.testPermissionFromPrincipal(PRINCIPAL1, key),
+        Services.perms.UNKNOWN_ACTION,
+        "Should not have allowed the page"
+      );
+
+      await testExtension.unload();
+
+      is(
+        Services.perms.testPermissionFromPrincipal(extensionPrincipal, key),
+        Services.perms.UNKNOWN_ACTION,
+        "Should have cleared the extension's normal principal permission"
+      );
+      is(
+        Services.perms.testPermissionFromPrincipal(
+          extensionPrivatePrincipal,
+          key
+        ),
+        Services.perms.UNKNOWN_ACTION,
+        "Should have cleared the private browsing principal"
+      );
+    }
+  );
+
+  await BrowserTestUtils.closeWindow(win);
+});
+
+/**
+ * Tests that we do not show the permission dialog for extension content scripts
+ * when the page already has permission.
+ */
+add_task(async function test_extension_allowed_content() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    let testExtension;
+
+    let key = getSkipProtoDialogPermissionKey(scheme);
+    Services.perms.addFromPrincipal(
+      PRINCIPAL1,
+      key,
+      Services.perms.ALLOW_ACTION,
+      Services.perms.EXPIRE_NEVER
+    );
+
+    await testOpenProto(browser, scheme, {
+      triggerLoad: async () => {
+        let uri = `${scheme}://test`;
+
+        const EXTENSION_DATA = {
+          manifest: {
+            content_scripts: [
+              {
+                matches: [browser.currentURI.spec],
+                js: ["navigate.js"],
+              },
+            ],
+          },
+          files: {
+            "navigate.js": `window.location.href = "${uri}";`,
+          },
+        };
+
+        testExtension = ExtensionTestUtils.loadExtension(EXTENSION_DATA);
+        await testExtension.startup();
+      },
+      chooserDialogOptions: {
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+
+    let extensionPrincipal =
+      Services.scriptSecurityManager.createContentPrincipal(
+        Services.io.newURI(`moz-extension://${testExtension.uuid}/`),
+        {}
+      );
+
+    is(
+      Services.perms.testPermissionFromPrincipal(extensionPrincipal, key),
+      Services.perms.UNKNOWN_ACTION,
+      "Should not have permanently allowed the extension"
+    );
+
+    await testExtension.unload();
+    Services.perms.removeFromPrincipal(PRINCIPAL1, key);
+  });
+});
+
+/**
+ * Tests that we do not show the permission dialog for extension content scripts
+ * when the extension already has permission.
+ */
+add_task(async function test_extension_allowed_extension() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    let testExtension;
+
+    let key = getSkipProtoDialogPermissionKey(scheme);
+
+    await testOpenProto(browser, scheme, {
+      triggerLoad: async () => {
+        const EXTENSION_DATA = {
+          manifest: {
+            permissions: [`${ORIGIN1}/*`],
+          },
+          background() {
+            browser.test.onMessage.addListener(async (msg, uri) => {
+              switch (msg) {
+                case "engage":
+                  browser.tabs.executeScript({
+                    code: `window.location.href = "${uri}";`,
+                  });
+                  break;
+                default:
+                  browser.test.fail(`Unexpected message received: ${msg}`);
+              }
+            });
+          },
+        };
+
+        testExtension = ExtensionTestUtils.loadExtension(EXTENSION_DATA);
+        await testExtension.startup();
+
+        let extensionPrincipal =
+          Services.scriptSecurityManager.createContentPrincipal(
+            Services.io.newURI(`moz-extension://${testExtension.uuid}/`),
+            {}
+          );
+        Services.perms.addFromPrincipal(
+          extensionPrincipal,
+          key,
+          Services.perms.ALLOW_ACTION,
+          Services.perms.EXPIRE_NEVER
+        );
+
+        testExtension.sendMessage("engage", `${scheme}://test`);
+      },
+      chooserDialogOptions: {
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+
+    await testExtension.unload();
+    Services.perms.removeFromPrincipal(PRINCIPAL1, key);
+  });
+});
+
+/**
+ * Tests that we show the permission dialog for extensions directly opening a
+ * protocol.
+ */
+add_task(async function test_extension_principal() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab(ORIGIN1, async browser => {
+    let testExtension;
+
+    await testOpenProto(browser, scheme, {
+      triggerLoad: async () => {
+        const EXTENSION_DATA = {
+          background() {
+            browser.test.onMessage.addListener(async (msg, url) => {
+              switch (msg) {
+                case "engage":
+                  browser.tabs.update({
+                    url,
+                  });
+                  break;
+                default:
+                  browser.test.fail(`Unexpected message received: ${msg}`);
+              }
+            });
+          },
+        };
+
+        testExtension = ExtensionTestUtils.loadExtension(EXTENSION_DATA);
+        await testExtension.startup();
+        testExtension.sendMessage("engage", `${scheme}://test`);
+      },
+      permDialogOptions: {
+        hasCheckbox: true,
+        chooserIsNext: true,
+        hasChangeApp: false,
+        actionCheckbox: true,
+        actionConfirm: true,
+        checkContents: dialogEl => {
+          let description = dialogEl.querySelector("#description");
+          let { id, args } =
+            description.ownerDocument.l10n.getAttributes(description);
+          is(
+            id,
+            "permission-dialog-description-extension",
+            "Should be using the correct string."
+          );
+          is(
+            args.extension,
+            "Generated extension",
+            "Should have the correct extension name."
+          );
+        },
+      },
+      chooserDialogOptions: {
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+
+    let extensionPrincipal =
+      Services.scriptSecurityManager.createContentPrincipal(
+        Services.io.newURI(`moz-extension://${testExtension.uuid}/`),
+        {}
+      );
+
+    let key = getSkipProtoDialogPermissionKey(scheme);
+    is(
+      Services.perms.testPermissionFromPrincipal(extensionPrincipal, key),
+      Services.perms.ALLOW_ACTION,
+      "Should have permanently allowed the extension"
+    );
+    is(
+      Services.perms.testPermissionFromPrincipal(PRINCIPAL1, key),
+      Services.perms.UNKNOWN_ACTION,
+      "Should not have allowed the page"
+    );
+
+    await testExtension.unload();
+  });
+});
+
+/**
+ * Test that we use the redirect principal for the dialog when applicable.
+ */
+add_task(async function test_redirect_principal() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+    await testOpenProto(browser, scheme, {
+      triggerLoad: useServerRedirect("location"),
+      permDialogOptions: {
+        checkboxOrigin: ORIGIN1,
+        chooserIsNext: true,
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+});
+
+/**
+ * Test that we use the redirect principal for the dialog for refresh headers.
+ */
+add_task(async function test_redirect_principal() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+    await testOpenProto(browser, scheme, {
+      triggerLoad: useServerRedirect("refresh"),
+      permDialogOptions: {
+        checkboxOrigin: ORIGIN1,
+        chooserIsNext: true,
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+});
+
+/**
+ * Test that we use the redirect principal for the dialog for meta refreshes.
+ */
+add_task(async function test_redirect_principal() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+    await testOpenProto(browser, scheme, {
+      triggerLoad: useServerRedirect("meta-refresh"),
+      permDialogOptions: {
+        checkboxOrigin: ORIGIN1,
+        chooserIsNext: true,
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+});
+
+/**
+ * Test that we use the redirect principal for the dialog for JS redirects.
+ */
+add_task(async function test_redirect_principal_js() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+    await testOpenProto(browser, scheme, {
+      triggerLoad: () => {
+        let uri = `${scheme}://test`;
+
+        let innerParams = new URLSearchParams();
+        innerParams.set("uri", uri);
+        let params = new URLSearchParams();
+        params.set(
+          "uri",
+          "https://example.com/" +
+            ROOT_PATH +
+            "script_redirect.html?" +
+            innerParams.toString()
+        );
+        uri =
+          "https://example.org/" +
+          ROOT_PATH +
+          "script_redirect.html?" +
+          params.toString();
+        BrowserTestUtils.startLoadingURIString(browser, uri);
+      },
+      permDialogOptions: {
+        checkboxOrigin: ORIGIN1,
+        chooserIsNext: true,
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+});
+
+/**
+ * Test that we use the redirect principal for the dialog for link clicks.
+ */
+add_task(async function test_redirect_principal_links() {
+  let scheme = TEST_PROTOS[0];
+  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+    await testOpenProto(browser, scheme, {
+      triggerLoad: async () => {
+        let uri = `${scheme}://test`;
+
+        let params = new URLSearchParams();
+        params.set("uri", uri);
+        uri =
+          "https://example.com/" +
+          ROOT_PATH +
+          "redirect_helper.sjs?" +
+          params.toString();
+        await ContentTask.spawn(browser, { uri }, args => {
+          let textLink = content.document.createElement("a");
+          textLink.href = args.uri;
+          textLink.textContent = "click me";
+          content.document.body.appendChild(textLink);
+          textLink.click();
+        });
+      },
+      permDialogOptions: {
+        checkboxOrigin: ORIGIN1,
+        chooserIsNext: true,
+        hasCheckbox: true,
+        actionConfirm: false, // Cancel dialog
+      },
+    });
+  });
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_protocol_custom_sandbox.js b/uriloader/exthandler/tests/mochitest/browser_protocol_custom_sandbox.js
new file mode 100644
index 0000000000..bc4b13730a
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_protocol_custom_sandbox.js
@@ -0,0 +1,140 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests navigation to external protocol from sandboxed iframes.
+ */
+
+"use strict";
+
+requestLongerTimeout(2);
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["dom.block_external_protocol_navigation_from_sandbox", true]],
+  });
+
+  await setupMailHandler();
+});
+
+add_task(async function test_sandbox_disabled() {
+  await runExtProtocolSandboxTest({ blocked: false, sandbox: null });
+});
+
+add_task(async function test_sandbox_allowed() {
+  let flags = [
+    "allow-popups",
+    "allow-top-navigation",
+    "allow-top-navigation-by-user-activation",
+    "allow-top-navigation-to-custom-protocols",
+  ];
+
+  for (let flag of flags) {
+    await runExtProtocolSandboxTest({
+      blocked: false,
+      sandbox: `allow-scripts ${flag}`,
+    });
+  }
+});
+
+add_task(async function test_sandbox_blocked() {
+  let flags = [
+    "",
+    "allow-same-origin",
+    "allow-forms",
+    "allow-scripts",
+    "allow-pointer-lock",
+    "allow-orientation-lock",
+    "allow-modals",
+    "allow-popups-to-escape-sandbox",
+    "allow-presentation",
+    "allow-storage-access-by-user-activation",
+    "allow-downloads",
+  ];
+
+  for (let flag of flags) {
+    await runExtProtocolSandboxTest({
+      blocked: true,
+      sandbox: `allow-scripts ${flag}`,
+    });
+  }
+});
+
+add_task(async function test_sandbox_blocked_triggers() {
+  info(
+    "For sandboxed frames external protocol navigation is blocked, no matter how it is triggered."
+  );
+  for (let triggerMethod of [
+    "trustedClick",
+    "untrustedClick",
+    "trustedLocationAPI",
+    "untrustedLocationAPI",
+    "frameSrc",
+  ]) {
+    await runExtProtocolSandboxTest({
+      blocked: true,
+      sandbox: "allow-scripts",
+      triggerMethod,
+    });
+  }
+
+  info(
+    "When allow-top-navigation-by-user-activation navigation to external protocols with transient user activations is allowed."
+  );
+  await runExtProtocolSandboxTest({
+    blocked: false,
+    sandbox: "allow-scripts allow-top-navigation-by-user-activation",
+    triggerMethod: "trustedClick",
+  });
+
+  await runExtProtocolSandboxTest({
+    blocked: true,
+    sandbox: "allow-scripts allow-top-navigation-by-user-activation",
+    triggerMethod: "untrustedClick",
+  });
+
+  await runExtProtocolSandboxTest({
+    blocked: true,
+    sandbox: "allow-scripts allow-top-navigation-by-user-activation",
+    triggerMethod: "untrustedLocationAPI",
+  });
+
+  await runExtProtocolSandboxTest({
+    blocked: true,
+    sandbox: "allow-scripts allow-top-navigation-by-user-activation",
+    triggerMethod: "frameSrc",
+  });
+});
+
+add_task(async function test_sandbox_combination() {
+  await runExtProtocolSandboxTest({
+    blocked: false,
+    sandbox:
+      "allow-scripts allow-downloads allow-top-navigation-to-custom-protocols",
+  });
+
+  await runExtProtocolSandboxTest({
+    blocked: false,
+    sandbox:
+      "allow-scripts allow-top-navigation allow-top-navigation-to-custom-protocols",
+  });
+
+  await runExtProtocolSandboxTest({
+    blocked: true,
+    sandbox: "allow-scripts allow-modals",
+  });
+});
+
+add_task(async function test_sandbox_iframe_redirect() {
+  await runExtProtocolSandboxTest({
+    blocked: true,
+    sandbox: "allow-scripts",
+    triggerMethod: "frameSrcRedirect",
+  });
+
+  await runExtProtocolSandboxTest({
+    blocked: false,
+    sandbox: "allow-scripts allow-top-navigation-to-custom-protocols",
+    triggerMethod: "frameSrcRedirect",
+  });
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_protocol_custom_sandbox_csp.js b/uriloader/exthandler/tests/mochitest/browser_protocol_custom_sandbox_csp.js
new file mode 100644
index 0000000000..03d2fe8cf5
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_protocol_custom_sandbox_csp.js
@@ -0,0 +1,38 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests navigation to external protocol from csp-sandboxed iframes.
+ */
+
+"use strict";
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["dom.block_external_protocol_navigation_from_sandbox", true]],
+  });
+
+  await setupMailHandler();
+});
+
+add_task(async function test_sandbox_csp() {
+  for (let triggerMethod of [
+    "trustedClick",
+    "untrustedClick",
+    "trustedLocationAPI",
+    "untrustedLocationAPI",
+  ]) {
+    await runExtProtocolSandboxTest({
+      blocked: false,
+      sandbox: "allow-scripts",
+      useCSPSandbox: true,
+      triggerMethod,
+    });
+  }
+
+  await runExtProtocolSandboxTest({
+    blocked: false,
+    sandbox: "allow-scripts allow-top-navigation-to-custom-protocols",
+    useCSPSandbox: true,
+  });
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_protocolhandler_loop.js b/uriloader/exthandler/tests/mochitest/browser_protocolhandler_loop.js
new file mode 100644
index 0000000000..d906d9c55b
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_protocolhandler_loop.js
@@ -0,0 +1,77 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_helperapp() {
+  // Set up the test infrastructure:
+  const kProt = "foopydoopydoo";
+  const extProtocolSvc = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  const handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+  let handlerInfo = extProtocolSvc.getProtocolHandlerInfo(kProt);
+  if (handlerSvc.exists(handlerInfo)) {
+    handlerSvc.fillHandlerInfo(handlerInfo, "");
+  }
+  // Say we want to use a specific app:
+  handlerInfo.preferredAction = Ci.nsIHandlerInfo.useHelperApp;
+  handlerInfo.alwaysAskBeforeHandling = false;
+
+  // Say it's us:
+  let selfFile = Services.dirsvc.get("XREExeF", Ci.nsIFile);
+  // Make sure it's the .app
+  if (AppConstants.platform == "macosx") {
+    while (
+      !selfFile.leafName.endsWith(".app") &&
+      !selfFile.leafName.endsWith(".app/")
+    ) {
+      selfFile = selfFile.parent;
+    }
+  }
+  let selfHandlerApp = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  selfHandlerApp.executable = selfFile;
+  handlerInfo.possibleApplicationHandlers.appendElement(selfHandlerApp);
+  handlerInfo.preferredApplicationHandler = selfHandlerApp;
+  handlerSvc.store(handlerInfo);
+
+  await BrowserTestUtils.withNewTab("about:blank", async browser => {
+    // Now, do some safety stubbing. If we do end up recursing we spawn
+    // infinite tabs. We definitely don't want that. Avoid it by stubbing
+    // our external URL handling bits:
+    let oldAddTab = gBrowser.addTab;
+    registerCleanupFunction(() => (gBrowser.addTab = oldAddTab));
+    let wrongThingHappenedPromise = new Promise(resolve => {
+      gBrowser.addTab = function (aURI) {
+        ok(false, "Tried to open unexpected URL in a tab: " + aURI);
+        resolve(null);
+        // Pass a dummy object to avoid upsetting BrowserContentHandler -
+        // if it thinks opening the tab failed, it tries to open a window instead,
+        // which we can't prevent as easily, and at which point we still end up
+        // with runaway tabs.
+        return {};
+      };
+    });
+
+    let askedUserPromise = waitForProtocolAppChooserDialog(browser, true);
+
+    gBrowser.fixupAndLoadURIString(kProt + ":test", {
+      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+      loadFlags: Ci.nsIWebNavigation.LOAD_FLAGS_FROM_EXTERNAL,
+    });
+    let dialog = await Promise.race([
+      wrongThingHappenedPromise,
+      askedUserPromise,
+    ]);
+    ok(dialog, "Should have gotten a dialog");
+
+    let closePromise = waitForProtocolAppChooserDialog(browser, false);
+    dialog.close();
+    await closePromise;
+    askedUserPromise = null;
+  });
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_remember_download_option.js b/uriloader/exthandler/tests/mochitest/browser_remember_download_option.js
new file mode 100644
index 0000000000..f3fedab69c
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_remember_download_option.js
@@ -0,0 +1,61 @@
+add_task(async function () {
+  // create mocked objects
+  let launcher = createMockedObjects(true);
+
+  // open helper app dialog with mocked launcher
+  let dlg = await openHelperAppDialog(launcher);
+
+  let doc = dlg.document;
+  let dialogElement = doc.getElementById("unknownContentType");
+
+  // Set remember choice
+  ok(
+    !doc.getElementById("rememberChoice").checked,
+    "Remember choice checkbox should be not checked."
+  );
+  doc.getElementById("rememberChoice").checked = true;
+
+  // Make sure the mock handler information is not in nsIHandlerService
+  ok(
+    !gHandlerSvc.exists(launcher.MIMEInfo),
+    "Should not be in nsIHandlerService."
+  );
+
+  // close the dialog by pushing the ok button.
+  let dialogClosedPromise = BrowserTestUtils.windowClosed(dlg);
+  // Make sure the ok button is enabled, since the ok button might be disabled by
+  // EnableDelayHelper mechanism. Please refer the detailed
+  // https://searchfox.org/mozilla-central/source/toolkit/components/prompts/src/PromptUtils.sys.mjs#51
+  dialogElement.getButton("accept").disabled = false;
+  dialogElement.acceptDialog();
+  await dialogClosedPromise;
+
+  // check the mocked handler information is saved in nsIHandlerService
+  ok(gHandlerSvc.exists(launcher.MIMEInfo), "Should be in nsIHandlerService.");
+  // check the extension.
+  var mimeType = gHandlerSvc.getTypeFromExtension("abc");
+  is(mimeType, launcher.MIMEInfo.type, "Got correct mime type.");
+  for (let handlerInfo of gHandlerSvc.enumerate()) {
+    if (handlerInfo.type == launcher.MIMEInfo.type) {
+      // check the alwaysAskBeforeHandling
+      ok(
+        !handlerInfo.alwaysAskBeforeHandling,
+        "Should turn off the always ask."
+      );
+      // check the preferredApplicationHandler
+      ok(
+        handlerInfo.preferredApplicationHandler.equals(
+          launcher.MIMEInfo.preferredApplicationHandler
+        ),
+        "Should be equal to the mockedHandlerApp."
+      );
+      // check the perferredAction
+      is(
+        handlerInfo.preferredAction,
+        launcher.MIMEInfo.preferredAction,
+        "Should be equal to Ci.nsIHandlerInfo.useHelperApp."
+      );
+      break;
+    }
+  }
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_save_filenames.js b/uriloader/exthandler/tests/mochitest/browser_save_filenames.js
new file mode 100644
index 0000000000..46750d5c87
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_save_filenames.js
@@ -0,0 +1,860 @@
+// There are at least seven different ways in a which a file can be saved or downloaded. This
+// test ensures that the filename is determined correctly when saving in these ways. The seven
+// ways are:
+//   - save the file individually from the File menu
+//   - save as complete web page (this uses a different codepath than the previous one)
+//   - dragging an image to the local file system
+//   - copy an image and paste it as a file to the local file system (windows only)
+//   - open a link with content-disposition set to attachment
+//   - open a link with the download attribute
+//   - save a link or image from the context menu
+
+requestLongerTimeout(8);
+
+let types = {
+  text: "text/plain",
+  html: "text/html",
+  png: "image/png",
+  jpeg: "image/jpeg",
+  webp: "image/webp",
+  otherimage: "image/unknown",
+  // Other js types (application/javascript and text/javascript) are handled by the system
+  // inconsistently, but application/x-javascript is handled by the external helper app service,
+  // so it is used here for this test.
+  js: "application/x-javascript",
+  binary: "application/octet-stream",
+  nonsense: "application/x-nonsense",
+  zip: "application/zip",
+  json: "application/json",
+  tar: "application/x-tar",
+};
+
+const PNG_DATA = atob(
+  "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEUAA" +
+    "ACnej3aAAAAAXRSTlMAQObYZgAAAApJREFUCNdjYAAAAAIAAeIhvDMAAAAASUVORK5CYII="
+);
+
+const JPEG_DATA = atob(
+  "/9j/4AAQSkZJRgABAQEAYABgAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRofHh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4z" +
+    "NDL/2wBDAQkJCQwLDBgNDRgyIRwhMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjL/wAARCAABAAEDASIAAhEB" +
+    "AxEB/8QAHwAAAQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQRBRIhMUEGE1FhByJxFDKBkaEII0KxwRVS" +
+    "0fAkM2JyggkKFhcYGRolJicoKSo0NTY3ODk6Q0RFRkdISUpTVFVWV1hZWmNkZWZnaGlqc3R1dnd4eXqDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ipqrKz" +
+    "tLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uHi4+Tl5ufo6erx8vP09fb3+Pn6/8QAHwEAAwEBAQEBAQEBAQAAAAAAAAECAwQFBgcICQoL/8QAtREAAgEC" +
+    "BAQDBAcFBAQAAQJ3AAECAxEEBSExBhJBUQdhcRMiMoEIFEKRobHBCSMzUvAVYnLRChYkNOEl8RcYGRomJygpKjU2Nzg5OkNERUZHSElKU1RVVldYWVpj" +
+    "ZGVmZ2hpanN0dXZ3eHl6goOEhYaHiImKkpOUlZaXmJmaoqOkpaanqKmqsrO0tba3uLm6wsPExcbHyMnK0tPU1dbX2Nna4uPk5ebn6Onq8vP09fb3+Pn6" +
+    "/9oADAMBAAIRAxEAPwD3+iiigD//2Q=="
+);
+
+const WEBP_DATA = atob(
+  "UklGRiIAAABXRUJQVlA4TBUAAAAvY8AYAAfQ/4j+B4CE8H+/ENH/VCIA"
+);
+
+const DEFAULT_FILENAME =
+  AppConstants.platform == "win" ? "Untitled.htm" : "Untitled.html";
+
+const PROMISE_FILENAME_TYPE = "application/x-moz-file-promise-dest-filename";
+
+let MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+
+let expectedItems;
+let sendAsAttachment = false;
+let httpServer = null;
+
+function handleRequest(aRequest, aResponse) {
+  const queryString = new URLSearchParams(aRequest.queryString);
+  let type = queryString.get("type");
+  let filename = queryString.get("filename");
+  let dispname = queryString.get("dispname");
+
+  aResponse.setStatusLine(aRequest.httpVersion, 200);
+  if (type) {
+    aResponse.setHeader("Content-Type", types[type]);
+  }
+
+  if (dispname) {
+    let dispositionType = sendAsAttachment ? "attachment" : "inline";
+    aResponse.setHeader(
+      "Content-Disposition",
+      dispositionType + ';name="' + dispname + '"'
+    );
+  } else if (filename) {
+    let dispositionType = sendAsAttachment ? "attachment" : "inline";
+    aResponse.setHeader(
+      "Content-Disposition",
+      dispositionType + ';filename="' + filename + '"'
+    );
+  } else if (sendAsAttachment) {
+    aResponse.setHeader("Content-Disposition", "attachment");
+  }
+
+  if (type == "png") {
+    aResponse.write(PNG_DATA);
+  } else if (type == "jpeg") {
+    aResponse.write(JPEG_DATA);
+  } else if (type == "webp") {
+    aResponse.write(WEBP_DATA);
+  } else if (type == "html") {
+    aResponse.write(
+      "<html><head><title>file.inv</title></head><body>File</body></html>"
+    );
+  } else {
+    aResponse.write("// Some Text");
+  }
+}
+
+function handleBasicImageRequest(aRequest, aResponse) {
+  aResponse.setHeader("Content-Type", "image/png");
+  aResponse.write(PNG_DATA);
+}
+
+function handleRedirect(aRequest, aResponse) {
+  const queryString = new URLSearchParams(aRequest.queryString);
+  let filename = queryString.get("filename");
+
+  aResponse.setStatusLine(aRequest.httpVersion, 302);
+  aResponse.setHeader("Location", "/bell" + filename[0] + "?" + queryString);
+}
+
+// nsIFile::CreateUnique crops long filenames if the path is too long, but
+// we don't know exactly how long depending on the full path length, so
+// for those save methods that use CreateUnique, instead just verify that
+// the filename starts with the right string and has the correct extension.
+function checkShortenedFilename(actual, expected) {
+  if (actual.length < expected.length) {
+    let actualDot = actual.lastIndexOf(".");
+    let actualExtension = actual.substring(actualDot);
+    let expectedExtension = expected.substring(expected.lastIndexOf("."));
+    if (
+      actualExtension == expectedExtension &&
+      expected.startsWith(actual.substring(0, actualDot))
+    ) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+add_setup(async function () {
+  const { HttpServer } = ChromeUtils.importESModule(
+    "resource://testing-common/httpd.sys.mjs"
+  );
+  httpServer = new HttpServer();
+  httpServer.start(8000);
+
+  // Need to load the page from localhost:8000 as the download attribute
+  // only applies to links from the same domain.
+  let saveFilenamesPage = await IOUtils.getFile(
+    Services.dirsvc.get("CurWorkD", Ci.nsIFile).path,
+    ..."browser/uriloader/exthandler/tests/mochitest/save_filenames.html".split(
+      "/"
+    )
+  );
+  httpServer.registerFile("/save_filenames.html", saveFilenamesPage);
+
+  // A variety of different scripts are set up to better ensure uniqueness.
+  httpServer.registerPathHandler("/save_filename.sjs", handleRequest);
+  httpServer.registerPathHandler("/save_thename.sjs", handleRequest);
+  httpServer.registerPathHandler("/getdata.png", handleRequest);
+  httpServer.registerPathHandler("/base", handleRequest);
+  httpServer.registerPathHandler("/basedata", handleRequest);
+  httpServer.registerPathHandler("/basetext", handleRequest);
+  httpServer.registerPathHandler("/text2.txt", handleRequest);
+  httpServer.registerPathHandler("/text3.gonk", handleRequest);
+  httpServer.registerPathHandler("/basic.png", handleBasicImageRequest);
+  httpServer.registerPathHandler("/aquamarine.jpeg", handleBasicImageRequest);
+  httpServer.registerPathHandler("/lazuli.exe", handleBasicImageRequest);
+  httpServer.registerPathHandler("/redir", handleRedirect);
+  httpServer.registerPathHandler("/bellr", handleRequest);
+  httpServer.registerPathHandler("/bellg", handleRequest);
+  httpServer.registerPathHandler("/bellb", handleRequest);
+  httpServer.registerPathHandler("/executable.exe", handleRequest);
+
+  await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "http://localhost:8000/save_filenames.html"
+  );
+
+  expectedItems = await getItems("items");
+});
+
+function getItems(parentid) {
+  return SpecialPowers.spawn(
+    gBrowser.selectedBrowser,
+    [parentid, AppConstants.platform],
+    (id, platform) => {
+      let elements = [];
+      let elem = content.document.getElementById(id).firstElementChild;
+      while (elem) {
+        let filename =
+          elem.dataset["filenamePlatform" + platform] || elem.dataset.filename;
+        let url = elem.getAttribute("src") || elem.getAttribute("href");
+        let draggable =
+          elem.localName == "img" && elem.dataset.nodrag != "true";
+        let unknown = elem.dataset.unknown;
+        let noattach = elem.dataset.noattach;
+        let savepagename = elem.dataset.savepagename;
+        let pickedfilename = elem.dataset.pickedfilename;
+        elements.push({
+          draggable,
+          unknown,
+          filename,
+          url,
+          noattach,
+          savepagename,
+          pickedfilename,
+        });
+        elem = elem.nextElementSibling;
+      }
+      return elements;
+    }
+  );
+}
+
+function getDirectoryEntries(dir) {
+  let files = [];
+  let entries = dir.directoryEntries;
+  while (true) {
+    let file = entries.nextFile;
+    if (!file) {
+      break;
+    }
+    files.push(file.leafName);
+  }
+  entries.close();
+  return files;
+}
+
+// This test saves the document as a complete web page and verifies
+// that the resources are saved with the correct filename.
