Adding upstream version 124.0.1.upstream/124.0.1

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

31 files changed, 14661 insertions, 0 deletions

diff --git a/toolkit/components/downloads/DownloadCore.sys.mjs b/toolkit/components/downloads/DownloadCore.sys.mjs
new file mode 100644
index 0000000000..1711