+add_task(async function save_document() {
+  let browser = gBrowser.selectedBrowser;
+
+  let tmp = SpecialPowers.Services.dirsvc.get("TmpD", Ci.nsIFile);
+  const baseFilename = "test_save_filenames_" + Date.now();
+
+  let tmpFile = tmp.clone();
+  tmpFile.append(baseFilename + "_document.html");
+  let tmpDir = tmp.clone();
+  tmpDir.append(baseFilename + "_document_files");
+
+  MockFilePicker.displayDirectory = tmpDir;
+  MockFilePicker.showCallback = function (fp) {
+    MockFilePicker.setFiles([tmpFile]);
+    MockFilePicker.filterIndex = 0; // kSaveAsType_Complete
+  };
+
+  let downloadsList = await Downloads.getList(Downloads.PUBLIC);
+  let savePromise = promiseDownloadFinished(downloadsList);
+  saveBrowser(browser);
+  await savePromise;
+
+  let filesSaved = getDirectoryEntries(tmpDir);
+
+  for (let idx = 0; idx < expectedItems.length; idx++) {
+    let filename = expectedItems[idx].filename;
+    if (idx == 66 && AppConstants.platform == "win") {
+      // This is special-cased on Windows. The default filename will be used, since
+      // the filename is invalid, but since the previous test file has the same issue,
+      // this second file will be saved with a number suffix added to it.
+      filename = "Untitled_002";
+    }
+
+    let file = tmpDir.clone();
+    file.append(filename);
+
+    let fileIdx = -1;
+    // Use checkShortenedFilename to check long filenames.
+    if (filename.length > 240) {
+      for (let t = 0; t < filesSaved.length; t++) {
+        if (
+          filesSaved[t].length > 60 &&
+          checkShortenedFilename(filesSaved[t], filename)
+        ) {
+          fileIdx = t;
+          break;
+        }
+      }
+    } else {
+      fileIdx = filesSaved.indexOf(filename);
+    }
+
+    ok(
+      fileIdx >= 0,
+      "file i" +
+        idx +
+        " " +
+        filename +
+        " was saved with the correct name using saveDocument"
+    );
+    if (fileIdx >= 0) {
+      // If found, remove it from the list. At end of the test, the
+      // list should be empty.
+      filesSaved.splice(fileIdx, 1);
+    }
+  }
+
+  is(filesSaved.length, 0, "all files accounted for");
+  tmpDir.remove(true);
+  tmpFile.remove(false);
+  downloadsList.removeFinished();
+});
+
+// This test simulates dragging the images in the document and ensuring that
+// the correct filename is used for each one.
+// On Mac, the data is added in the parent process instead, so we cannot
+// test dragging directly.
+if (AppConstants.platform != "macosx") {
+  add_task(async function drag_files() {
+    let browser = gBrowser.selectedBrowser;
+
+    await SpecialPowers.spawn(browser, [PROMISE_FILENAME_TYPE], type => {
+      content.addEventListener("dragstart", event => {
+        content.draggedFile = event.dataTransfer.getData(type);
+        event.preventDefault();
+      });
+    });
+
+    for (let idx = 0; idx < expectedItems.length; idx++) {
+      if (!expectedItems[idx].draggable) {
+        // You can't drag non-images and invalid images.
+        continue;
+      }
+
+      await BrowserTestUtils.synthesizeMouse(
+        "#i" + idx,
+        1,
+        1,
+        { type: "mousedown" },
+        browser
+      );
+      await BrowserTestUtils.synthesizeMouse(
+        "#i" + idx,
+        11,
+        11,
+        { type: "mousemove" },
+        browser
+      );
+      await BrowserTestUtils.synthesizeMouse(
+        "#i" + idx,
+        20,
+        20,
+        { type: "mousemove" },
+        browser
+      );
+      await BrowserTestUtils.synthesizeMouse(
+        "#i" + idx,
+        20,
+        20,
+        { type: "mouseup" },
+        browser
+      );
+
+      let draggedFile = await SpecialPowers.spawn(browser, [], () => {
+        let file = content.draggedFile;
+        content.draggedFile = null;
+        return file;
+      });
+
+      is(
+        draggedFile,
+        expectedItems[idx].filename,
+        "i" +
+          idx +
+          " " +
+          expectedItems[idx].filename +
+          " was saved with the correct name when dragging"
+      );
+    }
+  });
+}
+
+// This test checks that copying an image provides the right filename
+// for pasting to the local file system. This is only implemented on Windows.
+const imageAsFileEnabled = SpecialPowers.getBoolPref(
+  "clipboard.imageAsFile.enabled",
+  false
+);
+if (AppConstants.platform == "win" && imageAsFileEnabled) {
+  add_task(async function copy_image() {
+    for (let idx = 0; idx < expectedItems.length; idx++) {
+      if (!expectedItems[idx].draggable) {
+        // You can't context-click on non-images.
+        continue;
+      }
+
+      let data = await SpecialPowers.spawn(
+        gBrowser.selectedBrowser,
+        [idx, PROMISE_FILENAME_TYPE],
+        (imagenum, type) => {
+          // No need to wait for the data to be really on the clipboard, we only
+          // need the promise data added when the command is performed.
+          SpecialPowers.setCommandNode(
+            content,
+            content.document.getElementById("i" + imagenum)
+          );
+          SpecialPowers.doCommand(content, "cmd_copyImageContents");
+
+          return SpecialPowers.getClipboardData(type);
+        }
+      );
+
+      is(
+        data,
+        expectedItems[idx].filename,
+        "i" +
+          idx +
+          " " +
+          expectedItems[idx].filename +
+          " was saved with the correct name when copying"
+      );
+    }
+  });
+}
+
+// This test checks the default filename selected when selecting to save
+// a file from either the context menu or what would happen when save page
+// as was selected from the file menu. Note that this tests a filename assigned
+// when using content-disposition: inline.
+add_task(async function saveas_files() {
+  // Iterate over each item and try saving them from the context menu,
+  // and the Save Page As command on the File menu.
+  for (let testname of ["context menu", "save page as"]) {
+    for (let idx = 0; idx < expectedItems.length; idx++) {
+      let menu;
+      if (testname == "context menu") {
+        if (!expectedItems[idx].draggable) {
+          // You can't context-click on non-images.
+          continue;
+        }
+
+        menu = document.getElementById("contentAreaContextMenu");
+        let popupShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
+        BrowserTestUtils.synthesizeMouse(
+          "#i" + idx,
+          5,
+          5,
+          { type: "contextmenu", button: 2 },
+          gBrowser.selectedBrowser
+        );
+        await popupShown;
+      } else {
+        if (expectedItems[idx].unknown == "typeonly") {
+          // Items marked with unknown="typeonly" have unknown content types and
+          // will be downloaded instead of opened in a tab.
+          let list = await Downloads.getList(Downloads.PUBLIC);
+          let downloadFinishedPromise = promiseDownloadFinished(list);
+
+          await BrowserTestUtils.openNewForegroundTab({
+            gBrowser,
+            opening: expectedItems[idx].url,
+            waitForLoad: false,
+            waitForStateStop: true,
+          });
+
+          let download = await downloadFinishedPromise;
+
+          let filename = PathUtils.filename(download.target.path);
+
+          let expectedFilename = expectedItems[idx].filename;
+          if (expectedFilename.length > 240) {
+            ok(
+              checkShortenedFilename(filename, expectedFilename),
+              "open link" +
+                idx +
+                " " +
+                expectedFilename +
+                " was downloaded with the correct name when opened as a url (with long name)"
+            );
+          } else {
+            is(
+              filename,
+              expectedFilename,
+              "open link" +
+                idx +
+                " " +
+                expectedFilename +
+                " was downloaded with the correct name when opened as a url"
+            );
+          }
+
+          try {
+            await IOUtils.remove(download.target.path);
+          } catch (ex) {}
+
+          await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+          continue;
+        }
+
+        await BrowserTestUtils.openNewForegroundTab({
+          gBrowser,
+          opening: expectedItems[idx].url,
+          waitForLoad: false,
+          waitForStateStop: true,
+        });
+      }
+
+      let filename = await new Promise(resolve => {
+        MockFilePicker.showCallback = function (fp) {
+          setTimeout(() => {
+            resolve(fp.defaultString);
+          }, 0);
+          return Ci.nsIFilePicker.returnCancel;
+        };
+
+        if (testname == "context menu") {
+          let menuitem = document.getElementById("context-saveimage");
+          menu.activateItem(menuitem);
+        } else if (testname == "save page as") {
+          document.getElementById("Browser:SavePage").doCommand();
+        }
+      });
+
+      // Trying to open an unknown or binary type will just open a blank
+      // page, so trying to save will just save the blank page with the
+      // filename Untitled.html.
+      let expectedFilename = expectedItems[idx].unknown
+        ? DEFAULT_FILENAME
+        : expectedItems[idx].savepagename || expectedItems[idx].filename;
+
+      // When saving via contentAreaUtils.js, the content disposition name
+      // field is used as an alternate.
+      if (expectedFilename == "save_thename.png") {
+        expectedFilename = "withname.png";
+      }
+
+      is(
+        filename,
+        expectedFilename,
+        "i" +
+          idx +
+          " " +
+          expectedFilename +
+          " was saved with the correct name " +
+          testname
+      );
+
+      if (testname == "save page as") {
+        await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+      }
+    }
+  }
+});
+
+// This test checks that the filename is saved correctly when it
+// has been modified within the file picker.
+add_task(async function saveas_files_modified_in_filepicker() {
+  let items = await getItems("modifieditems");
+  for (let idx = 0; idx < items.length; idx++) {
+    await BrowserTestUtils.openNewForegroundTab({
+      gBrowser,
+      opening: items[idx].url,
+      waitForLoad: false,
+      waitForStateStop: true,
+    });
+
+    let savedFile = SpecialPowers.Services.dirsvc.get("TmpD", Ci.nsIFile);
+
+    let downloadsList = await Downloads.getList(Downloads.PUBLIC);
+    let savePromise = promiseDownloadFinished(downloadsList);
+
+    await new Promise(resolve => {
+      MockFilePicker.displayDirectory = savedFile;
+
+      MockFilePicker.showCallback = function (fp) {
+        MockFilePicker.filterIndex = 0; // kSaveAsType_Complete
+        savedFile.append(items[idx].pickedfilename);
+        MockFilePicker.setFiles([savedFile]);
+        setTimeout(() => {
+          resolve(items[idx].pickedfilename);
+        }, 0);
+
+        return Ci.nsIFilePicker.returnOK;
+      };
+
+      document.getElementById("Browser:SavePage").doCommand();
+    });
+
+    await savePromise;
+
+    savedFile.leafName = items[idx].filename;
+    ok(
+      savedFile.exists(),
+      "i" +
+        idx +
+        " '" +
+        savedFile.leafName +
+        "' was saved when modified with the correct name "
+    );
+    if (savedFile.exists()) {
+      savedFile.remove(false);
+    }
+
+    await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+  }
+});
+
+// This test checks that links that result in files with
+// content-disposition: attachment are saved with the right filenames.
+add_task(async function save_links() {
+  sendAsAttachment = true;
+
+  // Create some links based on each image and insert them into the document.
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], () => {
+    let doc = content.document;
+    let insertPos = doc.getElementById("attachment-links");
+
+    let idx = 0;
+    let elem = doc.getElementById("items").firstElementChild;
+    while (elem) {
+      let attachmentlink = doc.createElement("a");
+      attachmentlink.id = "attachmentlink" + idx;
+      attachmentlink.href = elem.localName == "object" ? elem.data : elem.src;
+      attachmentlink.textContent = elem.dataset.filename;
+      insertPos.appendChild(attachmentlink);
+      insertPos.appendChild(doc.createTextNode("  "));
+
+      elem = elem.nextElementSibling;
+      idx++;
+    }
+  });
+
+  let list = await Downloads.getList(Downloads.PUBLIC);
+
+  for (let idx = 0; idx < expectedItems.length; idx++) {
+    // Skip the items that won't have a content-disposition.
+    if (expectedItems[idx].noattach) {
+      continue;
+    }
+
+    let downloadFinishedPromise = promiseDownloadFinished(list);
+
+    BrowserTestUtils.synthesizeMouse(
+      "#attachmentlink" + idx,
+      5,
+      5,
+      {},
+      gBrowser.selectedBrowser
+    );
+
+    let download = await downloadFinishedPromise;
+
+    let filename = PathUtils.filename(download.target.path);
+
+    let expectedFilename = expectedItems[idx].filename;
+    // Use checkShortenedFilename to check long filenames.
+    if (expectedItems[idx].filename.length > 240) {
+      ok(
+        checkShortenedFilename(filename, expectedFilename),
+        "attachmentlink" +
+          idx +
+          " " +
+          expectedFilename +
+          " was saved with the correct name when opened as attachment (with long name)"
+      );
+    } else {
+      is(
+        filename,
+        expectedFilename,
+        "attachmentlink" +
+          idx +
+          " " +
+          expectedFilename +
+          " was saved with the correct name when opened as attachment"
+      );
+    }
+
+    try {
+      await IOUtils.remove(download.target.path);
+    } catch (ex) {}
+  }
+
+  sendAsAttachment = false;
+});
+
+// This test checks some cases where links to images are saved using Save Link As,
+// and when opening them in a new tab and then using Save Page As.
+add_task(async function saveas_image_links() {
+  let links = await getItems("links");
+
+  // Iterate over each link and try saving the links from the context menu,
+  // and then after opening a new tab for that link and then selecting
+  // the Save Page As command on the File menu.
+  for (let testname of ["save link as", "save link then save page as"]) {
+    for (let idx = 0; idx < links.length; idx++) {
+      let menu = document.getElementById("contentAreaContextMenu");
+      let popupShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
+      BrowserTestUtils.synthesizeMouse(
+        "#link" + idx,
+        5,
+        5,
+        { type: "contextmenu", button: 2 },
+        gBrowser.selectedBrowser
+      );
+      await popupShown;
+
+      let promptPromise = new Promise(resolve => {
+        MockFilePicker.showCallback = function (fp) {
+          setTimeout(() => {
+            resolve(fp.defaultString);
+          }, 0);
+          return Ci.nsIFilePicker.returnCancel;
+        };
+      });
+
+      if (testname == "save link as") {
+        let menuitem = document.getElementById("context-savelink");
+        menu.activateItem(menuitem);
+      } else {
+        let newTabPromise = BrowserTestUtils.waitForNewTab(gBrowser);
+
+        let menuitem = document.getElementById("context-openlinkintab");
+        menu.activateItem(menuitem);
+
+        let tab = await newTabPromise;
+        await BrowserTestUtils.switchTab(gBrowser, tab);
+
+        document.getElementById("Browser:SavePage").doCommand();
+      }
+
+      let filename = await promptPromise;
+
+      let expectedFilename = links[idx].filename;
+      // Only codepaths that go through contentAreaUtils.js use the
+      // name from the content disposition.
+      if (testname == "save link as" && expectedFilename == "four.png") {
+        expectedFilename = "save_filename.png";
+      }
+
+      is(
+        filename,
+        expectedFilename,
+        "i" +
+          idx +
+          " " +
+          expectedFilename +
+          " link was saved with the correct name " +
+          testname
+      );
+
+      if (testname == "save link then save page as") {
+        await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+      }
+    }
+  }
+});
+
+// This test checks that links that with a download attribute
+// are saved with the right filenames.
+add_task(async function save_download_links() {
+  let downloads = await getItems("downloads");
+
+  let list = await Downloads.getList(Downloads.PUBLIC);
+  for (let idx = 0; idx < downloads.length; idx++) {
+    let downloadFinishedPromise = promiseDownloadFinished(list);
+
+    BrowserTestUtils.synthesizeMouse(
+      "#download" + idx,
+      2,
+      2,
+      {},
+      gBrowser.selectedBrowser
+    );
+
+    let download = await downloadFinishedPromise;
+
+    let filename = PathUtils.filename(download.target.path);
+
+    if (downloads[idx].filename.length > 240) {
+      ok(
+        checkShortenedFilename(filename, downloads[idx].filename),
+        "download" +
+          idx +
+          " " +
+          downloads[idx].filename +
+          " was saved with the correct name when link has download attribute"
+      );
+    } else {
+      if (idx == 66 && filename == "Untitled(1)") {
+        // Sometimes, the previous test's file still exists or wasn't created in time
+        // and a non-duplicated name is created. Allow this rather than figuring out
+        // how to avoid it since it doesn't affect what is being tested here.
+        filename = "Untitled";
+      }
+
+      is(
+        filename,
+        downloads[idx].filename,
+        "download" +
+          idx +
+          " " +
+          downloads[idx].filename +
+          " was saved with the correct name when link has download attribute"
+      );
+    }
+
+    try {
+      await IOUtils.remove(download.target.path);
+    } catch (ex) {}
+  }
+});
+
+// This test verifies that invalid extensions are not removed when they
+// have been entered in the file picker.
+add_task(async function save_page_with_invalid_after_filepicker() {
+  await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "http://localhost:8000/save_filename.sjs?type=html&filename=invfile.lnk"
+  );
+
+  let filename = await new Promise(resolve => {
+    MockFilePicker.showCallback = function (fp) {
+      let expectedFilename =
+        AppConstants.platform == "win" ? "invfile.lnk.htm" : "invfile.lnk.html";
+      is(fp.defaultString, expectedFilename, "supplied filename is correct");
+      setTimeout(() => {
+        resolve("otherfile.local");
+      }, 0);
+      return Ci.nsIFilePicker.returnCancel;
+    };
+
+    document.getElementById("Browser:SavePage").doCommand();
+  });
+
+  is(filename, "otherfile.local", "lnk extension has been preserved");
+
+  await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+});
+
+add_task(async function save_page_with_invalid_extension() {
+  await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "http://localhost:8000/save_filename.sjs?type=html"
+  );
+
+  let filename = await new Promise(resolve => {
+    MockFilePicker.showCallback = function (fp) {
+      setTimeout(() => {
+        resolve(fp.defaultString);
+      }, 0);
+      return Ci.nsIFilePicker.returnCancel;
+    };
+
+    document.getElementById("Browser:SavePage").doCommand();
+  });
+
+  is(
+    filename,
+    AppConstants.platform == "win" ? "file.inv.htm" : "file.inv.html",
+    "html extension has been added"
+  );
+
+  await BrowserTestUtils.removeTab(gBrowser.selectedTab);
+});
+
+add_task(async () => {
+  BrowserTestUtils.removeTab(gBrowser.selectedTab);
+  MockFilePicker.cleanup();
+  await new Promise(resolve => httpServer.stop(resolve));
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_shows_where_to_save_dialog.js b/uriloader/exthandler/tests/mochitest/browser_shows_where_to_save_dialog.js
new file mode 100644
index 0000000000..6bf375dfff
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_shows_where_to_save_dialog.js
@@ -0,0 +1,303 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { DownloadIntegration } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+const { handleInternally, useHelperApp, useSystemDefault, saveToDisk } =
+  Ci.nsIHandlerInfo;
+
+let MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["browser.download.always_ask_before_handling_new_types", false],
+      ["browser.download.useDownloadDir", false],
+    ],
+  });
+
+  registerCleanupFunction(async () => {
+    let hiddenPromise = BrowserTestUtils.waitForEvent(
+      DownloadsPanel.panel,
+      "popuphidden"
+    );
+    DownloadsPanel.hidePanel();
+    await hiddenPromise;
+    MockFilePicker.cleanup();
+  });
+});
+
+// This test ensures that a "Save as..." filepicker dialog is shown for a file
+// if useDownloadDir ("Always ask where to save files") is set to false and
+// the filetype is set to save to disk.
+add_task(async function aDownloadSavedToDiskPromptsForFolder() {
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  ensureMIMEState(
+    { preferredAction: saveToDisk },
+    { type: "text/plain", ext: "txt" }
+  );
+  registerCleanupFunction(async () => {
+    await publicList.removeFinished();
+  });
+  let filePickerShownPromise = new Promise(resolve => {
+    MockFilePicker.showCallback = function (fp) {
+      setTimeout(resolve, 0);
+      return Ci.nsIFilePicker.returnCancel;
+    };
+  });
+
+  let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_txt_attachment_test.txt",
+    waitForLoad: false,
+    waitForStateStop: true,
+  });
+
+  info("Waiting on filepicker.");
+  await filePickerShownPromise;
+  ok(true, "filepicker should have been shown");
+
+  BrowserTestUtils.removeTab(loadingTab);
+});
+
+// This test ensures that downloads configured to open internally create only
+// one file destination when saved via the filepicker, and don't prompt.
+add_task(async function testFilesHandledInternally() {
+  let dir = await setupFilePickerDirectory();
+
+  ensureMIMEState(
+    { preferredAction: handleInternally },
+    { type: "image/webp", ext: "webp" }
+  );
+
+  let filePickerShown = false;
+  MockFilePicker.showCallback = function (fp) {
+    filePickerShown = true;
+    return Ci.nsIFilePicker.returnCancel;
+  };
+
+  let thirdTabPromise = BrowserTestUtils.waitForNewTab(
+    gBrowser,
+    url => {
+      info("Got load for " + url);
+      return url.endsWith("file_green.webp") && url.startsWith("file:");
+    },
+    true,
+    true
+  );
+  let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_green.webp",
+    waitForLoad: false,
+    waitForStateStop: true,
+  });
+
+  let openedTab = await thirdTabPromise;
+  ok(!filePickerShown, "file picker should not have shown up.");
+
+  assertCorrectFile(dir, "file_green.webp");
+
+  // Cleanup
+  BrowserTestUtils.removeTab(loadingTab);
+  BrowserTestUtils.removeTab(openedTab);
+});
+
+// This test ensures that downloads configured to open with a system default
+// app create only one file destination and don't open the filepicker.
+add_task(async function testFilesHandledBySystemDefaultApp() {
+  let dir = await setupFilePickerDirectory();
+
+  ensureMIMEState({ preferredAction: useSystemDefault });
+
+  let filePickerShown = false;
+  MockFilePicker.showCallback = function (fp) {
+    filePickerShown = true;
+    return Ci.nsIFilePicker.returnCancel;
+  };
+
+  let oldLaunchFile = DownloadIntegration.launchFile;
+  let launchFileCalled = new Promise(resolve => {
+    DownloadIntegration.launchFile = async (file, mimeInfo) => {
+      is(
+        useSystemDefault,
+        mimeInfo.preferredAction,
+        "The file should be launched with a system app handler."
+      );
+      resolve();
+    };
+  });
+
+  let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_pdf_application_pdf.pdf",
+    waitForLoad: false,
+    waitForStateStop: true,
+  });
+
+  await launchFileCalled;
+  ok(!filePickerShown, "file picker should not have shown up.");
+
+  assertCorrectFile(dir, "file_pdf_application_pdf.pdf");
+
+  // Cleanup
+  BrowserTestUtils.removeTab(loadingTab);
+  DownloadIntegration.launchFile = oldLaunchFile;
+});
+
+// This test ensures that downloads configured to open with a helper app create
+// only one file destination when saved via the filepicker.
+add_task(async function testFilesHandledByHelperApp() {
+  let dir = await setupFilePickerDirectory();
+
+  // Create a custom helper app so we can check that a launcherPath is
+  // configured for the serialized download.
+  let appHandler = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  appHandler.name = "Dummy Test Handler";
+  appHandler.executable = Services.dirsvc.get("ProfD", Ci.nsIFile);
+  appHandler.executable.append("helper_handler_test.exe");
+
+  if (!appHandler.executable.exists()) {
+    appHandler.executable.create(Ci.nsIFile.NORMAL_FILE_TYPE, 0o777);
+  }
+
+  ensureMIMEState({
+    preferredAction: useHelperApp,
+    preferredHandlerApp: appHandler,
+  });
+
+  let filePickerShown = false;
+  MockFilePicker.showCallback = function (fp) {
+    filePickerShown = true;
+    return Ci.nsIFilePicker.returnCancel;
+  };
+
+  let publicDownloads = await Downloads.getList(Downloads.PUBLIC);
+  let downloadFinishedPromise = new Promise(resolve => {
+    publicDownloads.addView({
+      onDownloadChanged(download) {
+        if (download.succeeded || download.error) {
+          ok(
+            download.launcherPath.includes("helper_handler_test.exe"),
+            "Launcher path is available."
+          );
+          resolve();
+        }
+      },
+    });
+  });
+
+  let oldLaunchFile = DownloadIntegration.launchFile;
+  let launchFileCalled = new Promise(resolve => {
+    DownloadIntegration.launchFile = async (file, mimeInfo) => {
+      is(
+        useHelperApp,
+        mimeInfo.preferredAction,
+        "The file should be launched with a helper app handler."
+      );
+      resolve();
+    };
+  });
+
+  let loadingTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: TEST_PATH + "file_pdf_application_pdf.pdf",
+    waitForLoad: false,
+    waitForStateStop: true,
+  });
+
+  await downloadFinishedPromise;
+  await launchFileCalled;
+  ok(!filePickerShown, "file picker should not have shown up.");
+  assertCorrectFile(dir, "file_pdf_application_pdf.pdf");
+
+  // Cleanup
+  BrowserTestUtils.removeTab(loadingTab);
+  DownloadIntegration.launchFile = oldLaunchFile;
+});
+
+async function setupFilePickerDirectory() {
+  let saveDir = createSaveDir();
+  Services.prefs.setComplexValue("browser.download.dir", Ci.nsIFile, saveDir);
+  Services.prefs.setIntPref("browser.download.folderList", 2);
+
+  MockFilePicker.displayDirectory = saveDir;
+  MockFilePicker.returnValue = MockFilePicker.returnOK;
+  MockFilePicker.showCallback = function (fp) {
+    let file = saveDir.clone();
+    file.append(fp.defaultString);
+    MockFilePicker.setFiles([file]);
+  };
+
+  registerCleanupFunction(async () => {
+    Services.prefs.clearUserPref("browser.download.dir");
+    Services.prefs.clearUserPref("browser.download.folderList");
+    let publicList = await Downloads.getList(Downloads.PUBLIC);
+    let unfinishedDownloads = new Set(
+      (await publicList.getAll()).filter(dl => !dl.succeeded && !dl.error)
+    );
+    if (unfinishedDownloads.size) {
+      info(`Have ${unfinishedDownloads.size} unfinished downloads, waiting.`);
+      await new Promise(resolve => {
+        let view = {
+          onChanged(dl) {
+            if (unfinishedDownloads.has(dl) && (dl.succeeded || dl.error)) {
+              unfinishedDownloads.delete(dl);
+              info(`Removed another download.`);
+              if (!unfinishedDownloads.size) {
+                publicList.removeView(view);
+                resolve();
+              }
+            }
+          },
+        };
+        publicList.addView(view);
+      });
+    }
+    try {
+      await IOUtils.remove(saveDir.path, { recursive: true });
+    } catch (e) {
+      console.error(e);
+    }
+  });
+
+  return saveDir;
+}
+
+function assertCorrectFile(saveDir, filename) {
+  info("Make sure additional files haven't been created.");
+  let iter = saveDir.directoryEntries;
+  let file = iter.nextFile;
+  ok(file.path.includes(filename), "Download has correct filename");
+  ok(!iter.nextFile, "Only one file was created.");
+}
+
+function createSaveDir() {
+  info("Creating save directory.");
+  let time = new Date().getTime();
+  let saveDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  saveDir.append(time);
+  return saveDir;
+}
+
+function ensureMIMEState(
+  { preferredAction, preferredHandlerApp = null },
+  { type = "application/pdf", ext = "pdf" } = {}
+) {
+  const mimeInfo = gMimeSvc.getFromTypeAndExtension(type, ext);
+  mimeInfo.preferredAction = preferredAction;
+  mimeInfo.preferredApplicationHandler = preferredHandlerApp;
+  mimeInfo.alwaysAskBeforeHandling = false;
+  gHandlerSvc.store(mimeInfo);
+}
diff --git a/uriloader/exthandler/tests/mochitest/browser_standalone_application_chooser_window_fallback.js b/uriloader/exthandler/tests/mochitest/browser_standalone_application_chooser_window_fallback.js
new file mode 100644
index 0000000000..40496fb3b2
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_standalone_application_chooser_window_fallback.js
@@ -0,0 +1,67 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Test the fallback fixed in bug 1875460, if the modern tab dialog box is not
+// supported.
+
+const TEST_URL =
+  "https://example.com/browser/" +
+  "uriloader/exthandler/tests/mochitest/FTPprotocolHandler.html";
+
+add_task(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["security.external_protocol_requires_permission", false]],
+  });
+
+  // Load a page with an FTP link.
+  let browser = gBrowser.selectedBrowser;
+  BrowserTestUtils.startLoadingURIString(browser, TEST_URL);
+  await BrowserTestUtils.browserLoaded(browser, false, TEST_URL);
+
+  // Make sure no handler is set, forcing the dialog to show.
+  let protoSvc = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  let protoInfo = protoSvc.getProtocolHandlerInfo("ftp");
+  ok(!protoInfo.preferredApplicationHandler, "no preferred handler is set");
+  let handlers = protoInfo.possibleApplicationHandlers;
+  is(0, handlers.length, "no handler registered for ftp");
+  protoInfo.alwaysAskBeforeHandling = true;
+  let handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+  handlerSvc.store(protoInfo);
+
+  // Delete getTabDialogBox from gBrowser, to test the fallback to the standalone
+  // application chooser window.
+  let _getTabDialogBox = gBrowser.getTabDialogBox;
+  delete gBrowser.getTabDialogBox;
+
+  let appChooserDialogOpenPromise = BrowserTestUtils.domWindowOpened(
+    null,
+    async win => {
+      await BrowserTestUtils.waitForEvent(win, "load");
+      Assert.ok(
+        win.document.documentURI ==
+          "chrome://mozapps/content/handling/appChooser.xhtml",
+        "application chooser dialog opened"
+      );
+      return true;
+    }
+  );
+  let link = "#link";
+  await BrowserTestUtils.synthesizeMouseAtCenter(link, {}, browser);
+  let appChooserDialog = await appChooserDialogOpenPromise;
+
+  let appChooserDialogClosePromise =
+    BrowserTestUtils.domWindowClosed(appChooserDialog);
+  let dialog = appChooserDialog.document.getElementsByTagName("dialog")[0];
+  let cancelButton = dialog.getButton("cancel");
+  cancelButton.click();
+  await appChooserDialogClosePromise;
+
+  // Restore the original getTabDialogBox(), to not affect other tests.
+  gBrowser.getTabDialogBox = _getTabDialogBox;
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_txt_download_save_as.js b/uriloader/exthandler/tests/mochitest/browser_txt_download_save_as.js
new file mode 100644
index 0000000000..7155c35fd9
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_txt_download_save_as.js
@@ -0,0 +1,168 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { DownloadIntegration } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+const HandlerService = Cc[
+  "@mozilla.org/uriloader/handler-service;1"
+].getService(Ci.nsIHandlerService);
+const MIMEService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "https://example.com"
+);
+
+const {
+  saveToDisk,
+  alwaysAsk,
+  handleInternally,
+  useHelperApp,
+  useSystemDefault,
+} = Ci.nsIHandlerInfo;
+const testDir = createTemporarySaveDirectory();
+const MockFilePicker = SpecialPowers.MockFilePicker;
+MockFilePicker.init(window);
+
+Services.scriptloader.loadSubScript(
+  "chrome://mochitests/content/browser/toolkit/content/tests/browser/common/mockTransfer.js",
+  this
+);
+
+async function testSaveAsDownload() {
+  await BrowserTestUtils.withNewTab(
+    `data:text/html,<a id="test-link" href="${TEST_PATH}/file_txt_attachment_test.txt">Test TXT Link</a>`,
+    async browser => {
+      let menu = document.getElementById("contentAreaContextMenu");
+      ok(menu, "Context menu exists on the page");
+
+      let popupShown = BrowserTestUtils.waitForEvent(menu, "popupshown");
+      BrowserTestUtils.synthesizeMouseAtCenter(
+        "a#test-link",
+        { type: "contextmenu", button: 2 },
+        browser
+      );
+      await popupShown;
+      info("Context menu popup was successfully displayed");
+
+      let filePickerPromise = setupFilePicker();
+
+      info("Clicking Save As... context menu");
+      let menuitem = menu.querySelector("#context-savelink");
+      menu.activateItem(menuitem);
+      await filePickerPromise;
+    }
+  );
+}
+
+async function setupFilePicker() {
+  return new Promise(resolve => {
+    MockFilePicker.returnValue = MockFilePicker.returnOK;
+    MockFilePicker.displayDirectory = testDir;
+    MockFilePicker.showCallback = fp => {
+      ok(true, "filepicker should be visible");
+      ok(
+        fp.defaultExtension === "txt",
+        "Default extension in filepicker should be txt"
+      );
+      ok(
+        fp.defaultString === "file_txt_attachment_test.txt",
+        "Default string name in filepicker should have the correct file name"
+      );
+      const destFile = testDir.clone();
+      destFile.append(fp.defaultString);
+      MockFilePicker.setFiles([destFile]);
+
+      mockTransferCallback = success => {
+        ok(success, "File should have been downloaded successfully");
+        ok(destFile.exists(), "File should exist in test directory");
+        resolve(destFile);
+      };
+    };
+  });
+}
+
+add_setup(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["browser.download.always_ask_before_handling_new_types", false],
+      ["browser.download.useDownloadDir", false],
+    ],
+  });
+  mockTransferRegisterer.register();
+
+  let oldLaunchFile = DownloadIntegration.launchFile;
+  DownloadIntegration.launchFile = () => {
+    ok(false, "Download should not have launched");
+  };
+
+  registerCleanupFunction(async () => {
+    DownloadIntegration.launchFile = oldLaunchFile;
+    mockTransferRegisterer.unregister();
+
+    // We only want to run MockFilerPicker.cleanup after the entire test is run.
+    // Otherwise, we cannot use MockFilePicker for each preferredAction.
+    MockFilePicker.cleanup();
+
+    testDir.remove(true);
+    ok(!testDir.exists(), "Test directory should be removed");
+  });
+});
+
+/**
+ * Tests that selecting the context menu item `Save Link As…` on a txt file link
+ * opens the file picker and only downloads the file without any launches when
+ * browser.download.always_ask_before_handling_new_types is disabled.
+ */
+add_task(async function test_txt_save_as_link() {
+  let mimeInfo;
+
+  for (let preferredAction of [
+    saveToDisk,
+    alwaysAsk,
+    handleInternally,
+    useHelperApp,
+    useSystemDefault,
+  ]) {
+    mimeInfo = MIMEService.getFromTypeAndExtension("text/plain", "txt");
+    mimeInfo.alwaysAskBeforeHandling = preferredAction === alwaysAsk;
+    mimeInfo.preferredAction = preferredAction;
+    HandlerService.store(mimeInfo);
+
+    info(
+      `Setting up filepicker with preferredAction ${preferredAction} and ask = ${mimeInfo.alwaysAskBeforeHandling}`
+    );
+    await testSaveAsDownload(mimeInfo);
+  }
+});
+
+/**
+ * Tests that selecting the context menu item `Save Link As…` on a txt file link
+ * opens the file picker and only downloads the file without any launches when
+ * browser.download.always_ask_before_handling_new_types is disabled. For this
+ * particular test, set alwaysAskBeforeHandling to true.
+ */
+add_task(async function test_txt_save_as_link_alwaysAskBeforeHandling() {
+  let mimeInfo;
+
+  for (let preferredAction of [
+    saveToDisk,
+    alwaysAsk,
+    handleInternally,
+    useHelperApp,
+    useSystemDefault,
+  ]) {
+    mimeInfo = MIMEService.getFromTypeAndExtension("text/plain", "txt");
+    mimeInfo.alwaysAskBeforeHandling = true;
+    mimeInfo.preferredAction = preferredAction;
+    HandlerService.store(mimeInfo);
+
+    info(
+      `Setting up filepicker with preferredAction ${preferredAction} and ask = ${mimeInfo.alwaysAskBeforeHandling}`
+    );
+    await testSaveAsDownload(mimeInfo);
+  }
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_web_handler_app_pinned_tab.js b/uriloader/exthandler/tests/mochitest/browser_web_handler_app_pinned_tab.js
new file mode 100644
index 0000000000..a9d4803fca
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_web_handler_app_pinned_tab.js
@@ -0,0 +1,85 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+ChromeUtils.defineESModuleGetters(this, {
+  UrlbarTestUtils: "resource://testing-common/UrlbarTestUtils.sys.mjs",
+});
+
+let testURL =
+  "http://mochi.test:8888/browser/" +
+  "uriloader/exthandler/tests/mochitest/mailto.html";
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "gExternalProtocolService",
+  "@mozilla.org/uriloader/external-protocol-service;1",
+  "nsIExternalProtocolService"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "gHandlerService",
+  "@mozilla.org/uriloader/handler-service;1",
+  "nsIHandlerService"
+);
+
+let prevAlwaysAskBeforeHandling;
+let prevPreferredAction;
+let prevPreferredApplicationHandler;
+
+add_setup(async function () {
+  let handler = gExternalProtocolService.getProtocolHandlerInfo("mailto", {});
+
+  // Create a fake mail handler
+  const APP_NAME = "ExMail";
+  const HANDLER_URL = "https://example.com/?extsrc=mailto&url=%s";
+  let app = Cc["@mozilla.org/uriloader/web-handler-app;1"].createInstance(
+    Ci.nsIWebHandlerApp
+  );
+  app.uriTemplate = HANDLER_URL;
+  app.name = APP_NAME;
+
+  // Store defaults
+  prevAlwaysAskBeforeHandling = handler.alwaysAskBeforeHandling;
+  prevPreferredAction = handler.preferredAction;
+  prevPreferredApplicationHandler = handler.preferredApplicationHandler;
+
+  // Set the fake app as default
+  handler.alwaysAskBeforeHandling = false;
+  handler.preferredAction = Ci.nsIHandlerInfo.useHelperApp;
+  handler.preferredApplicationHandler = app;
+  gHandlerService.store(handler);
+});
+
+registerCleanupFunction(async function () {
+  let handler = gExternalProtocolService.getProtocolHandlerInfo("mailto", {});
+  handler.alwaysAskBeforeHandling = prevAlwaysAskBeforeHandling;
+  handler.preferredAction = prevPreferredAction;
+  handler.preferredApplicationHandler = prevPreferredApplicationHandler;
+  gHandlerService.store(handler);
+});
+
+add_task(async function () {
+  const expectedURL =
+    "https://example.com/?extsrc=mailto&url=mailto%3Amail%40example.com";
+
+  // Load a page with mailto handler.
+  let browser = gBrowser.selectedBrowser;
+  BrowserTestUtils.startLoadingURIString(browser, testURL);
+  await BrowserTestUtils.browserLoaded(browser, false, testURL);
+
+  // Pin as an app tab
+  gBrowser.pinTab(gBrowser.selectedTab);
+
+  // Click the link and check the new tab is correct
+  let promiseTabOpened = BrowserTestUtils.waitForNewTab(gBrowser, expectedURL);
+  await BrowserTestUtils.synthesizeMouseAtCenter("#link", {}, browser);
+  let tab = await promiseTabOpened;
+  is(
+    gURLBar.value,
+    UrlbarTestUtils.trimURL(expectedURL),
+    "the mailto web handler is opened in a new tab"
+  );
+  BrowserTestUtils.removeTab(tab);
+});
diff --git a/uriloader/exthandler/tests/mochitest/browser_web_protocol_handlers.js b/uriloader/exthandler/tests/mochitest/browser_web_protocol_handlers.js
new file mode 100644
index 0000000000..828d02d24b
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/browser_web_protocol_handlers.js
@@ -0,0 +1,130 @@
+ChromeUtils.defineESModuleGetters(this, {
+  UrlbarTestUtils: "resource://testing-common/UrlbarTestUtils.sys.mjs",
+});
+
+let testURL =
+  "https://example.com/browser/" +
+  "uriloader/exthandler/tests/mochitest/protocolHandler.html";
+
+add_task(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["security.external_protocol_requires_permission", false]],
+  });
+
+  // Load a page registering a protocol handler.
+  let browser = gBrowser.selectedBrowser;
+  BrowserTestUtils.startLoadingURIString(browser, testURL);
+  await BrowserTestUtils.browserLoaded(browser, false, testURL);
+
+  // Register the protocol handler by clicking the notificationbar button.
+  let notificationValue = "Protocol Registration: web+testprotocol";
+  let getNotification = () =>
+    gBrowser.getNotificationBox().getNotificationWithValue(notificationValue);
+  await BrowserTestUtils.waitForCondition(getNotification);
+  let notification = getNotification();
+  let button = notification.buttonContainer.querySelector("button");
+  ok(button, "got registration button");
+  button.click();
+
+  // Set the new handler as default.
+  const protoSvc = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+  let protoInfo = protoSvc.getProtocolHandlerInfo("web+testprotocol");
+  is(
+    protoInfo.preferredAction,
+    protoInfo.useHelperApp,
+    "using a helper application is the preferred action"
+  );
+  ok(!protoInfo.preferredApplicationHandler, "no preferred handler is set");
+  let handlers = protoInfo.possibleApplicationHandlers;
+  is(1, handlers.length, "only one handler registered for web+testprotocol");
+  let handler = handlers.queryElementAt(0, Ci.nsIHandlerApp);
+  ok(handler instanceof Ci.nsIWebHandlerApp, "the handler is a web handler");
+  is(
+    handler.uriTemplate,
+    "https://example.com/foobar?uri=%s",
+    "correct url template"
+  );
+  protoInfo.preferredApplicationHandler = handler;
+  protoInfo.alwaysAskBeforeHandling = false;
+  const handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+  handlerSvc.store(protoInfo);
+
+  const expectedURL =
+    "https://example.com/foobar?uri=web%2Btestprotocol%3Atest";
+
+  // Create a framed link:
+  await SpecialPowers.spawn(browser, [], async function () {
+    let iframe = content.document.createElement("iframe");
+    iframe.src = `data:text/html,<a href="web+testprotocol:test">Click me</a>`;
+    content.document.body.append(iframe);
+    // Can't return this promise because it resolves to the event object.
+    await ContentTaskUtils.waitForEvent(iframe, "load");
+    iframe.contentDocument.querySelector("a").click();
+  });
+  let kidContext = browser.browsingContext.children[0];
+  await TestUtils.waitForCondition(() => {
+    let spec = kidContext.currentWindowGlobal?.documentURI?.spec || "";
+    return spec == expectedURL;
+  });
+  is(
+    kidContext.currentWindowGlobal.documentURI.spec,
+    expectedURL,
+    "Should load in frame."
+  );
+
+  // Middle-click a testprotocol link and check the new tab is correct
+  let link = "#link";
+
+  let promiseTabOpened = BrowserTestUtils.waitForNewTab(gBrowser, expectedURL);
+  await BrowserTestUtils.synthesizeMouseAtCenter(link, { button: 1 }, browser);
+  let tab = await promiseTabOpened;
+  gBrowser.selectedTab = tab;
+  is(
+    gURLBar.value,
+    UrlbarTestUtils.trimURL(expectedURL),
+    "the expected URL is displayed in the location bar"
+  );
+  BrowserTestUtils.removeTab(tab);
+
+  // Shift-click the testprotocol link and check the new window.
+  let newWindowPromise = BrowserTestUtils.waitForNewWindow({
+    url: expectedURL,
+  });
+  await BrowserTestUtils.synthesizeMouseAtCenter(
+    link,
+    { shiftKey: true },
+    browser
+  );
+  let win = await newWindowPromise;
+  await BrowserTestUtils.waitForCondition(
+    () => win.gBrowser.currentURI.spec == expectedURL
+  );
+  is(
+    win.gURLBar.value,
+    UrlbarTestUtils.trimURL(expectedURL),
+    "the expected URL is displayed in the location bar"
+  );
+  await BrowserTestUtils.closeWindow(win);
+
+  // Click the testprotocol link and check the url in the current tab.
+  let loadPromise = BrowserTestUtils.browserLoaded(browser);
+  await BrowserTestUtils.synthesizeMouseAtCenter(link, {}, browser);
+  await loadPromise;
+  await BrowserTestUtils.waitForCondition(
+    () => gURLBar.value != UrlbarTestUtils.trimURL(testURL)
+  );
+  is(
+    gURLBar.value,
+    UrlbarTestUtils.trimURL(expectedURL),
+    "the expected URL is displayed in the location bar"
+  );
+
+  // Cleanup.
+  protoInfo.preferredApplicationHandler = null;
+  handlers.removeElementAt(0);
+  handlerSvc.store(protoInfo);
+});
diff --git a/uriloader/exthandler/tests/mochitest/download.bin b/uriloader/exthandler/tests/mochitest/download.bin
new file mode 100644
index 0000000000..0e4b0c7bae
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/download.bin
@@ -0,0 +1 @@
+abc123
diff --git a/uriloader/exthandler/tests/mochitest/download.sjs b/uriloader/exthandler/tests/mochitest/download.sjs
new file mode 100644
index 0000000000..d02d2b7355
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/download.sjs
@@ -0,0 +1,42 @@
+"use strict";
+
+function actuallyHandleRequest(req, res) {
+  res.setHeader("Content-Type", "application/octet-stream", false);
+  res.write("abc123");
+  res.finish();
+}
+
+function handleRequest(req, res) {
+  if (req.queryString.includes("finish")) {
+    res.write("OK");
+    let downloadReq = null;
+    getObjectState("downloadReq", o => {
+      downloadReq = o;
+    });
+    // Two possibilities: either the download request has already reached us, or not.
+    if (downloadReq) {
+      downloadReq.wrappedJSObject.callback();
+    } else {
+      // Set a variable to allow the request to complete immediately:
+      setState("finishReq", "true");
+    }
+  } else if (req.queryString.includes("reset")) {
+    res.write("OK");
+    setObjectState("downloadReq", null);
+    setState("finishReq", "false");
+  } else {
+    res.processAsync();
+    if (getState("finishReq") === "true") {
+      actuallyHandleRequest(req, res);
+    } else {
+      let o = {
+        callback() {
+          actuallyHandleRequest(req, res);
+        },
+      };
+      o.wrappedJSObject = o;
+      o.QueryInterface = () => o;
+      setObjectState("downloadReq", o);
+    }
+  }
+}
diff --git a/uriloader/exthandler/tests/mochitest/download_page.html b/uriloader/exthandler/tests/mochitest/download_page.html
new file mode 100644
index 0000000000..5a264888fa
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/download_page.html
@@ -0,0 +1,22 @@
+<!-- Any copyright is dedicated to the Public Domain.
+   - http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE html>
+<html>
+<head>
+  <meta charset=UTF-8>
+  <title>Test page for link clicking</title>
+  <script type="text/javascript">
+    function launch_download(extra) {
+      window.open("download.sjs", "_blank", "height=100,width=100" + extra);
+    }
+  </script>
+</head>
+<body>
+  <a href="download.bin" id="regular_load">regular load</a>
+  <a href="download.bin" id="target_blank" target="_blank" rel="opener">target blank</a>
+  <a href="#" onclick="launch_download(''); return false" id="new_window">new window</a>
+  <a href="#" onclick="window.open('download_page.html?newwin'); return false" id="open_in_new_tab">click to reopen</a>
+  <a href="download.bin" id="target_blank_no_opener" rel="noopener" target="_blank">target blank (noopener)</a>
+  <a href="#" onclick="window.open('download.bin', '_blank', 'noopener'); return false" id="open_in_new_tab_no_opener">click to reopen (noopener)</a>
+  <a href="#" onclick="launch_download(',noopener'); return false" id="new_window_no_opener">new window (noopener)</a>
+</body>
diff --git a/uriloader/exthandler/tests/mochitest/file_as.exe b/uriloader/exthandler/tests/mochitest/file_as.exe
new file mode 100644
index 0000000000..f2f5ab47f3
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_as.exe
@@ -0,0 +1 @@
+Not actually an executable... but let's pretend!
diff --git a/uriloader/exthandler/tests/mochitest/file_as.exe^headers^ b/uriloader/exthandler/tests/mochitest/file_as.exe^headers^
new file mode 100644
index 0000000000..89f22e30be
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_as.exe^headers^
@@ -0,0 +1,2 @@
+Content-Type: binary/octet-stream
+Content-Disposition: attachment
diff --git a/uriloader/exthandler/tests/mochitest/file_external_protocol_iframe.html b/uriloader/exthandler/tests/mochitest/file_external_protocol_iframe.html
new file mode 100644
index 0000000000..eb2fb74441
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_external_protocol_iframe.html
@@ -0,0 +1 @@
+<iframe src="mailto:foo@bar.com"></iframe>
diff --git a/uriloader/exthandler/tests/mochitest/file_green.webp b/uriloader/exthandler/tests/mochitest/file_green.webp
new file mode 100644
index 0000000000..04b7f003b4
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_green.webp
diff --git a/uriloader/exthandler/tests/mochitest/file_green.webp^headers^ b/uriloader/exthandler/tests/mochitest/file_green.webp^headers^
new file mode 100644
index 0000000000..3f6afd6625
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_green.webp^headers^
@@ -0,0 +1,3 @@
+Content-Disposition: attachment; filename=file_green.webp
+Content-Type: image/webp
+
diff --git a/uriloader/exthandler/tests/mochitest/file_image_svgxml.svg b/uriloader/exthandler/tests/mochitest/file_image_svgxml.svg
new file mode 100644
index 0000000000..b730c4c492
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_image_svgxml.svg
@@ -0,0 +1,3 @@
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg">
+  <rect x="5" y="5" width="10" height="10" fill="white"/>
+</svg>
diff --git a/uriloader/exthandler/tests/mochitest/file_image_svgxml.svg^headers^ b/uriloader/exthandler/tests/mochitest/file_image_svgxml.svg^headers^
new file mode 100644
index 0000000000..5279ae8636
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_image_svgxml.svg^headers^
@@ -0,0 +1,2 @@
+content-disposition: attachment; filename=file_image_svgxml_svg; filename*=UTF-8''file_image_svgxml.svg
+content-type: image/svg+xml
diff --git a/uriloader/exthandler/tests/mochitest/file_nested_protocol_request.html b/uriloader/exthandler/tests/mochitest/file_nested_protocol_request.html
new file mode 100644
index 0000000000..b1bb863f89
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_nested_protocol_request.html
@@ -0,0 +1 @@
+<iframe srcdoc="<a href='mailto:help@example.com'>Mail someone</a>"></iframe>
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_application_octet_stream.pdf b/uriloader/exthandler/tests/mochitest/file_pdf_application_octet_stream.pdf
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_application_octet_stream.pdf
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_application_octet_stream.pdf^headers^ b/uriloader/exthandler/tests/mochitest/file_pdf_application_octet_stream.pdf^headers^
new file mode 100644
index 0000000000..9e8cb41cba
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_application_octet_stream.pdf^headers^
@@ -0,0 +1,2 @@
+Content-Disposition: attachment; filename="file_pdf_application_octet_stream.pdf"; filename*=UTF-8''file_pdf_application_octet_stream.pdf
+Content-Type: application/octet-stream
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_application_pdf.pdf b/uriloader/exthandler/tests/mochitest/file_pdf_application_pdf.pdf
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_application_pdf.pdf
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_application_pdf.pdf^headers^ b/uriloader/exthandler/tests/mochitest/file_pdf_application_pdf.pdf^headers^
new file mode 100644
index 0000000000..d1d59b9754
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_application_pdf.pdf^headers^
@@ -0,0 +1,2 @@
+content-disposition: attachment; filename=file_pdf_application_pdf.pdf; filename*=UTF-8''file_pdf_application_pdf.pdf
+content-type: application/pdf
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_application_unknown.pdf b/uriloader/exthandler/tests/mochitest/file_pdf_application_unknown.pdf
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_application_unknown.pdf
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_application_unknown.pdf^headers^ b/uriloader/exthandler/tests/mochitest/file_pdf_application_unknown.pdf^headers^
new file mode 100644
index 0000000000..157c0e0943
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_application_unknown.pdf^headers^
@@ -0,0 +1,2 @@
+content-disposition: attachment; filename=file_pdf_application_unknown.pdf; filename*=UTF-8''file_pdf_application_unknown.pdf
+content-type: application/unknown
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_binary_octet_stream.pdf b/uriloader/exthandler/tests/mochitest/file_pdf_binary_octet_stream.pdf
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_binary_octet_stream.pdf
diff --git a/uriloader/exthandler/tests/mochitest/file_pdf_binary_octet_stream.pdf^headers^ b/uriloader/exthandler/tests/mochitest/file_pdf_binary_octet_stream.pdf^headers^
new file mode 100644
index 0000000000..6358f54f48
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_pdf_binary_octet_stream.pdf^headers^
@@ -0,0 +1,2 @@
+Content-Disposition: attachment; filename="file_pdf_binary_octet_stream.pdf"; filename*=UTF-8''file_pdf_binary_octet_stream.pdf
+Content-Type: binary/octet-stream
diff --git a/uriloader/exthandler/tests/mochitest/file_txt_attachment_test.txt b/uriloader/exthandler/tests/mochitest/file_txt_attachment_test.txt
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_txt_attachment_test.txt
diff --git a/uriloader/exthandler/tests/mochitest/file_txt_attachment_test.txt^headers^ b/uriloader/exthandler/tests/mochitest/file_txt_attachment_test.txt^headers^
new file mode 100644
index 0000000000..dcfed6af23
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_txt_attachment_test.txt^headers^
@@ -0,0 +1,2 @@
+Content-Disposition: attachment; filename=file_txt_attachment_test.txt
+Content-Type: text/plain
diff --git a/uriloader/exthandler/tests/mochitest/file_with@@funny_name.png b/uriloader/exthandler/tests/mochitest/file_with@@funny_name.png
new file mode 100644
index 0000000000..743292dc6f
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_with@@funny_name.png
diff --git a/uriloader/exthandler/tests/mochitest/file_with@@funny_name.png^headers^ b/uriloader/exthandler/tests/mochitest/file_with@@funny_name.png^headers^
new file mode 100644
index 0000000000..06e0cd957f
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_with@@funny_name.png^headers^
@@ -0,0 +1,2 @@
+Content-Disposition: inline; filename=file_with%40%40funny_name.png
+Content-Type: image/png
diff --git a/uriloader/exthandler/tests/mochitest/file_with[funny_name.webm b/uriloader/exthandler/tests/mochitest/file_with[funny_name.webm
new file mode 100644
index 0000000000..7bc738b8b4
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_with[funny_name.webm
diff --git a/uriloader/exthandler/tests/mochitest/file_with[funny_name.webm^headers^ b/uriloader/exthandler/tests/mochitest/file_with[funny_name.webm^headers^
new file mode 100644
index 0000000000..b77e9d3687
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_with[funny_name.webm^headers^
@@ -0,0 +1,2 @@
+Content-Disposition: inline; filename=file_with%5Bfunny_name.webm
+Content-Type: video/webm
diff --git a/uriloader/exthandler/tests/mochitest/file_xml_attachment_binary_octet_stream.xml b/uriloader/exthandler/tests/mochitest/file_xml_attachment_binary_octet_stream.xml
new file mode 100644
index 0000000000..3a5792586a
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_xml_attachment_binary_octet_stream.xml
@@ -0,0 +1,4 @@
+<?xml version = "1.0" encoding = "utf-8"?>
+
+<something>
+</something>
diff --git a/uriloader/exthandler/tests/mochitest/file_xml_attachment_binary_octet_stream.xml^headers^ b/uriloader/exthandler/tests/mochitest/file_xml_attachment_binary_octet_stream.xml^headers^
new file mode 100644
index 0000000000..5bdc4448e8
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_xml_attachment_binary_octet_stream.xml^headers^
@@ -0,0 +1,2 @@
+Content-Disposition: attachment
+Content-Type: binary/octet-stream
diff --git a/uriloader/exthandler/tests/mochitest/file_xml_attachment_test.xml b/uriloader/exthandler/tests/mochitest/file_xml_attachment_test.xml
new file mode 100644
index 0000000000..3a5792586a
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_xml_attachment_test.xml
@@ -0,0 +1,4 @@
+<?xml version = "1.0" encoding = "utf-8"?>
+
+<something>
+</something>
diff --git a/uriloader/exthandler/tests/mochitest/file_xml_attachment_test.xml^headers^ b/uriloader/exthandler/tests/mochitest/file_xml_attachment_test.xml^headers^
new file mode 100644
index 0000000000..ac0355d976
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/file_xml_attachment_test.xml^headers^
@@ -0,0 +1,2 @@
+Content-Disposition: attachment; filename=file_xml_attachment_test.xml
+Content-Type: text/xml
diff --git a/uriloader/exthandler/tests/mochitest/head.js b/uriloader/exthandler/tests/mochitest/head.js
new file mode 100644
index 0000000000..4b973da06d
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/head.js
@@ -0,0 +1,537 @@
+var { FileUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/FileUtils.sys.mjs"
+);
+var { HandlerServiceTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/HandlerServiceTestUtils.sys.mjs"
+);
+
+var gMimeSvc = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+var gHandlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+  Ci.nsIHandlerService
+);
+
+function createMockedHandlerApp() {
+  // Mock the executable
+  let mockedExecutable = new FileUtils.File(
+    PathUtils.join(PathUtils.tempDir, "mockedExecutable")
+  );
+  if (!mockedExecutable.exists()) {
+    mockedExecutable.create(Ci.nsIFile.NORMAL_FILE_TYPE, 0o755);
+  }
+
+  // Mock the handler app
+  let mockedHandlerApp = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  mockedHandlerApp.executable = mockedExecutable;
+  mockedHandlerApp.detailedDescription = "Mocked handler app";
+
+  registerCleanupFunction(function () {
+    // remove the mocked executable from disk.
+    if (mockedExecutable.exists()) {
+      mockedExecutable.remove(true);
+    }
+  });
+
+  return mockedHandlerApp;
+}
+
+function createMockedObjects(createHandlerApp) {
+  // Mock the mime info
+  let internalMockedMIME = gMimeSvc.getFromTypeAndExtension(
+    "text/x-test-handler",
+    null
+  );
+  internalMockedMIME.alwaysAskBeforeHandling = true;
+  internalMockedMIME.preferredAction = Ci.nsIHandlerInfo.useHelperApp;
+  internalMockedMIME.appendExtension("abc");
+  if (createHandlerApp) {
+    let mockedHandlerApp = createMockedHandlerApp();
+    internalMockedMIME.description = mockedHandlerApp.detailedDescription;
+    internalMockedMIME.possibleApplicationHandlers.appendElement(
+      mockedHandlerApp
+    );
+    internalMockedMIME.preferredApplicationHandler = mockedHandlerApp;
+  }
+
+  // Proxy for the mocked MIME info for faking the read-only attributes
+  let mockedMIME = new Proxy(internalMockedMIME, {
+    get(target, property) {
+      switch (property) {
+        case "hasDefaultHandler":
+          return true;
+        case "defaultDescription":
+          return "Default description";
+        default:
+          return target[property];
+      }
+    },
+  });
+
+  // Mock the launcher:
+  let mockedLauncher = {
+    MIMEInfo: mockedMIME,
+    source: Services.io.newURI("http://www.mozilla.org/"),
+    suggestedFileName: "test_download_dialog.abc",
+    targetFileIsExecutable: false,
+    saveToDisk() {},
+    cancel() {},
+    setDownloadToLaunch() {},
+    setWebProgressListener() {},
+    saveDestinationAvailable() {},
+    contentLength: 42,
+    targetFile: null, // never read
+    // PRTime is microseconds since epoch, Date.now() returns milliseconds:
+    timeDownloadStarted: Date.now() * 1000,
+    QueryInterface: ChromeUtils.generateQI([
+      "nsICancelable",
+      "nsIHelperAppLauncher",
+    ]),
+  };
+
+  registerCleanupFunction(function () {
+    // remove the mocked mime info from database.
+    let mockHandlerInfo = gMimeSvc.getFromTypeAndExtension(
+      "text/x-test-handler",
+      null
+    );
+    if (gHandlerSvc.exists(mockHandlerInfo)) {
+      gHandlerSvc.remove(mockHandlerInfo);
+    }
+  });
+
+  return mockedLauncher;
+}
+
+function createTemporarySaveDirectory() {
+  var saveDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  saveDir.append("testsavedir");
+  if (!saveDir.exists()) {
+    saveDir.create(Ci.nsIFile.DIRECTORY_TYPE, 0o755);
+  }
+  return saveDir;
+}
+
+async function openHelperAppDialog(launcher) {
+  let helperAppDialog = Cc[
+    "@mozilla.org/helperapplauncherdialog;1"
+  ].createInstance(Ci.nsIHelperAppLauncherDialog);
+
+  let helperAppDialogShownPromise = BrowserTestUtils.domWindowOpenedAndLoaded();
+  try {
+    helperAppDialog.show(launcher, window, "foopy");
+  } catch (ex) {
+    ok(
+      false,
+      "Trying to show unknownContentType.xhtml failed with exception: " + ex
+    );
+    console.error(ex);
+  }
+  let dlg = await helperAppDialogShownPromise;
+
+  is(
+    dlg.location.href,
+    "chrome://mozapps/content/downloads/unknownContentType.xhtml",
+    "Got correct dialog"
+  );
+
+  return dlg;
+}
+
+/**
+ * Wait for a subdialog event indicating a dialog either opened
+ * or was closed.
+ *
+ * First argument is the browser in which to listen. If a tabbrowser,
+ * we listen to subdialogs for any tab of that browser.
+ */
+async function waitForSubDialog(browser, url, state) {
+  let eventStr = state ? "dialogopen" : "dialogclose";
+
+  let eventTarget;
+
+  // Tabbrowser?
+  if (browser.tabContainer) {
+    eventTarget = browser.tabContainer.ownerDocument.documentElement;
+  } else {
+    // Individual browser. Get its box:
+    let tabDialogBox = browser.ownerGlobal.gBrowser.getTabDialogBox(browser);
+    eventTarget = tabDialogBox.getTabDialogManager()._dialogStack;
+  }
+
+  let checkFn;
+
+  if (state) {
+    checkFn = dialogEvent => dialogEvent.detail.dialog?._openedURL == url;
+  }
+
+  let event = await BrowserTestUtils.waitForEvent(
+    eventTarget,
+    eventStr,
+    true,
+    checkFn
+  );
+
+  let { dialog } = event.detail;
+
+  // If the dialog is closing wait for it to be fully closed before resolving
+  if (!state) {
+    await dialog._closingPromise;
+  }
+
+  return event.detail.dialog;
+}
+
+/**
+ * Wait for protocol permission dialog open/close.
+ * @param {MozBrowser} browser - Browser element the dialog belongs to.
+ * @param {boolean} state - true: dialog open, false: dialog close
+ * @returns {Promise<SubDialog>} - Returns a promise which resolves with the
+ * SubDialog object of the dialog which closed or opened.
+ */
+async function waitForProtocolPermissionDialog(browser, state) {
+  return waitForSubDialog(
+    browser,
+    "chrome://mozapps/content/handling/permissionDialog.xhtml",
+    state
+  );
+}
+
+/**
+ * Wait for protocol app chooser dialog open/close.
+ * @param {MozBrowser} browser - Browser element the dialog belongs to.
+ * @param {boolean} state - true: dialog open, false: dialog close
+ * @returns {Promise<SubDialog>} - Returns a promise which resolves with the
+ * SubDialog object of the dialog which closed or opened.
+ */
+async function waitForProtocolAppChooserDialog(browser, state) {
+  return waitForSubDialog(
+    browser,
+    "chrome://mozapps/content/handling/appChooser.xhtml",
+    state
+  );
+}
+
+async function promiseDownloadFinished(list, stopFromOpening) {
+  return new Promise(resolve => {
+    list.addView({
+      onDownloadChanged(download) {
+        if (stopFromOpening) {
+          download.launchWhenSucceeded = false;
+        }
+        info("Download changed!");
+        if (download.succeeded || download.error) {
+          info("Download succeeded or errored");
+          list.removeView(this);
+          resolve(download);
+        }
+      },
+    });
+  });
+}
+
+function setupMailHandler() {
+  let mailHandlerInfo = HandlerServiceTestUtils.getHandlerInfo("mailto");
+  let gOldMailHandlers = [];
+
+  // Remove extant web handlers because they have icons that
+  // we fetch from the web, which isn't allowed in tests.
+  let handlers = mailHandlerInfo.possibleApplicationHandlers;
+  for (let i = handlers.Count() - 1; i >= 0; i--) {
+    try {
+      let handler = handlers.queryElementAt(i, Ci.nsIWebHandlerApp);
+      gOldMailHandlers.push(handler);
+      // If we get here, this is a web handler app. Remove it:
+      handlers.removeElementAt(i);
+    } catch (ex) {}
+  }
+
+  let previousHandling = mailHandlerInfo.alwaysAskBeforeHandling;
+  mailHandlerInfo.alwaysAskBeforeHandling = true;
+
+  // Create a dummy web mail handler so we always know the mailto: protocol.
+  // Without this, the test fails on VMs without a default mailto: handler,
+  // because no dialog is ever shown, as we ignore subframe navigations to
+  // protocols that cannot be handled.
+  let dummy = Cc["@mozilla.org/uriloader/web-handler-app;1"].createInstance(
+    Ci.nsIWebHandlerApp
+  );
+  dummy.name = "Handler 1";
+  dummy.uriTemplate = "https://example.com/first/%s";
+  mailHandlerInfo.possibleApplicationHandlers.appendElement(dummy);
+
+  gHandlerSvc.store(mailHandlerInfo);
+  registerCleanupFunction(() => {
+    // Re-add the original protocol handlers:
+    let mailHandlers = mailHandlerInfo.possibleApplicationHandlers;
+    for (let i = handlers.Count() - 1; i >= 0; i--) {
+      try {
+        // See if this is a web handler. If it is, it'll throw, otherwise,
+        // we will remove it.
+        mailHandlers.queryElementAt(i, Ci.nsIWebHandlerApp);
+        mailHandlers.removeElementAt(i);
+      } catch (ex) {}
+    }
+    for (let h of gOldMailHandlers) {
+      mailHandlers.appendElement(h);
+    }
+    mailHandlerInfo.alwaysAskBeforeHandling = previousHandling;
+    gHandlerSvc.store(mailHandlerInfo);
+  });
+}
+
+let gDownloadDir;
+
+async function setDownloadDir() {
+  let tmpDir = PathUtils.join(
+    PathUtils.tempDir,
+    "testsavedir" + Math.floor(Math.random() * 2 ** 32)
+  );
+  // Create this dir if it doesn't exist (ignores existing dirs)
+  await IOUtils.makeDirectory(tmpDir);
+  registerCleanupFunction(async function () {
+    Services.prefs.clearUserPref("browser.download.folderList");
+    Services.prefs.clearUserPref("browser.download.dir");
+    try {
+      await IOUtils.remove(tmpDir, { recursive: true });
+    } catch (e) {
+      console.error(e);
+    }
+  });
+  Services.prefs.setIntPref("browser.download.folderList", 2);
+  Services.prefs.setCharPref("browser.download.dir", tmpDir);
+  return tmpDir;
+}
+
+add_setup(async function test_common_initialize() {
+  gDownloadDir = await setDownloadDir();
+  Services.prefs.setCharPref("browser.download.loglevel", "Debug");
+  registerCleanupFunction(function () {
+    Services.prefs.clearUserPref("browser.download.loglevel");
+  });
+});
+
+async function removeAllDownloads() {
+  let publicList = await Downloads.getList(Downloads.PUBLIC);
+  let downloads = await publicList.getAll();
+  for (let download of downloads) {
+    await publicList.remove(download);
+    if (await IOUtils.exists(download.target.path)) {
+      await download.finalize(true);
+    }
+  }
+}
+
+// Helpers for external protocol sandbox tests.
+const EXT_PROTO_URI_MAILTO = "mailto:test@example.com";
+
+/**
+ * Creates and iframe and navigate to an external protocol from the iframe.
+ * @param {MozBrowser} browser - Browser to spawn iframe in.
+ * @param {string} sandboxAttr - Sandbox attribute value for the iframe.
+ * @param {'trustedClick'|'untrustedClick'|'trustedLocationAPI'|'untrustedLocationAPI'|'frameSrc'|'frameSrcRedirect'} triggerMethod
+ *  - How to trigger the navigation to the external protocol.
+ */
+async function navigateExternalProtoFromIframe(
+  browser,
+  sandboxAttr,
+  useCSPSandbox = false,
+  triggerMethod = "trustedClick"
+) {
+  if (
+    ![
+      "trustedClick",
+      "untrustedClick",
+      "trustedLocationAPI",
+      "untrustedLocationAPI",
+      "frameSrc",
+      "frameSrcRedirect",
+    ].includes(triggerMethod)
+  ) {
+    throw new Error("Invalid trigger method " + triggerMethod);
+  }
+
+  // Construct the url to use as iframe src.
+  let testPath = getRootDirectory(gTestPath).replace(
+    "chrome://mochitests/content",
+    "https://example.com"
+  );
+  let frameSrc = testPath + "/protocol_custom_sandbox_helper.sjs";
+
+  // Load the external protocol directly via the frame src field.
+  if (triggerMethod == "frameSrc") {
+    frameSrc = EXT_PROTO_URI_MAILTO;
+  } else if (triggerMethod == "frameSrcRedirect") {
+    let url = new URL(frameSrc);
+    url.searchParams.set("redirectCustomProtocol", "true");
+    frameSrc = url.href;
+  }
+
+  // If enabled set the sandbox attributes via CSP header instead. To do
+  // this we need to pass the sandbox flags to the test server via query
+  // params.
+  if (useCSPSandbox) {
+    let url = new URL(frameSrc);
+    url.searchParams.set("cspSandbox", sandboxAttr);
+    frameSrc = url.href;
+
+    // If we use CSP sandbox attributes we shouldn't set any via iframe attribute.
+    sandboxAttr = null;
+  }
+
+  // Create a sandboxed iframe and navigate to the external protocol.
+  await SpecialPowers.spawn(
+    browser,
+    [sandboxAttr, frameSrc, EXT_PROTO_URI_MAILTO, triggerMethod],
+    async (sandbox, src, extProtoURI, trigger) => {
+      let frame = content.document.createElement("iframe");
+
+      if (sandbox != null) {
+        frame.sandbox = sandbox;
+      }
+
+      frame.src = src;
+
+      let useFrameSrc = trigger == "frameSrc" || trigger == "frameSrcRedirect";
+
+      // Create frame load promise.
+      let frameLoadPromise;
+      // We won't get a load event if we directly put the external protocol in
+      // the frame src.
+      if (!useFrameSrc) {
+        frameLoadPromise = ContentTaskUtils.waitForEvent(frame, "load", false);
+      }
+
+      content.document.body.appendChild(frame);
+      await frameLoadPromise;
+
+      if (!useFrameSrc) {
+        // Trigger the external protocol navigation in the iframe. We test
+        // navigation by clicking links and navigation via the history API.
+        await SpecialPowers.spawn(
+          frame,
+          [extProtoURI, trigger],
+          async (uri, trigger2) => {
+            let link = content.document.createElement("a");
+            link.innerText = "CLICK ME";
+            link.id = "extProtoLink";
+            content.document.body.appendChild(link);
+
+            if (trigger2 == "trustedClick" || trigger2 == "untrustedClick") {
+              link.href = uri;
+            } else if (
+              trigger2 == "trustedLocationAPI" ||
+              trigger2 == "untrustedLocationAPI"
+            ) {
+              link.setAttribute("onclick", `location.href = '${uri}'`);
+            }
+
+            if (
+              trigger2 == "untrustedClick" ||
+              trigger2 == "untrustedLocationAPI"
+            ) {
+              link.click();
+            } else if (
+              trigger2 == "trustedClick" ||
+              trigger2 == "trustedLocationAPI"
+            ) {
+              await ContentTaskUtils.waitForCondition(
+                () => link,
+                "wait for link to be present"
+              );
+              await EventUtils.synthesizeMouseAtCenter(link, {}, content);
+            }
+          }
+        );
+      }
+    }
+  );
+}
+
+/**
+ * Wait for the sandbox error message which is shown in the web console when an
+ * external protocol navigation from a sandboxed context is blocked.
+ * @returns {Promise} - Promise which resolves once message has been logged.
+ */
+function waitForExtProtocolSandboxError() {
+  return new Promise(resolve => {
+    Services.console.registerListener(function onMessage(msg) {
+      let { message, logLevel } = msg;
+      if (logLevel != Ci.nsIConsoleMessage.error) {
+        return;
+      }
+      if (
+        !message.includes(
+          `Blocked navigation to custom protocol “${EXT_PROTO_URI_MAILTO}” from a sandboxed context.`
+        )
+      ) {
+        return;
+      }
+      Services.console.unregisterListener(onMessage);
+      resolve();
+    });
+  });
+}
+
+/**
+ * Run the external protocol sandbox test using iframes.
+ * @param {Object} options
+ * @param {boolean} options.blocked - Whether the navigation should be blocked.
+ * @param {string} options.sandbox -   See {@link navigateExternalProtoFromIframe}.
+ * @param {string} options.useCSPSandbox -  See {@link navigateExternalProtoFromIframe}.
+ * @param {string} options.triggerMethod - See {@link navigateExternalProtoFromIframe}.
+ * @returns {Promise} - Promise which resolves once the test has finished.
+ */
+function runExtProtocolSandboxTest(options) {
+  let { blocked, sandbox, useCSPSandbox = false, triggerMethod } = options;
+
+  let testPath = getRootDirectory(gTestPath).replace(
+    "chrome://mochitests/content",
+    "https://example.com"
+  );
+
+  info("runSandboxTest options: " + JSON.stringify(options));
+  return BrowserTestUtils.withNewTab(
+    testPath + "/protocol_custom_sandbox_helper.sjs",
+    async browser => {
+      if (blocked) {
+        let errorPromise = waitForExtProtocolSandboxError();
+        await navigateExternalProtoFromIframe(
+          browser,
+          sandbox,
+          useCSPSandbox,
+          triggerMethod
+        );
+        await errorPromise;
+
+        ok(
+          errorPromise,
+          "Should not show the dialog for iframe with sandbox " + sandbox
+        );
+      } else {
+        let dialogWindowOpenPromise = waitForProtocolAppChooserDialog(
+          browser,
+          true
+        );
+        await navigateExternalProtoFromIframe(
+          browser,
+          sandbox,
+          useCSPSandbox,
+          triggerMethod
+        );
+        let dialog = await dialogWindowOpenPromise;
+
+        ok(dialog, "Should show the dialog for sandbox " + sandbox);
+
+        // Close dialog before closing the tab to avoid intermittent failures.
+        let dialogWindowClosePromise = waitForProtocolAppChooserDialog(
+          browser,
+          false
+        );
+
+        dialog.close();
+        await dialogWindowClosePromise;
+      }
+    }
+  );
+}
diff --git a/uriloader/exthandler/tests/mochitest/invalidCharFileExtension.sjs b/uriloader/exthandler/tests/mochitest/invalidCharFileExtension.sjs
new file mode 100644
index 0000000000..8afa04cfe0
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/invalidCharFileExtension.sjs
@@ -0,0 +1,18 @@
+/* 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/. */
+
+function handleRequest(request, response) {
+  response.setStatusLine(request.httpVersion, 200, "OK");
+
+  if (!request.queryString.match(/^name=/)) {
+    return;
+  }
+  var name = decodeURIComponent(request.queryString.substring(5));
+
+  response.setHeader("Content-Type", 'image/png; name="' + name + '"');
+  response.setHeader(
+    "Content-Disposition",
+    'attachment; filename="' + name + '"'
+  );
+}
diff --git a/uriloader/exthandler/tests/mochitest/mailto.html b/uriloader/exthandler/tests/mochitest/mailto.html
new file mode 100644
index 0000000000..d507697443
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/mailto.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>Mailto handler</title>
+    <meta content="text/html;charset=utf-8" http-equiv="Content-Type">
+    <meta content="utf-8" http-equiv="encoding">
+  </head>
+  <body>
+    <a id="link" href="mailto:mail@example.com">mailto link</a>
+  </body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/mime_type_download.sjs b/uriloader/exthandler/tests/mochitest/mime_type_download.sjs
new file mode 100644
index 0000000000..a12bfe9735
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/mime_type_download.sjs
@@ -0,0 +1,21 @@
+function handleRequest(request, response) {
+  "use strict";
+
+  let content = "";
+  let params = new URLSearchParams(request.queryString);
+  let extension = params.get("extension");
+  let contentType = params.get("contentType");
+  if (params.has("withHeader")) {
+    response.setHeader(
+      "Content-Disposition",
+      `attachment; filename="mime_type_download${
+        extension ? "." + extension : ""
+      }";`,
+      false
+    );
+  }
+  response.setHeader("Content-Type", contentType, false);
+  response.setHeader("Content-Length", "" + content.length, false);
+  response.setStatusLine(request.httpVersion, 200);
+  response.write(content);
+}
diff --git a/uriloader/exthandler/tests/mochitest/mochitest.toml b/uriloader/exthandler/tests/mochitest/mochitest.toml
new file mode 100644
index 0000000000..1e0afbcdde
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/mochitest.toml
@@ -0,0 +1,25 @@
+[DEFAULT]
+
+["test_invalidCharFileExtension.xhtml"]
+skip-if = ["os == 'android'"] # Bug 1525959
+support-files = [
+  "HelperAppLauncherDialog_chromeScript.js",
+  "invalidCharFileExtension.sjs",
+]
+
+["test_nullCharFile.xhtml"]
+skip-if = ["os == 'android'"] # Bug 1525959
+support-files = ["HelperAppLauncherDialog_chromeScript.js"]
+
+["test_unknown_ext_protocol_handlers.html"]
+
+["test_unsafeBidiChars.xhtml"]
+skip-if = [
+  "os == 'android'", # Bug 1525959
+  "http3",
+  "http2",
+]
+support-files = [
+  "HelperAppLauncherDialog_chromeScript.js",
+  "unsafeBidiFileName.sjs",
+]
diff --git a/uriloader/exthandler/tests/mochitest/protocolHandler.html b/uriloader/exthandler/tests/mochitest/protocolHandler.html
new file mode 100644
index 0000000000..eff8a53aab
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/protocolHandler.html
@@ -0,0 +1,16 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <title>Protocol handler</title>
+    <meta content="text/html;charset=utf-8" http-equiv="Content-Type">
+    <meta content="utf-8" http-equiv="encoding">
+  </head>
+  <body>
+    <script type="text/javascript">
+      navigator.registerProtocolHandler("web+testprotocol",
+          "https://example.com/foobar?uri=%s",
+          "Test Protocol");
+    </script>
+    <a id="link" href="web+testprotocol:test">testprotocol link</a>
+  </body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/protocol_custom_sandbox_helper.sjs b/uriloader/exthandler/tests/mochitest/protocol_custom_sandbox_helper.sjs
new file mode 100644
index 0000000000..d12a953b57
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/protocol_custom_sandbox_helper.sjs
@@ -0,0 +1,28 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function handleRequest(request, response) {
+  let query = new URLSearchParams(request.queryString);
+
+  // Set CSP sandbox attributes if caller requests any.
+  let cspSandbox = query.get("cspSandbox");
+  if (cspSandbox) {
+    response.setHeader(
+      "Content-Security-Policy",
+      "sandbox " + cspSandbox,
+      false
+    );
+  }
+
+  // Redirect to custom protocol via HTTP 302.
+  if (query.get("redirectCustomProtocol")) {
+    response.setStatusLine(request.httpVersion, 302, "Found");
+
+    response.setHeader("Location", "mailto:test@example.com", false);
+    response.write("Redirect!");
+    return;
+  }
+
+  response.setStatusLine(request.httpVersion, 200);
+  response.write("OK");
+}
diff --git a/uriloader/exthandler/tests/mochitest/redirect_helper.sjs b/uriloader/exthandler/tests/mochitest/redirect_helper.sjs
new file mode 100644
index 0000000000..cdf6415c54
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/redirect_helper.sjs
@@ -0,0 +1,28 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+function handleRequest(request, response) {
+  let params = new URLSearchParams(request.queryString);
+  let uri = params.get("uri");
+  let redirectType = params.get("redirectType") || "location";
+  switch (redirectType) {
+    case "refresh":
+      response.setStatusLine(request.httpVersion, 200, "OK");
+      response.setHeader("Refresh", "0; url=" + uri);
+      break;
+
+    case "meta-refresh":
+      response.setStatusLine(request.httpVersion, 200, "OK");
+      response.setHeader("Content-Type", "text/html");
+      response.write(`<meta http-equiv="refresh" content="0; url=${uri}">`);
+      break;
+
+    case "location":
+    // fall through
+    default:
+      response.setStatusLine(request.httpVersion, 302, "Moved Temporarily");
+      response.setHeader("Location", uri);
+  }
+}
diff --git a/uriloader/exthandler/tests/mochitest/save_filenames.html b/uriloader/exthandler/tests/mochitest/save_filenames.html
new file mode 100644
index 0000000000..8755186206
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/save_filenames.html
@@ -0,0 +1,377 @@
+<html>
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
+</head>
+<body>
+<style>
+  img { padding: 10px; border: 1px solid red; }
+  a { padding-left: 10px; }
+</style>
+
+<span id="items">
+
+<!-- simple filename -->
+<img id="i0" src="http://localhost:8000/basic.png"
+     data-noattach="true" data-filename="basic.png">
+
+<!-- simple filename with content disposition -->
+<img id="i1" src="http://localhost:8000/save_filename.sjs?type=png&filename=simple.png" data-filename="simple.png">
+
+<!-- invalid characters in the filename -->
+<img id="i2" src="http://localhost:8000/save_filename.sjs?type=png&filename=invalidfilename/a:b*c%63d.png" data-filename="invalidfilename_a b ccd.png">
+
+<!-- invalid extension for a png image -->
+<img id="i3" src="http://localhost:8000/save_filename.sjs?type=png&filename=invalidextension.pang" data-filename="invalidextension.png">
+
+<!-- jpeg extension for a png image -->
+<img id="i4" src="http://localhost:8000/save_filename.sjs?type=png&filename=reallyapng.jpeg" data-filename="reallyapng.png">
+
+<!-- txt extension for a png image -->
+<img id="i5" src="http://localhost:8000/save_filename.sjs?type=png&filename=nottext.txt" data-filename="nottext.png">
+
+<!-- no extension for a png image -->
+<img id="i6" src="http://localhost:8000/save_filename.sjs?type=png&filename=noext" data-filename="noext.png">
+
+<!-- empty extension for a png image -->
+<img id="i7" src="http://localhost:8000/save_filename.sjs?type=png&filename=noextdot." data-filename="noextdot.png">
+
+<!-- long filename -->
+<img id="i8" src="http://localhost:8000/save_filename.sjs?type=png&filename=averylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilename.png"
+     data-filename="averylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongf.png">
+
+<!-- long filename with invalid extension -->
+<img id="i9" src="http://localhost:8000/save_filename.sjs?type=png&filename=bverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilename.exe"
+     data-filename="bverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongf.png">
+
+<!-- long filename with invalid extension -->
+<img id="i10" src="http://localhost:8000/save_filename.sjs?type=png&filename=cverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilename.exe.jpg"
+     data-filename="cverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongfilenameverylongf.png">
+
+<!-- jpeg with jpg extension -->
+<img id="i11" src="http://localhost:8000/save_filename.sjs?type=jpeg&filename=thejpg.jpg" data-filename="thejpg.jpg">
+
+<!-- jpeg with jpeg extension -->
+<img id="i12" src="http://localhost:8000/save_filename.sjs?type=jpeg&filename=thejpg.jpeg" data-filename="thejpg.jpeg">
+
+<!-- jpeg with invalid extension -->
+<img id="i13" src="http://localhost:8000/save_filename.sjs?type=jpeg&filename=morejpg.exe" data-filename="morejpg.jpg" data-filename-platformlinux="morejpg.jpeg">
+
+<!-- jpeg with multiple extensions -->
+<img id="i14" src="http://localhost:8000/save_filename.sjs?type=jpeg&filename=anotherjpg.jpg.exe" data-filename="anotherjpg.jpg.jpg" data-filename-platformlinux="anotherjpg.jpg.jpeg">
+
+<!-- jpeg with no filename portion -->
+<img id="i15" src="http://localhost:8000/save_filename.sjs?type=jpeg&filename=.jpg"
+     data-filename="jpg.jpg" data-filename-platformlinux="jpg.jpeg">
+
+<!-- png with no filename portion and invalid extension -->
+<img id="i16" src="http://localhost:8000/save_filename.sjs?type=png&filename=.exe" data-filename="exe.png">
+
+<!-- png with escaped characters -->
+<img id="i17" src="http://localhost:8000/save_filename.sjs?type=png&filename=first%20file.png" data-filename="first file.png">
+
+<!-- png with more escaped characters -->
+<img id="i18" src="http://localhost:8000/save_filename.sjs?type=png&filename=second%32file%2Eexe" data-filename="second2file.png">
+
+<!-- unknown type with png extension -->
+<img id="i19" src="http://localhost:8000/save_filename.sjs?type=nonsense&filename=nonsense1.png"
+     data-nodrag="true" data-unknown="typeonly" data-filename="nonsense1.png">
+
+<!-- unknown type with exe extension -->
+<img id="i20" src="http://localhost:8000/save_filename.sjs?type=nonsense&filename=nonsense2.exe"
+     data-nodrag="true" data-unknown="typeonly" data-filename="nonsense2.exe">
+
+<!-- unknown type with no extension -->
+<img id="i21" src="http://localhost:8000/save_filename.sjs?type=nonsense&filename=nonsense3"
+     data-nodrag="true" data-unknown="typeonly" data-filename="nonsense3">
+
+<!-- simple script -->
+<script id="i22" src="http://localhost:8000/save_filename.sjs?type=js&filename=script1.js" data-filename="script1.js"></script>
+
+<!-- script with invalid extension. -->
+<script id="i23" src="http://localhost:8000/save_filename.sjs?type=js&filename=script2.exe"
+        data-filename="script2.exe" data-savepagename="script2.exe.js"></script>
+
+<!-- script with escaped characters -->
+<script id="i24" src="http://localhost:8000/save_filename.sjs?type=js&filename=script%20%33.exe"
+        data-filename="script 3.exe" data-savepagename="script 3.exe.js"></script>
+
+<!-- script with long filename -->
+<script id="i25" src="http://localhost:8000/save_filename.sjs?type=js&filename=script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789.js"
+        data-filename="script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script123456789script12345.js"></script>
+
+<!-- binary with exe extension -->
+<object id="i26" data="http://localhost:8000/save_filename.sjs?type=binary&filename=download1.exe"
+        data-unknown="true" data-filename="download1.exe"></object>
+
+<!-- binary with invalid extension -->
+<object id="i27" data="http://localhost:8000/save_filename.sjs?type=binary&filename=download2.png"
+        data-unknown="true" data-filename="download2.png"></object>
+
+<!-- binary with no extension -->
+<object id="i28" data="http://localhost:8000/save_filename.sjs?type=binary&filename=downloadnoext"
+        data-unknown="true" data-filename="downloadnoext"></object>
+
+<!-- binary with no other invalid characters -->
+<object id="i29" data="http://localhost:8000/save_filename.sjs?type=binary&filename=binary^%31%20exe.exe"
+        data-unknown="true" data-filename="binary^1 exe.exe"></object>
+
+<!-- unknown image type with no extension, but ending in png -->
+<img id="i30" src="http://localhost:8000/save_filename.sjs?type=otherimage&filename=specialpng"
+     data-unknown="typeonly" data-nodrag="true" data-filename="specialpng">
+
+<!-- unknown image type with no extension, but ending in many dots -->
+<img id="i31" src="http://localhost:8000/save_filename.sjs?type=otherimage&filename=extrapng..."
+     data-unknown="typeonly" data-nodrag="true" data-filename="extrapng">
+
+<!-- image type with no content-disposition filename specified -->
+<img id="i32" src="http://localhost:8000/save_filename.sjs?type=png" data-filename="save_filename.png">
+
+<!-- binary with no content-disposition filename specified -->
+<object id="i33" data="http://localhost:8000/save_filename.sjs?type=binary"
+        data-unknown="true" data-filename="save_filename.sjs"></object>
+
+<!-- image where url has png extension -->
+<img id="i34" src="http://localhost:8000/getdata.png?type=png&filename=override.png" data-filename="override.png">
+
+<!-- image where url has png extension but content disposition has incorrect extension -->
+<img id="i35" src="http://localhost:8000/getdata.png?type=png&filename=flower.jpeg" data-filename="flower.png">
+
+<!-- image where url has png extension but content disposition does not -->
+<img id="i36" src="http://localhost:8000/getdata.png?type=png&filename=ruby" data-filename="ruby.png">
+
+<!-- image where url has png extension but content disposition has invalid characters -->
+<img id="i37" src="http://localhost:8000/getdata.png?type=png&filename=sapphire/data" data-filename="sapphire_data.png">
+
+<!-- image where neither content disposition or url have an extension -->
+<img id="i38" src="http://localhost:8000/base?type=png&filename=emerald" data-filename="emerald.png">
+
+<!-- image where filename is not specified -->
+<img id="i39" src="http://localhost:8000/base?type=png" data-filename="base.png">
+
+<!-- simple script where url filename has no extension -->
+<script id="i40" src="http://localhost:8000/base?type=js&filename=script4.js" data-filename="script4.js"></script>
+
+<!-- script where url filename has no extension and invalid extension in content disposition filename -->
+<script id="i41" src="http://localhost:8000/base?type=js&filename=script5.exe"
+        data-filename="script5.exe" data-savepagename="script5.exe.js"></script>
+
+<!-- script where url filename has no extension and escaped characters in content disposition filename-->
+<script id="i42" src="http://localhost:8000/base?type=js&filename=script%20%36.exe"
+        data-filename="script 6.exe" data-savepagename="script 6.exe.js"></script>
+
+<!-- text where filename is present -->
+<img id="i43" src="http://localhost:8000/getdata.png?type=text&filename=readme.txt"
+     data-nodrag="true" data-filename="readme.txt">
+
+<!-- text where filename is present with a different extension -->
+<img id="i44" src="http://localhost:8000/getdata.png?type=text&filename=main.cpp"
+     data-nodrag="true" data-filename="main.cpp">
+
+<!-- text where extension is not present -->
+<img id="i45" src="http://localhost:8000/getdata.png?type=text&filename=readme"
+     data-nodrag="true" data-filename="readme">
+
+<!-- text where extension is not present and url does not have extension -->
+<img id="i46" src="http://localhost:8000/base?type=text&filename=info"
+     data-nodrag="true" data-filename="info">
+
+<!-- text where filename is not present -->
+<img id="i47" src="http://localhost:8000/basetext?type=text"
+     data-nodrag="true" data-filename="basetext">
+
+<!-- text where url has extension -->
+<img id="i48" src="http://localhost:8000/text2.txt?type=text"
+     data-nodrag="true" data-filename="text2.txt">
+
+<!-- text where url has extension -->
+<img id="i49" src="http://localhost:8000/text3.gonk?type=text"
+     data-nodrag="true" data-filename="text3.gonk">
+
+<!-- text with long filename -->
+<img id="i50" src="http://localhost:8000/text3.gonk?type=text&filename=text0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789text0123456789zztext0123456789zztext0123456789zztext01234567.exe.txt" data-nodrag="true" data-filename="text0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext0123456789zztext012345.txt">
+
+<!-- webp image -->
+<img id="i51" src="http://localhost:8000/save_filename.sjs?type=webp&filename=webpimage.webp"
+     data-filename="webpimage.webp">
+
+<!-- webp image with jpg extension -->
+<img id="i52" src="http://localhost:8000/save_filename.sjs?type=webp&filename=realwebpimage.jpg"
+     data-filename="realwebpimage.webp">
+
+<!-- no content type specified -->
+<img id="i53" src="http://localhost:8000/save_filename.sjs?&filename=notype.png"
+     data-nodrag="true" data-filename="notype.png">
+
+<!-- no content type specified. -->
+<img id="i54" src="http://localhost:8000/save_filename.sjs?&filename=notypebin.exe"
+     data-nodrag="true" data-filename="notypebin.exe">
+
+<!-- extension contains invalid characters -->
+<img id="i55" src="http://localhost:8000/save_filename.sjs?type=png&filename=extinvalid.a?*"
+     data-filename="extinvalid.png">
+
+<!-- filename with redirect and content disposition -->
+<img id="i56" src="http://localhost:8000/redir?type=png&filename=red.png" data-filename="red.png">
+
+<!-- filename with redirect and different type -->
+<img id="i57" src="http://localhost:8000/redir?type=jpeg&filename=green.png"
+     data-filename="green.jpg" data-filename-platformlinux="green.jpeg">
+
+<!-- filename with redirect and binary type -->
+<object id="i58" data="http://localhost:8000/redir?type=binary&filename=blue.png"
+        data-unknown="true" data-filename="blue.png"></object>
+
+<!-- filename in url with incorrect extension -->
+<img id="i59" src="http://localhost:8000/aquamarine.jpeg"
+     data-noattach="true" data-filename="aquamarine.png">
+
+<!-- filename in url with exe extension, but returns a png image -->
+<img id="i60" src="http://localhost:8000/lazuli.exe"
+     data-noattach="true" data-filename="lazuli.png">
+
+<!-- filename with leading, trailing and duplicate spaces -->
+<img id="i61" src="http://localhost:8000/save_filename.sjs?type=png&filename=  with  spaces.png  "
+     data-filename="with spaces.png">
+
+<!-- filename with leading and trailing periods -->
+<img id="i62" src="http://localhost:8000/save_filename.sjs?type=png&filename=..with..dots..png.."
+     data-filename="with..dots..png">
+
+<!-- filename with non-ascii character -->
+<img id="i63" src="http://localhost:8000/base?type=png&filename=s%C3%B6meescapes.%C3%B7ng" data-filename="sömeescapes.png">
+
+<!-- filename with content disposition name assigned. The name is only used
+     when selecting to manually save, otherwise it is ignored.  -->
+<img id="i64" src="http://localhost:8000/save_thename.sjs?type=png&dispname=withname"
+     data-filename="save_thename.png">
+
+<!-- reserved filename on Windows -->
+<img id="i65" src="http://localhost:8000/save_filename.sjs?type=text&filename=com1"
+     data-nodrag="true" data-filename="com1" data-filename-platformwin="Untitled">
+
+<!-- reserved filename with extension on Windows -->
+<img id="i66" src="http://localhost:8000/save_filename.sjs?type=text&filename=com2.any"
+     data-nodrag="true" data-filename="com2.any" data-filename-platformwin="Untitled">
+
+<!-- simple zip file -->
+<object id="i67" data="http://localhost:8000/save_filename.sjs?type=zip&filename=simple.zip" data-filename="simple.zip"
+        data-unknown="true"></object>
+
+<!-- simple zip file with differing extension -->
+<object id="i68" data="http://localhost:8000/save_filename.sjs?type=zip&filename=simple.jar" data-filename="simple.jar"
+        data-unknown="true"></object>
+
+<!-- simple zip file with no extension -->
+<object id="i69" data="http://localhost:8000/save_filename.sjs?type=zip&filename=simplepack" data-filename="simplepack.zip"
+        data-unknown="true"></object>
+
+<!-- simple json file -->
+<object id="i70" data="http://localhost:8000/save_filename.sjs?type=json&filename=simple.json" data-filename="simple.json"
+        data-unknown="true"></object>
+
+<!-- simple json file with differing extension -->
+<object id="i71" data="http://localhost:8000/save_filename.sjs?type=json&filename=simple.dat" data-filename="simple.dat"
+        data-unknown="true"></object>
+
+<!-- compressed file with .gz extension -->
+<img id="i72" src="http://localhost:8000/save_filename.sjs?type=png&filename=compressed.png.gz" data-filename="compressed.png.png">
+
+<!-- compressed file with .tar.gz extension -->
+<object id="i73" data="http://localhost:8000/save_filename.sjs?type=tar&filename=compressed2.tar.gz" data-filename="compressed2.tar.gz"
+        data-unknown="true"></object>
+
+<!-- compressed file with bittar.gz extension. There is no tar mime info on Windows so the filename is not changed. -->
+<object id="i74" data="http://localhost:8000/save_filename.sjs?type=tar&filename=compressed3.bittar.gz"
+        data-filename="compressed3.bittar.gz"
+        data-unknown="true"></object>
+
+<!-- compressed file with .tar.bz2 extension -->
+<object id="i75" data="http://localhost:8000/save_filename.sjs?type=tar&filename=buzz.tar.bz2" data-filename="buzz.tar.bz2"
+        data-unknown="true"></object>
+
+<!-- executable with no filename specified and an extension specified within the url -->
+<img id="i76" src="http://localhost:8000/executable.exe?type=nonsense"
+     data-nodrag="true" data-unknown="typeonly" data-filename="executable.exe">
+
+<!-- embedded child html -->
+<object id="i77" data="http://localhost:8000/save_filename.sjs?type=html&filename=child.par"
+        data-filename="child.par" data-unknown="true"></object>
+
+<!-- file starting with a dot with and unknown extension -->
+<img id="i78" src="http://localhost:8000/save_filename.sjs?type=png&filename=.extension" data-filename="extension.png">
+
+<!-- html file starting with a dot -->
+<object id="i79" data="http://localhost:8000/save_filename.sjs?type=html&filename=.alternate"
+        data-filename="alternate.html" data-filename-platformwin="alternate.htm" data-unknown="true"></object>
+
+<!-- unrecognized file type starting with a dot -->
+<object id="i80" data="http://localhost:8000/save_filename.sjs?type=otherimage&filename=.alternate" data-filename="alternate"
+        data-unknown="true"></object>
+
+<!-- filename with lnk extension -->
+<img id="i81" src="http://localhost:8000/save_filename.sjs?type=nonsense&filename=shortcut.lnk"
+     data-nodrag="true" data-unknown="typeonly"
+     data-filename="shortcut.lnk.download">
+
+<!-- long filename with lnk extension -->
+<img id="i82" src="http://localhost:8000/save_filename.sjs?type=nonsense&filename=longshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshort1234567890.lnk"
+     data-nodrag="true" data-unknown="typeonly"
+     data-filename="longshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshortcutnamelongshort1234567890.lnk.download">
+
+</span>
+
+<!-- This set is used to test filenames that are changed within the file picker. -->
+<span id="modifieditems">
+
+<!-- filename which is changed within the file picker -->
+<a id="mod0" href="http://localhost:8000/save_filename.sjs?type=png&filename=teal.png"
+     data-pickedfilename='cyan.png' data-filename='cyan.png'>
+
+<!-- filename which is changed to an invalid filename within the file picker -->
+<a id="mod1" href="http://localhost:8000/save_filename.sjs?type=png&filename=orange.png"
+     data-pickedfilename='"peach".png' data-filename='peach .png'>
+
+<!-- an invalid filename which is changed to another invalid filename within the file picker -->
+<a id="mod2" href="http://localhost:8000/save_filename.sjs?type=png&filename=%maroon%34.png"
+     data-pickedfilename='"violet".png' data-filename="violet .png">
+
+</span>
+
+<!-- This set is used to test the filename specified by the download attribute is validated correctly. -->
+<span id="downloads">
+  <a id="download0" href="http://localhost:8000/base" download="pearl.png" data-filename="pearl.png">Link</a>
+  <a id="download1" href="http://localhost:8000/save_filename.sjs?type=png" download="opal.jpeg" data-filename="opal.png">Link</a>
+  <a id="download2" href="http://localhost:8000/save_filename.sjs?type=jpeg"
+     download="amethyst.png" data-filename="amethyst.jpg"
+     data-filename-platformlinux="amethyst.jpeg">Link</a>
+  <a id="download3" href="http://localhost:8000/save_filename.sjs?type=text"
+     download="onyx.png" data-filename="onyx.png">Link</a>
+  <!-- The content-disposition overrides the download attribute. -->
+  <a id="download4" href="http://localhost:8000/save_filename.sjs?type=png&filename=fakename.jpeg" download="topaz.jpeg" data-filename="fakename.png">Link</a>
+  <a id="download5" href="http://localhost:8000/save_filename.sjs?type=png"
+     download="amber?.png" data-filename="amber .png">Link</a>
+  <a id="download6" href="http://localhost:8000/save_filename.sjs?type=jpeg"
+     download="jade.:*jpeg" data-filename="jade.jpg"
+     data-filename-platformlinux="jade.jpeg">Link</a>>
+  <a id="download7" href="http://localhost:8000/save_filename.sjs?type=png"
+     download="thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename.png"
+     data-filename="thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisaverylongfilename-thisisavery.png">Link</a>
+  <a id="download8" href="http://localhost:8000/base"
+      download="&Tab;&NewLine;&nbsp;&#11&#x180e;&#x180e;&#12   spa   ced.png &Tab;&NewLine;&nbsp;&#x180e;&#x180e;&#11&#12 "
+      data-filename="spa ced.png">Link</a>
+</span>
+
+<span id="links">
+  <a id="link0" href="http://localhost:8000/save_filename.sjs?type=png&filename=one.png" data-filename="one.png">One</a>
+  <a id="link1" href="http://localhost:8000/save_filename.sjs?type=png&filename=two.jpeg" data-filename="two.png">Two</a>
+  <a id="link2" href="http://localhost:8000/save_filename.sjs?type=png&filename=three.con" data-filename="three.png">Three</a>
+  <a id="link3" href="http://localhost:8000/save_filename.sjs?type=png&dispname=four" data-filename="four.png">Four</a>
+  <a id="link4" href="http://localhost:8000/save_filename.sjs?type=png&filename=five.local" data-filename="five.png">Five</a>
+</span>
+
+<!-- The content-disposition attachment generates links from the images/objects/scripts above
+     and inserts them here. -->
+<p id="attachment-links">
+</p>
+
+</body></html>
diff --git a/uriloader/exthandler/tests/mochitest/script_redirect.html b/uriloader/exthandler/tests/mochitest/script_redirect.html
new file mode 100644
index 0000000000..31e0dc6a7e
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/script_redirect.html
@@ -0,0 +1,5 @@
+<script>
+  let params = new URL(document.location).searchParams;
+  let uri = params.get("uri");
+  document.location = uri;
+</script>
diff --git a/uriloader/exthandler/tests/mochitest/test_invalidCharFileExtension.xhtml b/uriloader/exthandler/tests/mochitest/test_invalidCharFileExtension.xhtml
new file mode 100644
index 0000000000..177af3757f
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/test_invalidCharFileExtension.xhtml
@@ -0,0 +1,65 @@
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+  <title>Test for Handling of unsafe bidi chars</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<p id="display"></p>
+<iframe id="test"></iframe>
+<script type="text/javascript">
+var tests = [
+  ["test.png:large", "test.png"],
+  ["test.png/large", "test.png"],
+  [":test.png::large:", "test.png"],
+];
+
+add_task(async function() {
+  function promiseMessage(messageName) {
+    return new Promise(resolve => {
+      chromeScript.addMessageListener(messageName, function listener(data) {
+        chromeScript.removeMessageListener(messageName, listener);
+        resolve(data);
+      });
+    });
+  }
+
+  let url = SimpleTest.getTestFileURL("HelperAppLauncherDialog_chromeScript.js");
+  let chromeScript = SpecialPowers.loadChromeScript(url);
+
+  function wrongAPICallListener(msg) {
+    ok(
+      false,
+      `Called ${msg} when always ask pref was set to ${
+        Services.prefs.getBoolPref(
+          "browser.download.always_ask_before_handling_new_types"
+        )
+      }, which shouldn't happen.`
+    );
+  }
+  chromeScript.addMessageListener("wrongAPICall", wrongAPICallListener);
+
+  for (let prefVal of [false, true]) {
+    await SpecialPowers.pushPrefEnv({
+      set: [["browser.download.always_ask_before_handling_new_types", prefVal]]
+    });
+    // Run all the tests.
+    for (let [name, expected] of tests) {
+      let promiseName = promiseMessage("suggestedFileName");
+      document.getElementById("test").src =
+        "invalidCharFileExtension.sjs?name=" + encodeURIComponent(name);
+      is((await promiseName), expected, "got the expected sanitized name");
+    }
+  }
+
+  // Clean up.
+  let promise = promiseMessage("unregistered");
+  chromeScript.sendAsyncMessage("unregister");
+  await promise;
+
+  chromeScript.removeMessageListener("wrongAPICall", wrongAPICallListener);
+  chromeScript.destroy();
+});
+</script>
+</body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/test_nullCharFile.xhtml b/uriloader/exthandler/tests/mochitest/test_nullCharFile.xhtml
new file mode 100644
index 0000000000..b153395e81
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/test_nullCharFile.xhtml
@@ -0,0 +1,67 @@
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+  <title>Test for Handling of null char</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<p id="display"></p>
+<iframe id="test"></iframe>
+<script type="text/javascript">
+var tests = [
+  ["test.html\u0000.png", "test.html_.png"],
+  ["test.html.\u0000png", "test.html._png"],
+];
+
+add_task(async function() {
+  function promiseMessage(messageName) {
+    return new Promise(resolve => {
+      chromeScript.addMessageListener(messageName, function listener(data) {
+        chromeScript.removeMessageListener(messageName, listener);
+        resolve(data);
+      });
+    });
+  }
+
+  let url = SimpleTest.getTestFileURL("HelperAppLauncherDialog_chromeScript.js");
+  let chromeScript = SpecialPowers.loadChromeScript(url);
+
+  function wrongAPICallListener(msg) {
+    ok(
+      false,
+      `Called ${msg} when always ask pref was set to ${
+        Services.prefs.getBoolPref(
+          "browser.download.always_ask_before_handling_new_types"
+        )
+      }, which shouldn't happen.`
+    );
+  }
+  chromeScript.addMessageListener("wrongAPICall", wrongAPICallListener);
+
+  for (let prefVal of [false, true]) {
+    info("Pushing pref");
+    await SpecialPowers.pushPrefEnv({
+      set: [["browser.download.always_ask_before_handling_new_types", prefVal]]
+    });
+    for (let [name, expected] of tests) {
+      let promiseName = promiseMessage("suggestedFileName");
+      const a = document.createElement('a');
+      // Pass an unknown mimetype so we don't "correct" the extension:
+      a.href = "data:application/baconizer;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAAAAAA6fptVAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg==";
+      a.download = name;
+      a.dispatchEvent(new MouseEvent('click'));
+      is((await promiseName), expected, "got the expected sanitized name");
+    }
+  }
+
+  // Clean up.
+  let promise = promiseMessage("unregistered");
+  chromeScript.sendAsyncMessage("unregister");
+  await promise;
+
+  chromeScript.removeMessageListener("wrongAPICall", wrongAPICallListener);
+  chromeScript.destroy();
+});
+</script>
+</body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/test_spammy_page.html b/uriloader/exthandler/tests/mochitest/test_spammy_page.html
new file mode 100644
index 0000000000..b1e60a1e8e
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/test_spammy_page.html
@@ -0,0 +1,27 @@
+<!-- Any copyright is dedicated to the Public Domain.
+   - http://creativecommons.org/publicdomain/zero/1.0/ -->
+<!DOCTYPE html>
+<html>
+<head>
+	<meta charset="utf-8" />
+	<title>...</title>
+</head>
+<body>
+	<p> Hello, it's the spammy page! </p>
+<script type="text/javascript">
+	let count = 0;
+window.onload = window.onclick = function() {
+  if (count < 100) {
+	count++;
+	let l = document.createElement('a');
+	l.href = 'data:text/plain,some text';
+	l.download = 'sometext.pdf';
+
+	document.body.appendChild(l);
+	l.click();
+  }
+}
+</script>
+<a id="image" href="${TEST_ROOT}/file_with@@funny_name.png">Image</a>
+</body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/test_unknown_ext_protocol_handlers.html b/uriloader/exthandler/tests/mochitest/test_unknown_ext_protocol_handlers.html
new file mode 100644
index 0000000000..f8727db605
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/test_unknown_ext_protocol_handlers.html
@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <title>Test for no error reporting for unknown external protocols</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<iframe id="testFrame"></iframe>
+<script type="text/javascript">
+
+SimpleTest.waitForExplicitFinish();
+
+window.onload = () => {
+  let testFrame = document.getElementById("testFrame");
+
+  try {
+    testFrame.contentWindow.location.href = "unknownextproto:";
+    ok(true, "There is no error reporting for unknown external protocol navigation.");
+  } catch (e) {
+    ok(false, "There should be no error reporting for unknown external protocol navigation.");
+  }
+
+  SimpleTest.finish();
+};
+</script>
+</body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/test_unsafeBidiChars.xhtml b/uriloader/exthandler/tests/mochitest/test_unsafeBidiChars.xhtml
new file mode 100644
index 0000000000..34c6c956fd
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/test_unsafeBidiChars.xhtml
@@ -0,0 +1,89 @@
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+  <title>Test for Handling of unsafe bidi chars</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
+</head>
+<body>
+<p id="display"></p>
+<iframe id="test"></iframe>
+<script type="text/javascript">
+var unsafeBidiChars = [
+  "\xe2\x80\xaa", // LRE
+  "\xe2\x80\xab", // RLE
+  "\xe2\x80\xac", // PDF
+  "\xe2\x80\xad", // LRO
+  "\xe2\x80\xae",  // RLO
+];
+
+var tests = [
+  "{1}.test",
+  "{1}File.test",
+  "Fi{1}le.test",
+  "File{1}.test",
+  "File.{1}test",
+  "File.te{1}st",
+  "File.test{1}",
+  "File.{1}",
+];
+
+function replace(name, x) {
+  return name.replace(/\{1\}/, x);
+}
+
+function sanitize(name) {
+  return replace(name, "_");
+}
+
+add_task(async function() {
+  function promiseMessage(messageName) {
+    return new Promise(resolve => {
+      chromeScript.addMessageListener(messageName, function listener(data) {
+        chromeScript.removeMessageListener(messageName, listener);
+        resolve(data);
+      });
+    });
+  }
+
+  let url = SimpleTest.getTestFileURL("HelperAppLauncherDialog_chromeScript.js");
+  let chromeScript = SpecialPowers.loadChromeScript(url);
+
+  function wrongAPICallListener(msg) {
+    ok(
+      false,
+      `Called ${msg} when always ask pref was set to ${
+        Services.prefs.getBoolPref(
+          "browser.download.always_ask_before_handling_new_types"
+        )
+      }, which shouldn't happen.`
+    );
+  }
+  chromeScript.addMessageListener("wrongAPICall", wrongAPICallListener);
+
+  for (let prefVal of [false, true]) {
+    await SpecialPowers.pushPrefEnv({
+      set: [["browser.download.always_ask_before_handling_new_types", prefVal]]
+    });
+    for (let test of tests) {
+      for (let char of unsafeBidiChars) {
+        let promiseName = promiseMessage("suggestedFileName");
+        let name = replace(test, char);
+        let expected = sanitize(test);
+        document.getElementById("test").src =
+          "unsafeBidiFileName.sjs?name=" + encodeURIComponent(name);
+        is((await promiseName), expected, "got the expected sanitized name");
+      }
+    }
+  }
+
+  // Clean up.
+  let promise = promiseMessage("unregistered");
+  chromeScript.sendAsyncMessage("unregister");
+  await promise;
+
+  chromeScript.removeMessageListener("wrongAPICall", wrongAPICallListener);
+  chromeScript.destroy();
+});
+</script>
+</body>
+</html>
diff --git a/uriloader/exthandler/tests/mochitest/unsafeBidiFileName.sjs b/uriloader/exthandler/tests/mochitest/unsafeBidiFileName.sjs
new file mode 100644
index 0000000000..4f88ff6de2
--- /dev/null
+++ b/uriloader/exthandler/tests/mochitest/unsafeBidiFileName.sjs
@@ -0,0 +1,18 @@
+/* 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/. */
+
+function handleRequest(request, response) {
+  response.setStatusLine(request.httpVersion, 200, "OK");
+
+  if (!request.queryString.match(/^name=/)) {
+    return;
+  }
+  var name = decodeURIComponent(request.queryString.substring(5));
+
+  response.setHeader(
+    "Content-Type",
+    'application/octet-stream; name="' + name + '"'
+  );
+  response.setHeader("Content-Disposition", 'inline; filename="' + name + '"');
+}
diff --git a/uriloader/exthandler/tests/moz.build b/uriloader/exthandler/tests/moz.build
new file mode 100644
index 0000000000..95d7f5fe59
--- /dev/null
+++ b/uriloader/exthandler/tests/moz.build
@@ -0,0 +1,35 @@
+# -*- 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/.
+
+MOCHITEST_MANIFESTS += ["mochitest/mochitest.toml"]
+
+XPCSHELL_TESTS_MANIFESTS += ["unit/xpcshell.toml"]
+
+BROWSER_CHROME_MANIFESTS += ["mochitest/browser.toml"]
+
+TEST_DIRS += [
+    "gtest",
+]
+
+TESTING_JS_MODULES += [
+    "HandlerServiceTestUtils.sys.mjs",
+]
+
+GeckoSimplePrograms(
+    [
+        "WriteArgument",
+    ],
+    linkage=None,
+)
+
+if CONFIG["COMPILE_ENVIRONMENT"]:
+    TEST_HARNESS_FILES.xpcshell.uriloader.exthandler.tests.unit += [
+        "!WriteArgument%s" % CONFIG["BIN_SUFFIX"]
+    ]
+
+USE_LIBS += [
+    "nspr",
+]
diff --git a/uriloader/exthandler/tests/unit/handlers.json b/uriloader/exthandler/tests/unit/handlers.json
new file mode 100644
index 0000000000..51ce581d83
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/handlers.json
@@ -0,0 +1,83 @@
+{
+  "defaultHandlersVersion": {
+    "en-US": 999
+  },
+  "mimeTypes": {
+    "example/type.handleinternally": {
+      "unknownProperty": "preserved",
+      "action": 3,
+      "extensions": ["example_one"]
+    },
+    "example/type.savetodisk": {
+      "action": 0,
+      "ask": true,
+      "handlers": [
+        {
+          "name": "Example Default Handler",
+          "uriTemplate": "https://www.example.com/?url=%s"
+        }
+      ],
+      "extensions": ["example_two", "example_three"]
+    },
+    "example/type.usehelperapp": {
+      "action": 2,
+      "ask": true,
+      "handlers": [
+        {
+          "name": "Example Default Handler",
+          "uriTemplate": "https://www.example.com/?url=%s"
+        },
+        {
+          "name": "Example Possible Handler One",
+          "uriTemplate": "http://www.example.com/?id=1&url=%s"
+        },
+        {
+          "name": "Example Possible Handler Two",
+          "uriTemplate": "http://www.example.com/?id=2&url=%s"
+        }
+      ],
+      "extensions": ["example_two", "example_three"]
+    },
+    "example/type.usesystemdefault": {
+      "action": 4,
+      "handlers": [
+        null,
+        {
+          "name": "Example Possible Handler",
+          "uriTemplate": "http://www.example.com/?url=%s"
+        }
+      ]
+    }
+  },
+  "schemes": {
+    "examplescheme.usehelperapp": {
+      "action": 2,
+      "ask": true,
+      "handlers": [
+        {
+          "name": "Example Default Handler",
+          "uriTemplate": "https://www.example.com/?url=%s"
+        },
+        {
+          "name": "Example Possible Handler One",
+          "uriTemplate": "http://www.example.com/?id=1&url=%s"
+        },
+        {
+          "name": "Example Possible Handler Two",
+          "uriTemplate": "http://www.example.com/?id=2&url=%s"
+        }
+      ]
+    },
+    "examplescheme.usesystemdefault": {
+      "action": 4,
+      "handlers": [
+        null,
+        {
+          "name": "Example Possible Handler",
+          "uriTemplate": "http://www.example.com/?url=%s"
+        }
+      ]
+    }
+  },
+  "isDownloadsImprovementsAlreadyMigrated": true
+}
diff --git a/uriloader/exthandler/tests/unit/head.js b/uriloader/exthandler/tests/unit/head.js
new file mode 100644
index 0000000000..72d1ac87c5
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/head.js
@@ -0,0 +1,84 @@
+/* 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/. */
+
+/*
+ * Initialization for tests related to invoking external handler applications.
+ */
+
+"use strict";
+
+var { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+var { FileUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/FileUtils.sys.mjs"
+);
+var { NetUtil } = ChromeUtils.importESModule(
+  "resource://gre/modules/NetUtil.sys.mjs"
+);
+var { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+
+const { HandlerServiceTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/HandlerServiceTestUtils.sys.mjs"
+);
+var { TestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/TestUtils.sys.mjs"
+);
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "gHandlerService",
+  "@mozilla.org/uriloader/handler-service;1",
+  "nsIHandlerService"
+);
+
+do_get_profile();
+
+let jsonPath = PathUtils.join(PathUtils.profileDir, "handlers.json");
+
+/**
+ * Unloads the nsIHandlerService data store, so the back-end file can be
+ * accessed or modified, and the new data will be loaded at the next access.
+ */
+let unloadHandlerStore = async function () {
+  // If this function is called before the nsIHandlerService instance has been
+  // initialized for the first time, the observer below will not be registered.
+  // We have to force initialization to prevent the function from stalling.
+  gHandlerService;
+
+  let promise = TestUtils.topicObserved("handlersvc-json-replace-complete");
+  Services.obs.notifyObservers(null, "handlersvc-json-replace");
+  await promise;
+};
+
+/**
+ * Unloads the data store and deletes it.
+ */
+let deleteHandlerStore = async function () {
+  await unloadHandlerStore();
+
+  await IOUtils.remove(jsonPath, { ignoreAbsent: true });
+
+  Services.prefs.clearUserPref("gecko.handlerService.defaultHandlersVersion");
+};
+
+/**
+ * Unloads the data store and replaces it with the test data file.
+ */
+let copyTestDataToHandlerStore = async function () {
+  await unloadHandlerStore();
+
+  await IOUtils.copy(do_get_file("handlers.json").path, jsonPath);
+
+  Services.prefs.setIntPref("gecko.handlerService.defaultHandlersVersion", 100);
+};
+
+/**
+ * Ensures the files are removed and the services unloaded when the tests end.
+ */
+registerCleanupFunction(async function test_terminate() {
+  await deleteHandlerStore();
+});
diff --git a/uriloader/exthandler/tests/unit/mailcap b/uriloader/exthandler/tests/unit/mailcap
new file mode 100644
index 0000000000..dc93ef8042
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/mailcap
@@ -0,0 +1,2 @@
+text/plain; cat '%s'; needsterminal
+text/plain; sed '%s'
diff --git a/uriloader/exthandler/tests/unit/test_badMIMEType.js b/uriloader/exthandler/tests/unit/test_badMIMEType.js
new file mode 100644
index 0000000000..49c5e8d848
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_badMIMEType.js
@@ -0,0 +1,29 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 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/. */
+
+function run_test() {
+  // "text/plain" has an 0xFF character appended to it.  This means it's an
+  // invalid string, which is tricky to enter using a text editor (I used
+  // emacs' hexl-mode).  It also means an ordinary text editor might drop it
+  // or convert it to something that *is* valid (in UTF8).  So we measure
+  // its length to make sure this hasn't happened.
+  var badMimeType = "text/plainÿ";
+  Assert.equal(badMimeType.length, 11);
+  try {
+    Cc["@mozilla.org/mime;1"]
+      .getService(Ci.nsIMIMEService)
+      .getFromTypeAndExtension(badMimeType, "txt");
+  } catch (e) {
+    if (
+      !(e instanceof Ci.nsIException) ||
+      e.result != Cr.NS_ERROR_NOT_AVAILABLE
+    ) {
+      throw e;
+    }
+    // This is an expected exception, thrown if the type can't be determined
+  }
+  // Not crashing is good enough
+  Assert.equal(true, true);
+}
diff --git a/uriloader/exthandler/tests/unit/test_defaults_handlerService.js b/uriloader/exthandler/tests/unit/test_defaults_handlerService.js
new file mode 100644
index 0000000000..72d3779fd8
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_defaults_handlerService.js
@@ -0,0 +1,202 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "gExternalProtocolService",
+  "@mozilla.org/uriloader/external-protocol-service;1",
+  "nsIExternalProtocolService"
+);
+ChromeUtils.defineESModuleGetters(this, {
+  kHandlerList: "resource://gre/modules/handlers/HandlerList.sys.mjs",
+});
+
+add_task(async function test_check_defaults_get_added() {
+  let protocols = Object.keys(kHandlerList.default.schemes);
+  for (let protocol of protocols) {
+    let protocolHandlerCount =
+      kHandlerList.default.schemes[protocol].handlers.length;
+    Assert.ok(
+      gHandlerService.wrappedJSObject._store.data.schemes[protocol].stubEntry,
+      `Expect stub for ${protocol}`
+    );
+    let info = gExternalProtocolService.getProtocolHandlerInfo(protocol, {});
+    Assert.ok(
+      info,
+      `Should be able to get protocol handler info for ${protocol}`
+    );
+    let handlers = Array.from(
+      info.possibleApplicationHandlers.enumerate(Ci.nsIHandlerApp)
+    );
+    handlers = handlers.filter(h => h instanceof Ci.nsIWebHandlerApp);
+    Assert.equal(
+      handlers.length,
+      protocolHandlerCount,
+      `Default web handlers for ${protocol} should match`
+    );
+    let { alwaysAskBeforeHandling, preferredAction } = info;
+    // Actually store something, pretending there was a change:
+    let infoToWrite = gExternalProtocolService.getProtocolHandlerInfo(
+      protocol,
+      {}
+    );
+    gHandlerService.store(infoToWrite);
+    ok(
+      !gHandlerService.wrappedJSObject._store.data.schemes[protocol].stubEntry,
+      "Expect stub entry info to go away"
+    );
+
+    let newInfo = gExternalProtocolService.getProtocolHandlerInfo(protocol, {});
+    Assert.equal(
+      alwaysAskBeforeHandling,
+      newInfo.alwaysAskBeforeHandling,
+      protocol + " - always ask shouldn't change"
+    );
+    Assert.equal(
+      preferredAction,
+      newInfo.preferredAction,
+      protocol + " - preferred action shouldn't change"
+    );
+    await deleteHandlerStore();
+  }
+});
+
+add_task(async function test_check_default_modification() {
+  Assert.ok(
+    true,
+    JSON.stringify(gHandlerService.wrappedJSObject._store.data.schemes.mailto)
+  );
+  Assert.ok(
+    gHandlerService.wrappedJSObject._store.data.schemes.mailto.stubEntry,
+    "Expect stub for mailto"
+  );
+  let mailInfo = gExternalProtocolService.getProtocolHandlerInfo("mailto", {});
+  mailInfo.alwaysAskBeforeHandling = false;
+  mailInfo.preferredAction = Ci.nsIHandlerInfo.useSystemDefault;
+  gHandlerService.store(mailInfo);
+  Assert.ok(
+    !gHandlerService.wrappedJSObject._store.data.schemes.mailto.stubEntry,
+    "Stub entry should be removed immediately."
+  );
+  let newMail = gExternalProtocolService.getProtocolHandlerInfo("mailto", {});
+  Assert.equal(newMail.preferredAction, Ci.nsIHandlerInfo.useSystemDefault);
+  Assert.equal(newMail.alwaysAskBeforeHandling, false);
+  await deleteHandlerStore();
+});
+
+add_task(async function test_migrations() {
+  const kTestData = [
+    ["A", "http://compose.mail.yahoo.co.jp/ym/Compose?To=%s"],
+    ["B", "http://www.inbox.lv/rfc2368/?value=%s"],
+    ["C", "http://poczta.interia.pl/mh/?mailto=%s"],
+    ["D", "http://win.mail.ru/cgi-bin/sentmsg?mailto=%s"],
+  ];
+  // Set up the test handlers. This doesn't use prefs like the previous test,
+  // because we now refuse to import insecure handler prefs. They can only
+  // exist if they were added into the handler store before this restriction
+  // was created (bug 1526890).
+  gHandlerService.wrappedJSObject._injectDefaultProtocolHandlers();
+  let handler = gExternalProtocolService.getProtocolHandlerInfo("mailto");
+  while (handler.possibleApplicationHandlers.length) {
+    handler.possibleApplicationHandlers.removeElementAt(0);
+  }
+  for (let [name, uriTemplate] of kTestData) {
+    let app = Cc["@mozilla.org/uriloader/web-handler-app;1"].createInstance(
+      Ci.nsIWebHandlerApp
+    );
+    app.uriTemplate = uriTemplate;
+    app.name = name;
+    handler.possibleApplicationHandlers.appendElement(app);
+  }
+  gHandlerService.store(handler);
+
+  // Now migrate them:
+  Services.prefs.setCharPref("browser.handlers.migrations", "blah,secure-mail");
+  gHandlerService.wrappedJSObject._migrateProtocolHandlersIfNeeded();
+
+  // Now check the result:
+  handler = gExternalProtocolService.getProtocolHandlerInfo("mailto");
+
+  let expectedURIs = new Set([
+    "https://mail.yahoo.co.jp/compose/?To=%s",
+    "https://mail.inbox.lv/compose?to=%s",
+    "https://poczta.interia.pl/mh/?mailto=%s",
+    "https://e.mail.ru/cgi-bin/sentmsg?mailto=%s",
+  ]);
+
+  let possibleApplicationHandlers = Array.from(
+    handler.possibleApplicationHandlers.enumerate(Ci.nsIWebHandlerApp)
+  );
+  // Set iterators are stable to deletion, so this works:
+  for (let expected of expectedURIs) {
+    for (let app of possibleApplicationHandlers) {
+      if (app instanceof Ci.nsIWebHandlerApp && app.uriTemplate == expected) {
+        Assert.ok(true, "Found handler with URI " + expected);
+        // ... even when we remove items.
+        expectedURIs.delete(expected);
+        break;
+      }
+    }
+  }
+  Assert.equal(expectedURIs.size, 0, "Should have seen all the expected URIs.");
+
+  for (let app of possibleApplicationHandlers) {
+    if (app instanceof Ci.nsIWebHandlerApp) {
+      Assert.ok(
+        !kTestData.some(n => n[1] == app.uriTemplate),
+        "Should not be any of the original handlers"
+      );
+    }
+  }
+
+  Assert.ok(
+    !handler.preferredApplicationHandler,
+    "Shouldn't have preferred handler initially."
+  );
+  await deleteHandlerStore();
+});
+
+/**
+ * Check that non-https templates or ones without a '%s' are ignored.
+ */
+add_task(async function invalid_handlers_are_rejected() {
+  let schemes = kHandlerList.default.schemes;
+  schemes.myfancyinvalidstuff = {
+    handlers: [
+      {
+        name: "No template at all",
+      },
+      {
+        name: "Not secure",
+        uriTemplate: "http://example.com/%s",
+      },
+      {
+        name: "No replacement percent-s bit",
+        uriTemplate: "https://example.com/",
+      },
+      {
+        name: "Actually valid",
+        uriTemplate: "https://example.com/%s",
+      },
+    ],
+  };
+  gHandlerService.wrappedJSObject._injectDefaultProtocolHandlers();
+  // Now check the result:
+  let handler = gExternalProtocolService.getProtocolHandlerInfo(
+    "myfancyinvalidstuff"
+  );
+
+  let expectedURIs = ["https://example.com/%s"];
+
+  Assert.deepEqual(
+    Array.from(
+      handler.possibleApplicationHandlers.enumerate(Ci.nsIWebHandlerApp),
+      e => e.uriTemplate
+    ),
+    expectedURIs,
+    "Should have seen only 1 handler added."
+  );
+  await deleteHandlerStore();
+});
diff --git a/uriloader/exthandler/tests/unit/test_filename_sanitize.js b/uriloader/exthandler/tests/unit/test_filename_sanitize.js
new file mode 100644
index 0000000000..d8ab12c266
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_filename_sanitize.js
@@ -0,0 +1,398 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// This test verifies that
+// nsIMIMEService.validateFileNameForSaving sanitizes filenames
+// properly with different flags.
+
+"use strict";
+
+add_task(async function validate_filename_method() {
+  let mimeService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+
+  function checkFilename(filename, flags, mime = "image/png") {
+    return mimeService.validateFileNameForSaving(filename, mime, flags);
+  }
+
+  Assert.equal(checkFilename("basicfile.png", 0), "basicfile.png");
+  Assert.equal(checkFilename("  whitespace.png  ", 0), "whitespace.png");
+  Assert.equal(
+    checkFilename(" .whitespaceanddots.png...", 0),
+    "whitespaceanddots.png"
+  );
+  Assert.equal(
+    checkFilename("  \u00a0 \u00a0 extrawhitespace.png  \u00a0 \u00a0 ", 0),
+    "extrawhitespace.png"
+  );
+  Assert.equal(
+    checkFilename("  filename  with  whitespace.png  ", 0),
+    "filename with whitespace.png"
+  );
+  Assert.equal(checkFilename("\\path.png", 0), "_path.png");
+  Assert.equal(
+    checkFilename("\\path*and/$?~file.png", 0),
+    "_path and_$ ~file.png"
+  );
+  Assert.equal(
+    checkFilename(" \u180e whit\u180ee.png \u180e", 0),
+    "whit\u180ee.png"
+  );
+  Assert.equal(checkFilename("簡単簡単簡単", 0), "簡単簡単簡単.png");
+  Assert.equal(checkFilename(" happy\u061c\u2069.png", 0), "happy__.png");
+  Assert.equal(
+    checkFilename("12345678".repeat(31) + "abcdefgh.png", 0),
+    "12345678".repeat(31) + "ab.png"
+  );
+  Assert.equal(
+    checkFilename("簡単".repeat(41) + ".png", 0),
+    "簡単".repeat(41) + ".png"
+  );
+  Assert.equal(
+    checkFilename("a" + "簡単".repeat(42) + ".png", 0),
+    "a" + "簡単".repeat(40) + "簡.png"
+  );
+  Assert.equal(
+    checkFilename("a" + "簡単".repeat(56) + ".png", 0),
+    "a" + "簡単".repeat(40) + ".png"
+  );
+  Assert.equal(checkFilename("café.png", 0), "café.png");
+  Assert.equal(
+    checkFilename("café".repeat(50) + ".png", 0),
+    "café".repeat(50) + ".png"
+  );
+  Assert.equal(
+    checkFilename("café".repeat(51) + ".png", 0),
+    "café".repeat(49) + "caf.png"
+  );
+
+  Assert.equal(
+    checkFilename("\u{100001}\u{100002}.png", 0),
+    "\u{100001}\u{100002}.png"
+  );
+  Assert.equal(
+    checkFilename("\u{100001}\u{100002}".repeat(31) + ".png", 0),
+    "\u{100001}\u{100002}".repeat(31) + ".png"
+  );
+  Assert.equal(
+    checkFilename("\u{100001}\u{100002}".repeat(32) + ".png", 0),
+    "\u{100001}\u{100002}".repeat(30) + "\u{100001}.png"
+  );
+
+  Assert.equal(
+    checkFilename("noextensionfile".repeat(16), 0),
+    "noextensionfile".repeat(16) + ".png"
+  );
+  Assert.equal(
+    checkFilename("noextensionfile".repeat(17), 0),
+    "noextensionfile".repeat(16) + "noextensio.png"
+  );
+  Assert.equal(
+    checkFilename("noextensionfile".repeat(16) + "noextensionfil.", 0),
+    "noextensionfile".repeat(16) + "noextensio.png"
+  );
+
+  Assert.equal(checkFilename("  first  .png  ", 0), "first .png");
+  Assert.equal(
+    checkFilename(
+      "  second  .png  ",
+      mimeService.VALIDATE_DONT_COLLAPSE_WHITESPACE
+    ),
+    "second  .png"
+  );
+
+  // For whatever reason, the Android mime handler accepts the .jpeg
+  // extension for image/png, so skip this test there.
+  if (AppConstants.platform != "android") {
+    Assert.equal(checkFilename("thi/*rd.jpeg", 0), "thi_ rd.png");
+  }
+
+  Assert.equal(
+    checkFilename("f*\\ourth  file.jpg", mimeService.VALIDATE_SANITIZE_ONLY),
+    "f _ourth file.jpg"
+  );
+  Assert.equal(
+    checkFilename(
+      "f*\\ift  h.jpe*\\g",
+      mimeService.VALIDATE_SANITIZE_ONLY |
+        mimeService.VALIDATE_DONT_COLLAPSE_WHITESPACE
+    ),
+    "f _ift  h.jpe _g"
+  );
+  Assert.equal(checkFilename("sixth.j  pe/*g", 0), "sixth.png");
+
+  let repeatStr = "12345678".repeat(31);
+  Assert.equal(
+    checkFilename(
+      repeatStr + "seventh.png",
+      mimeService.VALIDATE_DONT_TRUNCATE
+    ),
+    repeatStr + "seventh.png"
+  );
+  Assert.equal(
+    checkFilename(repeatStr + "seventh.png", 0),
+    repeatStr + "se.png"
+  );
+
+  // no filename, so index is used by default.
+  Assert.equal(checkFilename(".png", 0), "png.png");
+
+  // sanitization only, so Untitled is not added, but initial period is stripped.
+  Assert.equal(
+    checkFilename(".png", mimeService.VALIDATE_SANITIZE_ONLY),
+    "png"
+  );
+
+  // correct .png extension is applied.
+  Assert.equal(checkFilename(".butterpecan.icecream", 0), "butterpecan.png");
+
+  // sanitization only, so extension is not modified, but initial period is stripped.
+  Assert.equal(
+    checkFilename(".butterpecan.icecream", mimeService.VALIDATE_SANITIZE_ONLY),
+    "butterpecan.icecream"
+  );
+
+  let ext = ".fairlyLongExtension";
+  Assert.equal(
+    checkFilename(repeatStr + ext, mimeService.VALIDATE_SANITIZE_ONLY),
+    repeatStr.substring(0, 254 - ext.length) + ext
+  );
+
+  ext = "lo%?n/ginvalid? ch\\ars";
+  Assert.equal(
+    checkFilename(repeatStr + ext, mimeService.VALIDATE_SANITIZE_ONLY),
+    repeatStr + "lo% n_"
+  );
+
+  ext = ".long/invalid%? ch\\ars";
+  Assert.equal(
+    checkFilename(repeatStr + ext, mimeService.VALIDATE_SANITIZE_ONLY),
+    repeatStr.substring(0, 233) + ".long_invalid% ch_ars"
+  );
+
+  Assert.equal(
+    checkFilename("test_ﾃｽﾄ_T\x83E\\S\x83T.png", 0),
+    "test_ﾃｽﾄ_T E_S T.png"
+  );
+  Assert.equal(
+    checkFilename("test_ﾃｽﾄ_T\x83E\\S\x83T.pﾃ\x83ng", 0),
+    "test_ﾃｽﾄ_T E_S T.png"
+  );
+
+  // Check we don't invalidate surrogate pairs when trimming.
+  Assert.equal(checkFilename("test😀", 0, ""), "test😀");
+  Assert.equal(checkFilename("test😀😀", 0, ""), "test😀😀");
+
+  // Now check some media types
+  Assert.equal(
+    mimeService.validateFileNameForSaving("video.ogg", "video/ogg", 0),
+    "video.ogg",
+    "video.ogg"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("video.ogv", "video/ogg", 0),
+    "video.ogv",
+    "video.ogv"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("video.ogt", "video/ogg", 0),
+    "video.ogv",
+    "video.ogt"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving("audio.mp3", "audio/mpeg", 0),
+    "audio.mp3",
+    "audio.mp3"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("audio.mpega", "audio/mpeg", 0),
+    "audio.mpega",
+    "audio.mpega"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("audio.mp2", "audio/mpeg", 0),
+    "audio.mp2",
+    "audio.mp2"
+  );
+
+  let expected = "audio.mp3";
+  if (AppConstants.platform == "linux") {
+    expected = "audio.mpga";
+  } else if (AppConstants.platform == "android") {
+    expected = "audio.mp4";
+  }
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving("audio.mp4", "audio/mpeg", 0),
+    expected,
+    "audio.mp4"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving("sound.m4a", "audio/mp4", 0),
+    "sound.m4a",
+    "sound.m4a"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("sound.m4b", "audio/mp4", 0),
+    AppConstants.platform == "android" ? "sound.m4a" : "sound.m4b",
+    "sound.m4b"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("sound.m4c", "audio/mp4", 0),
+    AppConstants.platform == "macosx" ? "sound.mp4" : "sound.m4a",
+    "sound.mpc"
+  );
+
+  // This has a long filename with a 13 character extension. The end of the filename should be
+  // cropped to fit into 255 bytes.
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "라이브9.9만 시청컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장24%102 000원 브랜드데이 앵콜 🎁 1.등 유산균 컬처렐 특가!",
+      "text/unknown",
+      mimeService.VALIDATE_SANITIZE_ONLY
+    ),
+    "라이브9.9만 시청컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 .등 유산균 컬처렐 특가!",
+    "very long filename with extension"
+  );
+
+  // This filename has a very long extension, almost the entire filename.
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "라이브9.9만 시청컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장24%102 000원 브랜드데이 앵콜 🎁 1등 유산균 컬처렐 특가!",
+      "text/unknown",
+      mimeService.VALIDATE_SANITIZE_ONLY
+    ),
+    "라이브9",
+    "another very long filename with long extension"
+  );
+
+  // This filename is cropped at 254 bytes.
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      ".라이브99만 시청컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장24%102 000원 브랜드데이 앵콜 🎁 1등 유산균 컬처렐 특가!",
+      "text/unknown",
+      mimeService.VALIDATE_SANITIZE_ONLY
+    ),
+    "라이브99만 시청컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장컬처렐 다이제스티브 3박스 - 3박스 더 (뚱랑이 굿즈 증정) - 선물용 쇼핑백 2장24%102 000원 브랜드데",
+    "very filename with extension only"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving("filename.LNK", "text/unknown", 0),
+    "filename.LNK.download",
+    "filename.LNK"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("filename.local", "text/unknown", 0),
+    "filename.local.download",
+    "filename.local"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("filename.url", "text/unknown", 0),
+    "filename.url.download",
+    "filename.url"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("filename.URl", "text/unknown", 0),
+    "filename.URl.download",
+    "filename.URl"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("filename.scf", "text/unknown", 0),
+    "filename.scf.download",
+    "filename.scf"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving("filename.sCF", "text/unknown", 0),
+    "filename.sCF.download",
+    "filename.sCF"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving("filename.lnk\n", "text/unknown", 0),
+    "filename.lnk.download",
+    "filename.lnk with newline"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.lnk\n  ",
+      "text/unknown",
+      0
+    ),
+    "filename.lnk.download",
+    "filename.lnk with newline"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.\n\t  lnk",
+      "text/unknown",
+      0
+    ),
+    "filename. lnk",
+    "filename.lnk with space and newline"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.local\u180e\u180e\u180e",
+      "text/unknown",
+      0
+    ),
+    "filename.local.download",
+    "filename.lnk with vowel separators"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.LNK",
+      "text/unknown",
+      mimeService.VALIDATE_SANITIZE_ONLY
+    ),
+    "filename.LNK.download",
+    "filename.LNK sanitize only"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.LNK\n",
+      "text/unknown",
+      mimeService.VALIDATE_ALLOW_INVALID_FILENAMES
+    ),
+    "filename.LNK",
+    "filename.LNK allow invalid"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.URL\n",
+      "text/unknown",
+      mimeService.VALIDATE_SANITIZE_ONLY |
+        mimeService.VALIDATE_ALLOW_INVALID_FILENAMES
+    ),
+    "filename.URL",
+    "filename.URL allow invalid, sanitize only"
+  );
+
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.desktop",
+      "text/unknown",
+      mimeService.VALIDATE_SANITIZE_ONLY
+    ),
+    "filename.desktop.download",
+    "filename.desktop sanitize only"
+  );
+  Assert.equal(
+    mimeService.validateFileNameForSaving(
+      "filename.DESKTOP\n",
+      "text/unknown",
+      mimeService.VALIDATE_SANITIZE_ONLY |
+        mimeService.VALIDATE_ALLOW_INVALID_FILENAMES
+    ),
+    "filename.DESKTOP",
+    "filename.DESKTOP allow invalid, sanitize only"
+  );
+});
diff --git a/uriloader/exthandler/tests/unit/test_getFromTypeAndExtension.js b/uriloader/exthandler/tests/unit/test_getFromTypeAndExtension.js
new file mode 100644
index 0000000000..6f4fe52a49
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_getFromTypeAndExtension.js
@@ -0,0 +1,17 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_utf8_extension() {
+  const mimeService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+  let someMIME = mimeService.getFromTypeAndExtension(
+    "application/x-nonsense",
+    ".тест"
+  );
+  Assert.stringContains(someMIME.description, "тест");
+  // primary extension isn't set on macOS or android, see bug 1721181
+  if (AppConstants.platform != "macosx" && AppConstants.platform != "android") {
+    Assert.equal(someMIME.primaryExtension, ".тест");
+  }
+});
diff --git a/uriloader/exthandler/tests/unit/test_getMIMEInfo_pdf.js b/uriloader/exthandler/tests/unit/test_getMIMEInfo_pdf.js
new file mode 100644
index 0000000000..03b0cea25e
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_getMIMEInfo_pdf.js
@@ -0,0 +1,30 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "gMIMEService",
+  "@mozilla.org/mime;1",
+  "nsIMIMEService"
+);
+
+// PDF files should always have a generic description instead
+// of relying on what is registered with the Operating System.
+add_task(async function test_check_unknown_mime_type() {
+  const mimeService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+  let pdfType = mimeService.getTypeFromExtension("pdf");
+  Assert.equal(pdfType, "application/pdf");
+  let extension = mimeService.getPrimaryExtension("application/pdf", "");
+  Assert.equal(extension, "pdf", "Expect pdf extension when given mime");
+  let mimeInfo = gMIMEService.getFromTypeAndExtension("", "pdf");
+  let stringBundle = Services.strings.createBundle(
+    "chrome://mozapps/locale/downloads/unknownContentType.properties"
+  );
+  Assert.equal(
+    mimeInfo.description,
+    stringBundle.GetStringFromName("pdfExtHandlerDescription"),
+    "PDF has generic description"
+  );
+});
diff --git a/uriloader/exthandler/tests/unit/test_getMIMEInfo_unknown_mime_type.js b/uriloader/exthandler/tests/unit/test_getMIMEInfo_unknown_mime_type.js
new file mode 100644
index 0000000000..9beef9d9c5
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_getMIMEInfo_unknown_mime_type.js
@@ -0,0 +1,32 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Zip files can be opened by Windows explorer, so we should always be able to
+// determine a description and default handler for them. However, things can
+// get messy if they are sent to us with a mime type other than what Windows
+// considers the "right" mimetype (application/x-zip-compressed), like
+// application/zip, which is what most places (IANA, macOS, probably all linux
+// distros, Apache, etc.) think is the "right" mimetype.
+add_task(async function test_check_unknown_mime_type() {
+  const mimeService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+  let zipType = mimeService.getTypeFromExtension("zip");
+  Assert.equal(zipType, "application/x-zip-compressed");
+  try {
+    let extension = mimeService.getPrimaryExtension("application/zip", "");
+    Assert.equal(
+      extension,
+      "zip",
+      "Expect our own info to provide an extension for zip files."
+    );
+  } catch (ex) {
+    Assert.ok(false, "We shouldn't throw when getting zip info.");
+  }
+  let found = {};
+  let mimeInfo = mimeService.getMIMEInfoFromOS("application/zip", "zip", found);
+  Assert.ok(
+    mimeInfo.hasDefaultHandler,
+    "Should have a default app for zip files"
+  );
+});
diff --git a/uriloader/exthandler/tests/unit/test_getTypeFromExtension_ext_to_type_mapping.js b/uriloader/exthandler/tests/unit/test_getTypeFromExtension_ext_to_type_mapping.js
new file mode 100644
index 0000000000..7202db58de
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_getTypeFromExtension_ext_to_type_mapping.js
@@ -0,0 +1,65 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 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/. */
+
+/**
+ * Test for bug 508030 <https://bugzilla.mozilla.org/show_bug.cgi?id=508030>:
+ * nsIMIMEService.getTypeFromExtension fails to find a match in the
+ * "ext-to-type-mapping" category if the provided extension is not lowercase.
+ */
+function run_test() {
+  // --- Common services ---
+
+  const mimeService = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+
+  const categoryManager = Services.catMan;
+
+  // --- Test procedure ---
+
+  const kTestExtension = "testextension";
+  const kTestExtensionMixedCase = "testExtensIon";
+  const kTestMimeType = "application/x-testextension";
+
+  // Ensure that the test extension is not initially recognized by the operating
+  // system or the "ext-to-type-mapping" category.
+  try {
+    // Try and get the MIME type associated with the extension.
+    mimeService.getTypeFromExtension(kTestExtension);
+    // The line above should have thrown an exception.
+    do_throw("nsIMIMEService.getTypeFromExtension succeeded unexpectedly");
+  } catch (e) {
+    if (
+      !(e instanceof Ci.nsIException) ||
+      e.result != Cr.NS_ERROR_NOT_AVAILABLE
+    ) {
+      throw e;
+    }
+    // This is an expected exception, thrown if the type can't be determined.
+    // Any other exception would cause the test to fail.
+  }
+
+  // Add a temporary category entry mapping the extension to the MIME type.
+  categoryManager.addCategoryEntry(
+    "ext-to-type-mapping",
+    kTestExtension,
+    kTestMimeType,
+    false,
+    true
+  );
+
+  // Check that the mapping is recognized in the simple case.
+  var type = mimeService.getTypeFromExtension(kTestExtension);
+  Assert.equal(type, kTestMimeType);
+
+  // Check that the mapping is recognized even if the extension has mixed case.
+  type = mimeService.getTypeFromExtension(kTestExtensionMixedCase);
+  Assert.equal(type, kTestMimeType);
+
+  // Clean up after ourselves.
+  categoryManager.deleteCategoryEntry(
+    "ext-to-type-mapping",
+    kTestExtension,
+    false
+  );
+}
diff --git a/uriloader/exthandler/tests/unit/test_getTypeFromExtension_with_empty_Content_Type.js b/uriloader/exthandler/tests/unit/test_getTypeFromExtension_with_empty_Content_Type.js
new file mode 100644
index 0000000000..5076a57738
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_getTypeFromExtension_with_empty_Content_Type.js
@@ -0,0 +1,218 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 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/. */
+
+/**
+ * Test for bug 484579 <https://bugzilla.mozilla.org/show_bug.cgi?id=484579>:
+ * nsIMIMEService.getTypeFromExtension may fail unexpectedly on Windows when
+ * "Content Type" is empty in the registry.
+ */
+
+// We must use a file extension that isn't listed in nsExternalHelperAppService's
+// defaultMimeEntries, otherwise the code takes a shortcut skipping the registry.
+const FILE_EXTENSION = ".nfo";
+// This is used to ensure the test properly used the mock, so that if we change
+// the underlying code, it won't be skipped.
+let gTestUsedOurMock = false;
+
+function run_test() {
+  // Activate the override of the file association data in the registry.
+  registerMockWindowsRegKeyFactory();
+
+  // Check the mock has been properly activated.
+  let regKey = Cc["@mozilla.org/windows-registry-key;1"].createInstance(
+    Ci.nsIWindowsRegKey
+  );
+  regKey.open(
+    Ci.nsIWindowsRegKey.ROOT_KEY_CLASSES_ROOT,
+    FILE_EXTENSION,
+    Ci.nsIWindowsRegKey.ACCESS_QUERY_VALUE
+  );
+  Assert.equal(
+    regKey.readStringValue("content type"),
+    "",
+    "Check the mock replied as expected."
+  );
+  Assert.ok(gTestUsedOurMock, "The test properly used the mock registry");
+  // Reset gTestUsedOurMock, because we just used it.
+  gTestUsedOurMock = false;
+  // Try and get the MIME type associated with the extension. If this
+  // operation does not throw an unexpected exception, the test succeeds.
+  Assert.throws(
+    () => {
+      Cc["@mozilla.org/mime;1"]
+        .getService(Ci.nsIMIMEService)
+        .getTypeFromExtension(FILE_EXTENSION);
+    },
+    /NS_ERROR_NOT_AVAILABLE/,
+    "Should throw a NOT_AVAILABLE exception"
+  );
+
+  Assert.ok(gTestUsedOurMock, "The test properly used the mock registry");
+}
+
+/**
+ * Constructs a new mock registry key by wrapping the provided object.
+ *
+ * This mock implementation is tailored for this test, and forces consumers
+ * of the readStringValue method to believe that the "Content Type" value of
+ * the FILE_EXTENSION key under HKEY_CLASSES_ROOT is an empty string.
+ *
+ * The same value read from "HKEY_LOCAL_MACHINE\SOFTWARE\Classes" is not
+ * affected.
+ *
+ * @param aWrappedObject   An actual nsIWindowsRegKey implementation.
+ */
+function MockWindowsRegKey(aWrappedObject) {
+  this._wrappedObject = aWrappedObject;
+
+  // This function creates a forwarding function for wrappedObject
+  function makeForwardingFunction(functionName) {
+    return function () {
+      return aWrappedObject[functionName].apply(aWrappedObject, arguments);
+    };
+  }
+
+  // Forward all the functions that are not explicitly overridden
+  for (var propertyName in aWrappedObject) {
+    if (!(propertyName in this)) {
+      if (typeof aWrappedObject[propertyName] == "function") {
+        this[propertyName] = makeForwardingFunction(propertyName);
+      } else {
+        this[propertyName] = aWrappedObject[propertyName];
+      }
+    }
+  }
+}
+
+MockWindowsRegKey.prototype = {
+  // --- Overridden nsISupports interface functions ---
+
+  QueryInterface: ChromeUtils.generateQI(["nsIWindowsRegKey"]),
+
+  // --- Overridden nsIWindowsRegKey interface functions ---
+
+  open(aRootKey, aRelPath, aMode) {
+    // Remember the provided root key and path
+    this._rootKey = aRootKey;
+    this._relPath = aRelPath;
+
+    // Create the actual registry key
+    return this._wrappedObject.open(aRootKey, aRelPath, aMode);
+  },
+
+  openChild(aRelPath, aMode) {
+    // Open the child key and wrap it
+    var innerKey = this._wrappedObject.openChild(aRelPath, aMode);
+    var key = new MockWindowsRegKey(innerKey);
+
+    // Set the properties of the child key and return it
+    key._rootKey = this._rootKey;
+    key._relPath = this._relPath + aRelPath;
+    return key;
+  },
+
+  createChild(aRelPath, aMode) {
+    // Create the child key and wrap it
+    var innerKey = this._wrappedObject.createChild(aRelPath, aMode);
+    var key = new MockWindowsRegKey(innerKey);
+
+    // Set the properties of the child key and return it
+    key._rootKey = this._rootKey;
+    key._relPath = this._relPath + aRelPath;
+    return key;
+  },
+
+  get childCount() {
+    return this._wrappedObject.childCount;
+  },
+
+  get valueCount() {
+    return this._wrappedObject.valueCount;
+  },
+
+  readStringValue(aName) {
+    // If this is the key under test, return a fake value
+    if (
+      this._rootKey == Ci.nsIWindowsRegKey.ROOT_KEY_CLASSES_ROOT &&
+      this._relPath.toLowerCase() == FILE_EXTENSION &&
+      aName.toLowerCase() == "content type"
+    ) {
+      gTestUsedOurMock = true;
+      return "";
+    }
+    // Return the real value from the registry
+    return this._wrappedObject.readStringValue(aName);
+  },
+};
+
+function registerMockWindowsRegKeyFactory() {
+  const kMockCID = Components.ID("{9b23dfe9-296b-4740-ba1c-d39c9a16e55e}");
+  const kWindowsRegKeyContractID = "@mozilla.org/windows-registry-key;1";
+  // Preserve the original CID.
+  let originalWindowsRegKeyCID = Cc[kWindowsRegKeyContractID].number;
+
+  // See bug 1694345 - nsNotifyAddrListener::CheckAdaptersAddresses might
+  // attempt to use the registry off the main thread, so we disable that
+  // feature while the mock registry is active.
+  let oldSuffixListPref = Services.prefs.getBoolPref(
+    "network.notify.dnsSuffixList"
+  );
+  Services.prefs.setBoolPref("network.notify.dnsSuffixList", false);
+
+  let oldCheckForProxiesPref = Services.prefs.getBoolPref(
+    "network.notify.checkForProxies"
+  );
+  Services.prefs.setBoolPref("network.notify.checkForProxies", false);
+
+  let oldCheckForNRPTPref = Services.prefs.getBoolPref(
+    "network.notify.checkForNRPT"
+  );
+  Services.prefs.setBoolPref("network.notify.checkForNRPT", false);
+
+  info("Create a mock RegKey factory");
+  let originalRegKey = Cc["@mozilla.org/windows-registry-key;1"].createInstance(
+    Ci.nsIWindowsRegKey
+  );
+  let mockWindowsRegKeyFactory = {
+    createInstance(iid) {
+      info("Create a mock wrapper around RegKey");
+      var key = new MockWindowsRegKey(originalRegKey);
+      return key.QueryInterface(iid);
+    },
+  };
+  info("Register the mock RegKey factory");
+  let registrar = Components.manager.QueryInterface(Ci.nsIComponentRegistrar);
+  registrar.registerFactory(
+    kMockCID,
+    "Mock Windows Registry Key Implementation",
+    kWindowsRegKeyContractID,
+    mockWindowsRegKeyFactory
+  );
+
+  registerCleanupFunction(() => {
+    // Free references to the mock factory
+    registrar.unregisterFactory(kMockCID, mockWindowsRegKeyFactory);
+    // Restore the original factory
+    registrar.registerFactory(
+      Components.ID(originalWindowsRegKeyCID),
+      "",
+      kWindowsRegKeyContractID,
+      null
+    );
+
+    Services.prefs.setBoolPref(
+      "network.notify.dnsSuffixList",
+      oldSuffixListPref
+    );
+    Services.prefs.setBoolPref(
+      "network.notify.checkForProxies",
+      oldCheckForProxiesPref
+    );
+    Services.prefs.setBoolPref(
+      "network.notify.checkForNRPT",
+      oldCheckForNRPTPref
+    );
+  });
+}
diff --git a/uriloader/exthandler/tests/unit/test_handlerService.js b/uriloader/exthandler/tests/unit/test_handlerService.js
new file mode 100644
index 0000000000..46a4d9fdd9
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_handlerService.js
@@ -0,0 +1,467 @@
+/* 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/. */
+
+function run_test() {
+  //* *************************************************************************//
+  // Constants
+
+  const handlerSvc = Cc["@mozilla.org/uriloader/handler-service;1"].getService(
+    Ci.nsIHandlerService
+  );
+
+  const mimeSvc = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+
+  const protoSvc = Cc[
+    "@mozilla.org/uriloader/external-protocol-service;1"
+  ].getService(Ci.nsIExternalProtocolService);
+
+  const prefSvc = Services.prefs;
+
+  let noMailto = false;
+  if (mozinfo.os == "win") {
+    // Check mailto handler from registry.
+    // If registry entry is nothing, no mailto handler
+    let regSvc = Cc["@mozilla.org/windows-registry-key;1"].createInstance(
+      Ci.nsIWindowsRegKey
+    );
+    try {
+      regSvc.open(regSvc.ROOT_KEY_CLASSES_ROOT, "mailto", regSvc.ACCESS_READ);
+      noMailto = false;
+    } catch (ex) {
+      noMailto = true;
+    }
+    regSvc.close();
+  }
+
+  if (mozinfo.os == "linux") {
+    // Check mailto handler from GIO
+    // If there isn't one, then we have no mailto handler
+    let gIOSvc = Cc["@mozilla.org/gio-service;1"].createInstance(
+      Ci.nsIGIOService
+    );
+    try {
+      gIOSvc.getAppForURIScheme("mailto");
+      noMailto = false;
+    } catch (ex) {
+      noMailto = true;
+    }
+  }
+
+  //* *************************************************************************//
+  // Sample Data
+
+  // It doesn't matter whether or not this nsIFile is actually executable,
+  // only that it has a path and exists.  Since we don't know any executable
+  // that exists on all platforms (except possibly the application being
+  // tested, but there doesn't seem to be a way to get a reference to that
+  // from the directory service), we use the temporary directory itself.
+  var executable = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  // XXX We could, of course, create an actual executable in the directory:
+  // executable.append("localhandler");
+  // if (!executable.exists())
+  //  executable.create(Ci.nsIFile.NORMAL_FILE_TYPE, 0o755);
+
+  var localHandler = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  localHandler.name = "Local Handler";
+  localHandler.executable = executable;
+
+  var webHandler = Cc[
+    "@mozilla.org/uriloader/web-handler-app;1"
+  ].createInstance(Ci.nsIWebHandlerApp);
+  webHandler.name = "Web Handler";
+  webHandler.uriTemplate = "http://www.example.com/?%s";
+
+  // FIXME: these tests create and manipulate enough variables that it would
+  // make sense to move each test into its own scope so we don't run the risk
+  // of one test stomping on another's data.
+
+  //* *************************************************************************//
+  // Test Default Properties
+
+  // Get a handler info for a MIME type that neither the application nor
+  // the OS knows about and make sure its properties are set to the proper
+  // default values.
+
+  var handlerInfo = mimeSvc.getFromTypeAndExtension("nonexistent/type", null);
+
+  // Make sure it's also an nsIHandlerInfo.
+  Assert.ok(handlerInfo instanceof Ci.nsIHandlerInfo);
+
+  Assert.equal(handlerInfo.type, "nonexistent/type");
+
+  // Deprecated property, but we should still make sure it's set correctly.
+  Assert.equal(handlerInfo.MIMEType, "nonexistent/type");
+
+  // These properties are the ones the handler service knows how to store.
+  Assert.equal(handlerInfo.preferredAction, Ci.nsIHandlerInfo.saveToDisk);
+  Assert.equal(handlerInfo.preferredApplicationHandler, null);
+  Assert.equal(handlerInfo.possibleApplicationHandlers.length, 0);
+  Assert.equal(
+    handlerInfo.alwaysAskBeforeHandling,
+    prefSvc.getBoolPref(
+      "browser.download.always_ask_before_handling_new_types",
+      false
+    )
+  );
+
+  // These properties are initialized to default values by the service,
+  // so we might as well make sure they're initialized to the right defaults.
+  Assert.equal(handlerInfo.description, "");
+  Assert.equal(handlerInfo.hasDefaultHandler, false);
+  Assert.equal(handlerInfo.defaultDescription, "");
+
+  const kExternalWarningDefault =
+    "network.protocol-handler.warn-external-default";
+  prefSvc.setBoolPref(kExternalWarningDefault, true);
+
+  // XXX add more thorough protocol info property checking
+
+  // no OS default handler exists
+  var protoInfo = protoSvc.getProtocolHandlerInfo("x-moz-rheet");
+  Assert.equal(protoInfo.preferredAction, protoInfo.alwaysAsk);
+  Assert.ok(protoInfo.alwaysAskBeforeHandling);
+
+  // OS default exists, injected default does not exist,
+  // explicit warning pref: false
+  const kExternalWarningPrefPrefix = "network.protocol-handler.warn-external.";
+  prefSvc.setBoolPref(kExternalWarningPrefPrefix + "http", false);
+  protoInfo = protoSvc.getProtocolHandlerInfo("http");
+  Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  // NOTE: this assertion will fail if the system executing the test does not
+  // have a handler registered for the http protocol. This is very unlikely to
+  // actually happen except on certain configurations of Linux, but one of
+  // those configurations is the default WSL Ubuntu install. So, if you are
+  // running this test locally and seeing a failure here, it might not be
+  // anything to really worry about.
+  Assert.ok(!protoInfo.alwaysAskBeforeHandling);
+
+  // OS default exists, injected default does not exist,
+  // explicit warning pref: true
+  prefSvc.setBoolPref(kExternalWarningPrefPrefix + "http", true);
+  protoInfo = protoSvc.getProtocolHandlerInfo("http");
+  // OS handler isn't included in possibleApplicationHandlers, so length is 0
+  // Once they become instances of nsILocalHandlerApp, this number will need
+  // to change.
+  Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  Assert.ok(protoInfo.alwaysAskBeforeHandling);
+
+  // OS default exists, injected default exists, explicit warning pref: false
+  prefSvc.setBoolPref(kExternalWarningPrefPrefix + "mailto", false);
+  protoInfo = protoSvc.getProtocolHandlerInfo("mailto");
+  if (AppConstants.MOZ_APP_NAME == "thunderbird") {
+    Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  } else {
+    Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
+  }
+
+  // Win7+ or Linux's GIO might not have a default mailto: handler
+  if (noMailto) {
+    Assert.ok(protoInfo.alwaysAskBeforeHandling);
+  } else {
+    Assert.ok(!protoInfo.alwaysAskBeforeHandling);
+  }
+
+  // OS default exists, injected default exists, explicit warning pref: true
+  prefSvc.setBoolPref(kExternalWarningPrefPrefix + "mailto", true);
+  protoInfo = protoSvc.getProtocolHandlerInfo("mailto");
+  if (AppConstants.MOZ_APP_NAME == "thunderbird") {
+    Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  } else {
+    Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
+    // Win7+ or Linux's GIO may have no default mailto: handler, so we'd ask
+    // anyway. Otherwise, the default handlers will not have stored preferred
+    // actions etc., so re-requesting them after the warning pref has changed
+    // will use the updated pref value. So both when we have and do not have
+    // a default mailto: handler, we'll ask:
+    Assert.ok(protoInfo.alwaysAskBeforeHandling);
+    // As soon as anyone actually stores updated defaults into the profile
+    // database, that default will stop tracking the warning pref.
+  }
+  // Now set the value stored in RDF to true, and the pref to false, to make
+  // sure we still get the right value. (Basically, same thing as above but
+  // with the values reversed.)
+  prefSvc.setBoolPref(kExternalWarningPrefPrefix + "mailto", false);
+  protoInfo.alwaysAskBeforeHandling = true;
+  handlerSvc.store(protoInfo);
+  protoInfo = protoSvc.getProtocolHandlerInfo("mailto");
+  if (AppConstants.MOZ_APP_NAME == "thunderbird") {
+    Assert.equal(0, protoInfo.possibleApplicationHandlers.length);
+  } else {
+    Assert.equal(1, protoInfo.possibleApplicationHandlers.length);
+    Assert.ok(protoInfo.alwaysAskBeforeHandling);
+  }
+
+  //* *************************************************************************//
+  // Test Round-Trip Data Integrity
+
+  // Test round-trip data integrity by setting the properties of the handler
+  // info object to different values, telling the handler service to store the
+  // object, and then retrieving a new info object for the same type and making
+  // sure its properties are identical.
+
+  handlerInfo.preferredAction = Ci.nsIHandlerInfo.useHelperApp;
+  handlerInfo.preferredApplicationHandler = localHandler;
+  handlerInfo.alwaysAskBeforeHandling = false;
+
+  handlerSvc.store(handlerInfo);
+
+  handlerInfo = mimeSvc.getFromTypeAndExtension("nonexistent/type", null);
+
+  Assert.equal(handlerInfo.preferredAction, Ci.nsIHandlerInfo.useHelperApp);
+
+  Assert.notEqual(handlerInfo.preferredApplicationHandler, null);
+  var preferredHandler = handlerInfo.preferredApplicationHandler;
+  Assert.equal(typeof preferredHandler, "object");
+  Assert.equal(preferredHandler.name, "Local Handler");
+  Assert.ok(preferredHandler instanceof Ci.nsILocalHandlerApp);
+  preferredHandler.QueryInterface(Ci.nsILocalHandlerApp);
+  Assert.equal(preferredHandler.executable.path, localHandler.executable.path);
+
+  Assert.ok(!handlerInfo.alwaysAskBeforeHandling);
+
+  // Make sure the handler service's enumerate method lists all known handlers.
+  var handlerInfo2 = mimeSvc.getFromTypeAndExtension("nonexistent/type2", null);
+  handlerSvc.store(handlerInfo2);
+  var handlerTypes = ["nonexistent/type", "nonexistent/type2"];
+  handlerTypes.push("mailto");
+  for (let handler of handlerSvc.enumerate()) {
+    Assert.notEqual(handlerTypes.indexOf(handler.type), -1);
+    handlerTypes.splice(handlerTypes.indexOf(handler.type), 1);
+  }
+  Assert.equal(handlerTypes.length, 0);
+  // Make sure the handler service's remove method removes a handler record.
+  handlerSvc.remove(handlerInfo2);
+  let handlers = handlerSvc.enumerate();
+  while (handlers.hasMoreElements()) {
+    Assert.notEqual(
+      handlers.getNext().QueryInterface(Ci.nsIHandlerInfo).type,
+      handlerInfo2.type
+    );
+  }
+
+  // Make sure we can store and retrieve a handler info object with no preferred
+  // handler.
+  var noPreferredHandlerInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/no-preferred-handler",
+    null
+  );
+  handlerSvc.store(noPreferredHandlerInfo);
+  noPreferredHandlerInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/no-preferred-handler",
+    null
+  );
+  Assert.equal(noPreferredHandlerInfo.preferredApplicationHandler, null);
+
+  // Make sure that the handler service removes an existing handler record
+  // if we store a handler info object with no preferred handler.
+  var removePreferredHandlerInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/rem-preferred-handler",
+    null
+  );
+  removePreferredHandlerInfo.preferredApplicationHandler = localHandler;
+  handlerSvc.store(removePreferredHandlerInfo);
+  removePreferredHandlerInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/rem-preferred-handler",
+    null
+  );
+  removePreferredHandlerInfo.preferredApplicationHandler = null;
+  handlerSvc.store(removePreferredHandlerInfo);
+  removePreferredHandlerInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/rem-preferred-handler",
+    null
+  );
+  Assert.equal(removePreferredHandlerInfo.preferredApplicationHandler, null);
+
+  // Make sure we can store and retrieve a handler info object with possible
+  // handlers.  We test both adding and removing handlers.
+
+  // Get a handler info and make sure it has no possible handlers.
+  var possibleHandlersInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/possible-handlers",
+    null
+  );
+  Assert.equal(possibleHandlersInfo.possibleApplicationHandlers.length, 0);
+
+  // Store and re-retrieve the handler and make sure it still has no possible
+  // handlers.
+  handlerSvc.store(possibleHandlersInfo);
+  possibleHandlersInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/possible-handlers",
+    null
+  );
+  Assert.equal(possibleHandlersInfo.possibleApplicationHandlers.length, 0);
+
+  // Add two handlers, store the object, re-retrieve it, and make sure it has
+  // two handlers.
+  possibleHandlersInfo.possibleApplicationHandlers.appendElement(localHandler);
+  possibleHandlersInfo.possibleApplicationHandlers.appendElement(webHandler);
+  handlerSvc.store(possibleHandlersInfo);
+  possibleHandlersInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/possible-handlers",
+    null
+  );
+  Assert.equal(possibleHandlersInfo.possibleApplicationHandlers.length, 2);
+
+  // Figure out which is the local and which is the web handler and the index
+  // in the array of the local handler, which is the one we're going to remove
+  // to test removal of a handler.
+  var handler1 =
+    possibleHandlersInfo.possibleApplicationHandlers.queryElementAt(
+      0,
+      Ci.nsIHandlerApp
+    );
+  var handler2 =
+    possibleHandlersInfo.possibleApplicationHandlers.queryElementAt(
+      1,
+      Ci.nsIHandlerApp
+    );
+  var localPossibleHandler, webPossibleHandler, localIndex;
+  if (handler1 instanceof Ci.nsILocalHandlerApp) {
+    [localPossibleHandler, webPossibleHandler, localIndex] = [
+      handler1,
+      handler2,
+      0,
+    ];
+  } else {
+    [localPossibleHandler, webPossibleHandler, localIndex] = [
+      handler2,
+      handler1,
+      1,
+    ];
+  }
+  localPossibleHandler.QueryInterface(Ci.nsILocalHandlerApp);
+  webPossibleHandler.QueryInterface(Ci.nsIWebHandlerApp);
+
+  // Make sure the two handlers are the ones we stored.
+  Assert.equal(localPossibleHandler.name, localHandler.name);
+  Assert.ok(localPossibleHandler.equals(localHandler));
+  Assert.equal(webPossibleHandler.name, webHandler.name);
+  Assert.ok(webPossibleHandler.equals(webHandler));
+
+  // Remove a handler, store the object, re-retrieve it, and make sure
+  // it only has one handler.
+  possibleHandlersInfo.possibleApplicationHandlers.removeElementAt(localIndex);
+  handlerSvc.store(possibleHandlersInfo);
+  possibleHandlersInfo = mimeSvc.getFromTypeAndExtension(
+    "nonexistent/possible-handlers",
+    null
+  );
+  Assert.equal(possibleHandlersInfo.possibleApplicationHandlers.length, 1);
+
+  // Make sure the handler is the one we didn't remove.
+  webPossibleHandler =
+    possibleHandlersInfo.possibleApplicationHandlers.queryElementAt(
+      0,
+      Ci.nsIWebHandlerApp
+    );
+  Assert.equal(webPossibleHandler.name, webHandler.name);
+  Assert.ok(webPossibleHandler.equals(webHandler));
+
+  // ////////////////////////////////////////////////////
+  // handler info command line parameters and equality
+  var localApp = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  var handlerApp = localApp.QueryInterface(Ci.nsIHandlerApp);
+
+  Assert.ok(handlerApp.equals(localApp));
+
+  localApp.executable = executable;
+
+  Assert.equal(0, localApp.parameterCount);
+  localApp.appendParameter("-test1");
+  Assert.equal(1, localApp.parameterCount);
+  localApp.appendParameter("-test2");
+  Assert.equal(2, localApp.parameterCount);
+  Assert.ok(localApp.parameterExists("-test1"));
+  Assert.ok(localApp.parameterExists("-test2"));
+  Assert.ok(!localApp.parameterExists("-false"));
+  localApp.clearParameters();
+  Assert.equal(0, localApp.parameterCount);
+
+  var localApp2 = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+
+  localApp2.executable = executable;
+
+  localApp.clearParameters();
+  Assert.ok(localApp.equals(localApp2));
+
+  // equal:
+  // cut -d 1 -f 2
+  // cut -d 1 -f 2
+
+  localApp.appendParameter("-test1");
+  localApp.appendParameter("-test2");
+  localApp.appendParameter("-test3");
+  localApp2.appendParameter("-test1");
+  localApp2.appendParameter("-test2");
+  localApp2.appendParameter("-test3");
+  Assert.ok(localApp.equals(localApp2));
+
+  // not equal:
+  // cut -d 1 -f 2
+  // cut -f 1 -d 2
+
+  localApp.clearParameters();
+  localApp2.clearParameters();
+
+  localApp.appendParameter("-test1");
+  localApp.appendParameter("-test2");
+  localApp.appendParameter("-test3");
+  localApp2.appendParameter("-test2");
+  localApp2.appendParameter("-test1");
+  localApp2.appendParameter("-test3");
+  Assert.ok(!localApp2.equals(localApp));
+
+  var str;
+  str = localApp.getParameter(0);
+  Assert.equal(str, "-test1");
+  str = localApp.getParameter(1);
+  Assert.equal(str, "-test2");
+  str = localApp.getParameter(2);
+  Assert.equal(str, "-test3");
+
+  // FIXME: test round trip integrity for a protocol.
+  // FIXME: test round trip integrity for a handler info with a web handler.
+
+  //* *************************************************************************//
+  // getTypeFromExtension tests
+
+  // test nonexistent extension
+  var lolType = handlerSvc.getTypeFromExtension("lolcat");
+  Assert.equal(lolType, "");
+
+  // add a handler for the extension
+  var lolHandler = mimeSvc.getFromTypeAndExtension("application/lolcat", null);
+
+  Assert.ok(!lolHandler.extensionExists("lolcat"));
+  lolHandler.preferredAction = Ci.nsIHandlerInfo.useHelperApp;
+  lolHandler.preferredApplicationHandler = localHandler;
+  lolHandler.alwaysAskBeforeHandling = false;
+  lolHandler.appendExtension("lolcat");
+
+  // store the handler
+  Assert.ok(!handlerSvc.exists(lolHandler));
+  handlerSvc.store(lolHandler);
+  Assert.ok(handlerSvc.exists(lolHandler));
+
+  // test now-existent extension
+  lolType = handlerSvc.getTypeFromExtension("lolcat");
+  Assert.equal(lolType, "application/lolcat");
+
+  // test mailcap entries with needsterminal are ignored on non-Windows non-Mac.
+  if (mozinfo.os != "win" && mozinfo.os != "mac") {
+    prefSvc.setStringPref(
+      "helpers.private_mailcap_file",
+      do_get_file("mailcap").path
+    );
+    handlerInfo = mimeSvc.getFromTypeAndExtension("text/plain", null);
+    Assert.equal(handlerInfo.preferredAction, Ci.nsIHandlerInfo.saveToDisk);
+    Assert.equal(handlerInfo.defaultDescription, "sed");
+  }
+}
diff --git a/uriloader/exthandler/tests/unit/test_handlerService_store.js b/uriloader/exthandler/tests/unit/test_handlerService_store.js
new file mode 100644
index 0000000000..27c8dc6c9e
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_handlerService_store.js
@@ -0,0 +1,757 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * Tests the nsIHandlerService interface.
+ */
+
+// Set up an nsIWebHandlerApp instance that can be used in multiple tests.
+let webHandlerApp = Cc[
+  "@mozilla.org/uriloader/web-handler-app;1"
+].createInstance(Ci.nsIWebHandlerApp);
+webHandlerApp.name = "Web Handler";
+webHandlerApp.uriTemplate = "https://www.example.com/?url=%s";
+let expectedWebHandlerApp = {
+  name: webHandlerApp.name,
+  uriTemplate: webHandlerApp.uriTemplate,
+};
+
+// Set up an nsILocalHandlerApp instance that can be used in multiple tests. The
+// executable should exist, but it doesn't need to point to an actual file, so
+// we simply initialize it to the path of an existing directory.
+let localHandlerApp = Cc[
+  "@mozilla.org/uriloader/local-handler-app;1"
+].createInstance(Ci.nsILocalHandlerApp);
+localHandlerApp.name = "Local Handler";
+localHandlerApp.executable = new FileUtils.File(PathUtils.tempDir);
+let expectedLocalHandlerApp = {
+  name: localHandlerApp.name,
+  executable: localHandlerApp.executable,
+};
+
+/**
+ * Returns a new nsIHandlerInfo instance initialized to known values that don't
+ * depend on the platform and are easier to verify later.
+ *
+ * @param type
+ *        Because the "preferredAction" is initialized to saveToDisk, this
+ *        should represent a MIME type rather than a protocol.
+ */
+function getKnownHandlerInfo(type) {
+  let handlerInfo = HandlerServiceTestUtils.getBlankHandlerInfo(type);
+  handlerInfo.preferredAction = Ci.nsIHandlerInfo.saveToDisk;
+  handlerInfo.alwaysAskBeforeHandling = false;
+  return handlerInfo;
+}
+
+/**
+ * Checks that the information stored in the handler service instance under
+ * testing matches the test data files.
+ */
+function assertAllHandlerInfosMatchTestData() {
+  let handlerInfos = HandlerServiceTestUtils.getAllHandlerInfos();
+
+  // It's important that the MIME types we check here do not exist at the
+  // operating system level, otherwise the list of handlers and file extensions
+  // will be merged. The current implementation avoids duplicate entries.
+
+  HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfos.shift(), {
+    type: "example/type.handleinternally",
+    preferredAction: Ci.nsIHandlerInfo.handleInternally,
+    alwaysAskBeforeHandling: false,
+    fileExtensions: ["example_one"],
+  });
+
+  HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfos.shift(), {
+    type: "example/type.savetodisk",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: true,
+    preferredApplicationHandler: {
+      name: "Example Default Handler",
+      uriTemplate: "https://www.example.com/?url=%s",
+    },
+    possibleApplicationHandlers: [
+      {
+        name: "Example Default Handler",
+        uriTemplate: "https://www.example.com/?url=%s",
+      },
+    ],
+    fileExtensions: ["example_two", "example_three"],
+  });
+
+  HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfos.shift(), {
+    type: "example/type.usehelperapp",
+    preferredAction: Ci.nsIHandlerInfo.useHelperApp,
+    alwaysAskBeforeHandling: true,
+    preferredApplicationHandler: {
+      name: "Example Default Handler",
+      uriTemplate: "https://www.example.com/?url=%s",
+    },
+    possibleApplicationHandlers: [
+      {
+        name: "Example Default Handler",
+        uriTemplate: "https://www.example.com/?url=%s",
+      },
+      {
+        name: "Example Possible Handler One",
+        uriTemplate: "http://www.example.com/?id=1&url=%s",
+      },
+      {
+        name: "Example Possible Handler Two",
+        uriTemplate: "http://www.example.com/?id=2&url=%s",
+      },
+    ],
+    fileExtensions: ["example_two", "example_three"],
+  });
+
+  HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfos.shift(), {
+    type: "example/type.usesystemdefault",
+    preferredAction: Ci.nsIHandlerInfo.useSystemDefault,
+    alwaysAskBeforeHandling: false,
+    possibleApplicationHandlers: [
+      {
+        name: "Example Possible Handler",
+        uriTemplate: "http://www.example.com/?url=%s",
+      },
+    ],
+  });
+
+  HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfos.shift(), {
+    type: "examplescheme.usehelperapp",
+    preferredAction: Ci.nsIHandlerInfo.useHelperApp,
+    alwaysAskBeforeHandling: true,
+    preferredApplicationHandler: {
+      name: "Example Default Handler",
+      uriTemplate: "https://www.example.com/?url=%s",
+    },
+    possibleApplicationHandlers: [
+      {
+        name: "Example Default Handler",
+        uriTemplate: "https://www.example.com/?url=%s",
+      },
+      {
+        name: "Example Possible Handler One",
+        uriTemplate: "http://www.example.com/?id=1&url=%s",
+      },
+      {
+        name: "Example Possible Handler Two",
+        uriTemplate: "http://www.example.com/?id=2&url=%s",
+      },
+    ],
+  });
+
+  HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfos.shift(), {
+    type: "examplescheme.usesystemdefault",
+    preferredAction: Ci.nsIHandlerInfo.useSystemDefault,
+    alwaysAskBeforeHandling: false,
+    possibleApplicationHandlers: [
+      {
+        name: "Example Possible Handler",
+        uriTemplate: "http://www.example.com/?url=%s",
+      },
+    ],
+  });
+
+  Assert.equal(handlerInfos.length, 0);
+}
+
+/**
+ * Loads data from a file in a predefined format, verifying that the format is
+ * recognized and all the known properties are loaded and saved.
+ */
+add_task(async function test_store_fillHandlerInfo_predefined() {
+  // Test that the file format used in previous versions can be loaded.
+  await copyTestDataToHandlerStore();
+  await assertAllHandlerInfosMatchTestData();
+
+  // Keep a copy of the nsIHandlerInfo instances, then delete the handler store
+  // and populate it with the known data. Since the handler store is empty, the
+  // default handlers for the current locale are also injected, so we have to
+  // delete them manually before adding the other nsIHandlerInfo instances.
+  let testHandlerInfos = HandlerServiceTestUtils.getAllHandlerInfos();
+  await deleteHandlerStore();
+  for (let handlerInfo of HandlerServiceTestUtils.getAllHandlerInfos()) {
+    gHandlerService.remove(handlerInfo);
+  }
+  for (let handlerInfo of testHandlerInfos) {
+    gHandlerService.store(handlerInfo);
+  }
+
+  // Test that the known data still matches after saving it and reloading.
+  await unloadHandlerStore();
+  await assertAllHandlerInfosMatchTestData();
+});
+
+/**
+ * Check that "store" is able to add new instances, that "remove" and "exists"
+ * work, and that "fillHandlerInfo" throws when the instance does not exist.
+ */
+add_task(async function test_store_remove_exists() {
+  // Test both MIME types and protocols.
+  for (let type of [
+    "example/type.usehelperapp",
+    "examplescheme.usehelperapp",
+  ]) {
+    // Create new nsIHandlerInfo instances before loading the test data.
+    await deleteHandlerStore();
+    let handlerInfoPresent = HandlerServiceTestUtils.getHandlerInfo(type);
+    let handlerInfoAbsent = HandlerServiceTestUtils.getHandlerInfo(type + "2");
+
+    // Set up known properties that we can verify later.
+    handlerInfoAbsent.preferredAction = Ci.nsIHandlerInfo.saveToDisk;
+    handlerInfoAbsent.alwaysAskBeforeHandling = false;
+
+    await copyTestDataToHandlerStore();
+
+    Assert.ok(gHandlerService.exists(handlerInfoPresent));
+    Assert.ok(!gHandlerService.exists(handlerInfoAbsent));
+
+    gHandlerService.store(handlerInfoAbsent);
+    gHandlerService.remove(handlerInfoPresent);
+
+    await unloadHandlerStore();
+
+    Assert.ok(!gHandlerService.exists(handlerInfoPresent));
+    Assert.ok(gHandlerService.exists(handlerInfoAbsent));
+
+    Assert.throws(
+      () => gHandlerService.fillHandlerInfo(handlerInfoPresent, ""),
+      ex => ex.result == Cr.NS_ERROR_NOT_AVAILABLE
+    );
+
+    let actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo(type + "2");
+    HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+      type: type + "2",
+      preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+      alwaysAskBeforeHandling: false,
+    });
+  }
+});
+
+/**
+ * Tests that it is possible to save an nsIHandlerInfo instance with a
+ * "preferredAction" that is either a valid or an unknown value, and the
+ * action always takes on an appropriate value when reloading.
+ */
+add_task(async function test_store_preferredAction() {
+  await deleteHandlerStore();
+
+  let handlerInfo = getKnownHandlerInfo("example/new");
+  // Valid action values should all remain unchanged across a refresh, except
+  // for alwaysAsk which may be overridden with useHelperApp depending on prefs.
+  // Invalid action values should always convert to useHelperApp.
+  const actions = [
+    {
+      preferred: Ci.nsIHandlerInfo.alwaysAsk,
+      expected: Ci.nsIHandlerInfo.alwaysAsk,
+    },
+    {
+      preferred: Ci.nsIHandlerInfo.handleInternally,
+      expected: Ci.nsIHandlerInfo.handleInternally,
+    },
+    { preferred: 999, expected: Ci.nsIHandlerInfo.useHelperApp },
+  ];
+
+  for (let action of actions) {
+    handlerInfo.preferredAction = action.preferred;
+    gHandlerService.store(handlerInfo);
+    gHandlerService.fillHandlerInfo(handlerInfo, "");
+    Assert.equal(handlerInfo.preferredAction, action.expected);
+  }
+});
+
+/**
+ * Tests that it is possible to save an nsIHandlerInfo instance containing an
+ * nsILocalHandlerApp instance pointing to an executable that doesn't exist, but
+ * this entry is ignored when reloading.
+ */
+add_task(async function test_store_localHandlerApp_missing() {
+  if (!("@mozilla.org/uriloader/dbus-handler-app;1" in Cc)) {
+    info("Skipping test because it does not apply to this platform.");
+    return;
+  }
+
+  let missingHandlerApp = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  missingHandlerApp.name = "Non-existing Handler";
+  missingHandlerApp.executable = new FileUtils.File(
+    PathUtils.join(PathUtils.tempDir, "nonexisting")
+  );
+
+  await deleteHandlerStore();
+
+  let handlerInfo = getKnownHandlerInfo("example/new");
+  handlerInfo.preferredApplicationHandler = missingHandlerApp;
+  handlerInfo.possibleApplicationHandlers.appendElement(missingHandlerApp);
+  handlerInfo.possibleApplicationHandlers.appendElement(webHandlerApp);
+  gHandlerService.store(handlerInfo);
+
+  await unloadHandlerStore();
+
+  let actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo("example/new");
+  HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+    type: "example/new",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: false,
+    possibleApplicationHandlers: [expectedWebHandlerApp],
+  });
+});
+
+/**
+ * Test saving and reloading an instance of nsIDBusHandlerApp.
+ */
+add_task(async function test_store_dBusHandlerApp() {
+  if (!("@mozilla.org/uriloader/dbus-handler-app;1" in Cc)) {
+    info("Skipping test because it does not apply to this platform.");
+    return;
+  }
+
+  // Set up an nsIDBusHandlerApp instance for testing.
+  let dBusHandlerApp = Cc[
+    "@mozilla.org/uriloader/dbus-handler-app;1"
+  ].createInstance(Ci.nsIDBusHandlerApp);
+  dBusHandlerApp.name = "DBus Handler";
+  dBusHandlerApp.service = "test.method.server";
+  dBusHandlerApp.method = "Method";
+  dBusHandlerApp.dBusInterface = "test.method.Type";
+  dBusHandlerApp.objectPath = "/test/method/Object";
+  let expectedDBusHandlerApp = {
+    name: dBusHandlerApp.name,
+    service: dBusHandlerApp.service,
+    method: dBusHandlerApp.method,
+    dBusInterface: dBusHandlerApp.dBusInterface,
+    objectPath: dBusHandlerApp.objectPath,
+  };
+
+  await deleteHandlerStore();
+
+  let handlerInfo = getKnownHandlerInfo("example/new");
+  handlerInfo.preferredApplicationHandler = dBusHandlerApp;
+  handlerInfo.possibleApplicationHandlers.appendElement(dBusHandlerApp);
+  gHandlerService.store(handlerInfo);
+
+  await unloadHandlerStore();
+
+  let actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo("example/new");
+  HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+    type: "example/new",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: false,
+    preferredApplicationHandler: expectedDBusHandlerApp,
+    possibleApplicationHandlers: [expectedDBusHandlerApp],
+  });
+});
+
+/**
+ * Tests that it is possible to save an nsIHandlerInfo instance with a
+ * "preferredApplicationHandler" and no "possibleApplicationHandlers", but the
+ * former is always included in the latter list when reloading.
+ */
+add_task(
+  async function test_store_possibleApplicationHandlers_includes_preferred() {
+    await deleteHandlerStore();
+
+    let handlerInfo = getKnownHandlerInfo("example/new");
+    handlerInfo.preferredApplicationHandler = localHandlerApp;
+    gHandlerService.store(handlerInfo);
+
+    await unloadHandlerStore();
+
+    let actualHandlerInfo =
+      HandlerServiceTestUtils.getHandlerInfo("example/new");
+    HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+      type: "example/new",
+      preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+      alwaysAskBeforeHandling: false,
+      preferredApplicationHandler: expectedLocalHandlerApp,
+      possibleApplicationHandlers: [expectedLocalHandlerApp],
+    });
+  }
+);
+
+/**
+ * Tests that it is possible to save an nsIHandlerInfo instance with a
+ * "preferredApplicationHandler" that is not the first element in
+ * "possibleApplicationHandlers", but the former is always included as the first
+ * element of the latter list when reloading.
+ */
+add_task(
+  async function test_store_possibleApplicationHandlers_preferred_first() {
+    await deleteHandlerStore();
+
+    let handlerInfo = getKnownHandlerInfo("example/new");
+    handlerInfo.preferredApplicationHandler = webHandlerApp;
+    // The preferred handler is appended after the other one.
+    handlerInfo.possibleApplicationHandlers.appendElement(localHandlerApp);
+    handlerInfo.possibleApplicationHandlers.appendElement(webHandlerApp);
+    gHandlerService.store(handlerInfo);
+
+    await unloadHandlerStore();
+
+    let actualHandlerInfo =
+      HandlerServiceTestUtils.getHandlerInfo("example/new");
+    HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+      type: "example/new",
+      preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+      alwaysAskBeforeHandling: false,
+      preferredApplicationHandler: expectedWebHandlerApp,
+      possibleApplicationHandlers: [
+        expectedWebHandlerApp,
+        expectedLocalHandlerApp,
+      ],
+    });
+  }
+);
+
+/**
+ * Tests that it is possible to save an nsIHandlerInfo instance with an
+ * uppercase file extension, but it is converted to lowercase when reloading.
+ */
+add_task(async function test_store_fileExtensions_lowercase() {
+  await deleteHandlerStore();
+
+  let handlerInfo = getKnownHandlerInfo("example/new");
+  handlerInfo.appendExtension("extension_test1");
+  handlerInfo.appendExtension("EXTENSION_test2");
+  gHandlerService.store(handlerInfo);
+
+  await unloadHandlerStore();
+
+  let actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo("example/new");
+  HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+    type: "example/new",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: false,
+    fileExtensions: ["extension_test1", "extension_test2"],
+  });
+});
+
+/**
+ * Tests that appendExtension doesn't add duplicates, and that anyway duplicates
+ * from possibleApplicationHandlers are removed when saving and reloading.
+ */
+add_task(async function test_store_no_duplicates() {
+  await deleteHandlerStore();
+
+  let handlerInfo = getKnownHandlerInfo("example/new");
+  handlerInfo.preferredApplicationHandler = webHandlerApp;
+  handlerInfo.possibleApplicationHandlers.appendElement(webHandlerApp);
+  handlerInfo.possibleApplicationHandlers.appendElement(localHandlerApp);
+  handlerInfo.possibleApplicationHandlers.appendElement(localHandlerApp);
+  handlerInfo.possibleApplicationHandlers.appendElement(webHandlerApp);
+  handlerInfo.appendExtension("extension_test1");
+  handlerInfo.appendExtension("extension_test2");
+  handlerInfo.appendExtension("extension_test1");
+  handlerInfo.appendExtension("EXTENSION_test1");
+  Assert.deepEqual(Array.from(handlerInfo.getFileExtensions()), [
+    "extension_test1",
+    "extension_test2",
+  ]);
+  gHandlerService.store(handlerInfo);
+
+  await unloadHandlerStore();
+
+  let actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo("example/new");
+  HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+    type: "example/new",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: false,
+    preferredApplicationHandler: expectedWebHandlerApp,
+    possibleApplicationHandlers: [
+      expectedWebHandlerApp,
+      expectedLocalHandlerApp,
+    ],
+    fileExtensions: ["extension_test1", "extension_test2"],
+  });
+});
+
+/**
+ * Tests that setFileExtensions doesn't add duplicates.
+ */
+add_task(async function test_setFileExtensions_no_duplicates() {
+  await deleteHandlerStore();
+
+  let handlerInfo = getKnownHandlerInfo("example/new");
+  handlerInfo.setFileExtensions("a,b,A,b,c,a");
+  let expected = ["a", "b", "c"];
+  Assert.deepEqual(Array.from(handlerInfo.getFileExtensions()), expected);
+  // Test empty extensions, also at begin and end.
+  handlerInfo.setFileExtensions(",a,,b,A,c,");
+  Assert.deepEqual(Array.from(handlerInfo.getFileExtensions()), expected);
+});
+
+/**
+ * Tests that "store" deletes properties that have their default values from
+ * the data store.
+ *
+ * File extensions are never deleted once they have been associated.
+ */
+add_task(async function test_store_deletes_properties_except_extensions() {
+  await deleteHandlerStore();
+
+  // Prepare an nsIHandlerInfo instance with all the properties set to values
+  // that will result in deletions. The preferredAction is also set to a defined
+  // value so we can more easily verify it later.
+  let handlerInfo = HandlerServiceTestUtils.getBlankHandlerInfo(
+    "example/type.savetodisk"
+  );
+  handlerInfo.preferredAction = Ci.nsIHandlerInfo.saveToDisk;
+  handlerInfo.alwaysAskBeforeHandling = false;
+
+  // All the properties for "example/type.savetodisk" are present in the test
+  // data, so we load the data before overwriting their values.
+  await copyTestDataToHandlerStore();
+  gHandlerService.store(handlerInfo);
+
+  // Now we can reload the data and verify that no extra values have been kept.
+  await unloadHandlerStore();
+  let actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo(
+    "example/type.savetodisk"
+  );
+  HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+    type: "example/type.savetodisk",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: false,
+    fileExtensions: ["example_two", "example_three"],
+  });
+});
+
+/**
+ * Tests the "overrideType" argument of "fillHandlerInfo".
+ */
+add_task(async function test_fillHandlerInfo_overrideType() {
+  // Test both MIME types and protocols.
+  for (let type of [
+    "example/type.usesystemdefault",
+    "examplescheme.usesystemdefault",
+  ]) {
+    await deleteHandlerStore();
+
+    // Create new nsIHandlerInfo instances before loading the test data.
+    let handlerInfoAbsent = HandlerServiceTestUtils.getHandlerInfo(type + "2");
+
+    // Fill the nsIHandlerInfo instance using the type that actually exists.
+    await copyTestDataToHandlerStore();
+    gHandlerService.fillHandlerInfo(handlerInfoAbsent, type);
+    HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfoAbsent, {
+      // While the data is populated from another type, the type is unchanged.
+      type: type + "2",
+      preferredAction: Ci.nsIHandlerInfo.useSystemDefault,
+      alwaysAskBeforeHandling: false,
+      possibleApplicationHandlers: [
+        {
+          name: "Example Possible Handler",
+          uriTemplate: "http://www.example.com/?url=%s",
+        },
+      ],
+    });
+  }
+});
+
+/**
+ * Tests "getTypeFromExtension" including unknown extensions.
+ */
+add_task(async function test_getTypeFromExtension() {
+  await copyTestDataToHandlerStore();
+
+  Assert.equal(gHandlerService.getTypeFromExtension(""), "");
+  Assert.equal(gHandlerService.getTypeFromExtension("example_unknown"), "");
+  Assert.equal(
+    gHandlerService.getTypeFromExtension("example_one"),
+    "example/type.handleinternally"
+  );
+  Assert.equal(
+    gHandlerService.getTypeFromExtension("EXAMPLE_one"),
+    "example/type.handleinternally"
+  );
+});
+
+/**
+ * Checks that the information stored in the handler service instance under
+ * testing matches the default handlers for the English locale.
+ */
+function assertAllHandlerInfosMatchDefaultHandlers() {
+  let handlerInfos = HandlerServiceTestUtils.getAllHandlerInfos();
+
+  HandlerServiceTestUtils.assertHandlerInfoMatches(handlerInfos.shift(), {
+    type: "mailto",
+    preferredActionOSDependent: true,
+    possibleApplicationHandlers: [
+      {
+        name: "Gmail",
+        uriTemplate: "https://mail.google.com/mail/?extsrc=mailto&url=%s",
+      },
+    ],
+  });
+
+  Assert.equal(handlerInfos.length, 0);
+}
+
+/**
+ * Tests the default protocol handlers imported from the locale-specific data.
+ */
+add_task(
+  { skip_if: () => AppConstants.MOZ_APP_NAME == "thunderbird" },
+  async function test_default_protocol_handlers() {
+    if (
+      !Services.prefs.getPrefType("gecko.handlerService.defaultHandlersVersion")
+    ) {
+      info("This platform or locale does not have default handlers.");
+      return;
+    }
+
+    // This will inject the default protocol handlers for the current locale.
+    await deleteHandlerStore();
+
+    await assertAllHandlerInfosMatchDefaultHandlers();
+  }
+);
+
+/**
+ * Tests that the default protocol handlers are not imported again from the
+ * locale-specific data if they already exist.
+ */
+add_task(
+  { skip_if: () => AppConstants.MOZ_APP_NAME == "thunderbird" },
+  async function test_default_protocol_handlers_no_duplicates() {
+    if (
+      !Services.prefs.getPrefType("gecko.handlerService.defaultHandlersVersion")
+    ) {
+      info("This platform or locale does not have default handlers.");
+      return;
+    }
+
+    // This will inject the default protocol handlers for the current locale.
+    await deleteHandlerStore();
+
+    // Clear the preference to force injecting again.
+    Services.prefs.clearUserPref("gecko.handlerService.defaultHandlersVersion");
+
+    await unloadHandlerStore();
+
+    // There should be no duplicate handlers in the protocols.
+    assertAllHandlerInfosMatchDefaultHandlers();
+  }
+);
+
+/**
+ * Ensures forward compatibility by checking that the "store" method preserves
+ * unknown properties in the test data.
+ */
+add_task(async function test_store_keeps_unknown_properties() {
+  // Create a new nsIHandlerInfo instance before loading the test data.
+  await deleteHandlerStore();
+  let handlerInfo = HandlerServiceTestUtils.getHandlerInfo(
+    "example/type.handleinternally"
+  );
+
+  await copyTestDataToHandlerStore();
+  gHandlerService.store(handlerInfo);
+
+  await unloadHandlerStore();
+  let data = await IOUtils.readJSON(jsonPath);
+  Assert.equal(
+    data.mimeTypes["example/type.handleinternally"].unknownProperty,
+    "preserved"
+  );
+});
+
+/**
+ * Runs the asyncInit method, ensuring that it successfully inits the store
+ * and calls the handlersvc-store-initialized topic.
+ */
+add_task(async function test_async_init() {
+  await deleteHandlerStore();
+  await copyTestDataToHandlerStore();
+  gHandlerService.asyncInit();
+  await TestUtils.topicObserved("handlersvc-store-initialized");
+  await assertAllHandlerInfosMatchTestData();
+
+  await unloadHandlerStore();
+});
+
+/**
+ * Races the asyncInit method against the sync init (implicit in enumerate),
+ * to ensure that the store will be synchronously initialized without any
+ * ill effects.
+ */
+add_task(async function test_race_async_init() {
+  await deleteHandlerStore();
+  await copyTestDataToHandlerStore();
+  let storeInitialized = false;
+  // Pass a callback to synchronously observe the topic, as a promise would
+  // resolve asynchronously
+  TestUtils.topicObserved("handlersvc-store-initialized", () => {
+    storeInitialized = true;
+    return true;
+  });
+  gHandlerService.asyncInit();
+  Assert.ok(!storeInitialized);
+  gHandlerService.enumerate();
+  Assert.ok(storeInitialized);
+  await assertAllHandlerInfosMatchTestData();
+
+  await unloadHandlerStore();
+});
+
+/**
+ * Test saving and reloading an instance of nsIGIOMimeApp.
+ */
+add_task(async function test_store_gioHandlerApp() {
+  if (!("@mozilla.org/gio-service;1" in Cc)) {
+    info("Skipping test because it does not apply to this platform.");
+    return;
+  }
+
+  // Create dummy exec file that following won't fail because file not found error
+  let dummyHandlerFile = await IOUtils.getFile(
+    PathUtils.tempDir,
+    "dummyHandler"
+  );
+  dummyHandlerFile.createUnique(
+    Ci.nsIFile.NORMAL_FILE_TYPE,
+    parseInt("777", 8)
+  );
+
+  // Set up an nsIGIOMimeApp instance for testing.
+  let handlerApp = Cc["@mozilla.org/gio-service;1"]
+    .getService(Ci.nsIGIOService)
+    .createAppFromCommand(dummyHandlerFile.path, "Dummy GIO handler");
+  let expectedGIOMimeHandlerApp = {
+    name: handlerApp.name,
+    command: handlerApp.command,
+  };
+
+  await deleteHandlerStore();
+
+  let handlerInfo = getKnownHandlerInfo("example/new");
+  handlerInfo.preferredApplicationHandler = handlerApp;
+  handlerInfo.possibleApplicationHandlers.appendElement(handlerApp);
+  handlerInfo.possibleApplicationHandlers.appendElement(webHandlerApp);
+  gHandlerService.store(handlerInfo);
+
+  await unloadHandlerStore();
+
+  let actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo("example/new");
+  HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+    type: "example/new",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: false,
+    preferredApplicationHandler: expectedGIOMimeHandlerApp,
+    possibleApplicationHandlers: [expectedGIOMimeHandlerApp, webHandlerApp],
+  });
+
+  await IOUtils.remove(dummyHandlerFile.path);
+
+  // After removing dummyHandlerFile, the handler should disappear from the
+  // list of possibleApplicationHandlers and preferredAppHandler should be null.
+  actualHandlerInfo = HandlerServiceTestUtils.getHandlerInfo("example/new");
+  HandlerServiceTestUtils.assertHandlerInfoMatches(actualHandlerInfo, {
+    type: "example/new",
+    preferredAction: Ci.nsIHandlerInfo.saveToDisk,
+    alwaysAskBeforeHandling: false,
+    preferredApplicationHandler: null,
+    possibleApplicationHandlers: [webHandlerApp],
+  });
+});
diff --git a/uriloader/exthandler/tests/unit/test_punycodeURIs.js b/uriloader/exthandler/tests/unit/test_punycodeURIs.js
new file mode 100644
index 0000000000..949c9914ec
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/test_punycodeURIs.js
@@ -0,0 +1,126 @@
+/* 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/. */
+
+// Encoded test URI to work on all platforms/independent of file encoding
+const kTestURI = "http://\u65e5\u672c\u8a93.jp/";
+const kExpectedURI = "http://xn--wgv71a309e.jp/";
+const kOutputFile = "result.txt";
+
+// Try several times in case the box we're running on is slow.
+const kMaxCheckExistAttempts = 30; // seconds
+var gCheckExistsAttempts = 0;
+
+const tempDir = do_get_tempdir();
+
+function checkFile() {
+  // This is where we expect the output
+  var tempFile = tempDir.clone();
+  tempFile.append(kOutputFile);
+
+  if (!tempFile.exists()) {
+    if (gCheckExistsAttempts >= kMaxCheckExistAttempts) {
+      do_throw(
+        "Expected File " +
+          tempFile.path +
+          " does not exist after " +
+          kMaxCheckExistAttempts +
+          " seconds"
+      );
+    } else {
+      ++gCheckExistsAttempts;
+      // Wait a bit longer then try again
+      do_timeout(1000, checkFile);
+      return;
+    }
+  }
+
+  // Now read it
+  var fstream = Cc["@mozilla.org/network/file-input-stream;1"].createInstance(
+    Ci.nsIFileInputStream
+  );
+  var sstream = Cc["@mozilla.org/scriptableinputstream;1"].createInstance(
+    Ci.nsIScriptableInputStream
+  );
+  fstream.init(tempFile, -1, 0, 0);
+  sstream.init(fstream);
+
+  // Read the first line only as that's the one we expect WriteArguments
+  // to be writing the argument to.
+  var data = sstream.read(4096);
+
+  sstream.close();
+  fstream.close();
+
+  // Now remove the old file
+  tempFile.remove(false);
+
+  // This currently fails on Mac with an argument like -psn_0_nnnnnn
+  // This seems to be to do with how the executable is called, but I couldn't
+  // find a way around it.
+  // Additionally the lack of OS detection in xpcshell tests sucks, so we'll
+  // have to check for the argument mac gives us.
+  if (data.substring(0, 7) != "-psn_0_") {
+    Assert.equal(data, kExpectedURI);
+  }
+
+  do_test_finished();
+}
+
+function run_test() {
+  if (mozinfo.os == "mac") {
+    dump("INFO | test_punycodeURIs.js | Skipping test on mac, bug 599475");
+    return;
+  }
+
+  // set up the uri to test with
+  var ioService = Services.io;
+
+  // set up the local handler object
+  var localHandler = Cc[
+    "@mozilla.org/uriloader/local-handler-app;1"
+  ].createInstance(Ci.nsILocalHandlerApp);
+  localHandler.name = "Test Local Handler App";
+
+  // WriteArgument will just dump its arguments to a file for us.
+  var processDir = do_get_cwd();
+  var exe = processDir.clone();
+  exe.append("WriteArgument");
+
+  if (!exe.exists()) {
+    // Maybe we are on windows
+    exe.leafName = "WriteArgument.exe";
+    if (!exe.exists()) {
+      do_throw("Could not locate the WriteArgument tests executable\n");
+    }
+  }
+
+  var outFile = tempDir.clone();
+  outFile.append(kOutputFile);
+
+  // Set an environment variable for WriteArgument to pick up
+  // The Write Argument file needs to know where its libraries are, so
+  // just force the path variable
+  // For mac
+  var greDir = Services.dirsvc.get("GreD", Ci.nsIFile);
+
+  Services.env.set("DYLD_LIBRARY_PATH", greDir.path);
+  // For Linux
+  Services.env.set("LD_LIBRARY_PATH", greDir.path);
+  // XXX: handle windows
+
+  // Now tell it where we want the file.
+  Services.env.set("WRITE_ARGUMENT_FILE", outFile.path);
+
+  var uri = ioService.newURI(kTestURI);
+
+  // Just check we've got these matching, if we haven't there's a problem
+  // with ascii spec or our test case.
+  Assert.equal(uri.asciiSpec, kExpectedURI);
+
+  localHandler.executable = exe;
+  localHandler.launchWithURI(uri);
+
+  do_test_pending();
+  do_timeout(1000, checkFile);
+}
diff --git a/uriloader/exthandler/tests/unit/xpcshell.toml b/uriloader/exthandler/tests/unit/xpcshell.toml
new file mode 100644
index 0000000000..216ad49555
--- /dev/null
+++ b/uriloader/exthandler/tests/unit/xpcshell.toml
@@ -0,0 +1,43 @@
+[DEFAULT]
+head = "head.js"
+run-sequentially = "Bug 912235 - Intermittent failures"
+firefox-appdir = "browser"
+
+["test_badMIMEType.js"]
+run-if = ["buildapp == 'browser'"]
+
+["test_defaults_handlerService.js"]
+# No default stored handlers on android given lack of support.
+# No default stored handlers on Thunderbird.
+skip-if = [
+  "os == 'android'",
+  "appname == 'thunderbird'",
+]
+
+["test_filename_sanitize.js"]
+skip-if = ["os == 'mac'"] # Bug 1817727
+
+["test_getFromTypeAndExtension.js"]
+
+["test_getMIMEInfo_pdf.js"]
+
+["test_getMIMEInfo_unknown_mime_type.js"]
+run-if = ["os == 'win'"] # Windows only test
+
+["test_getTypeFromExtension_ext_to_type_mapping.js"]
+
+["test_getTypeFromExtension_with_empty_Content_Type.js"]
+run-if = ["os == 'win'"] # Windows only test
+
+["test_handlerService.js"]
+support-files = ["mailcap"]
+# Bug 676997: test consistently fails on Android
+fail-if = ["os == 'android'"]
+
+["test_handlerService_store.js"]
+# Disabled for 1563343 -- the app should determine possible handlers in GV.
+fail-if = ["os == 'android'"]
+support-files = ["handlers.json"]
+
+["test_punycodeURIs.js"]
+skip-if = ["os == 'win' && msix"] # https://bugzilla.mozilla.org/show_bug.cgi?id=1809485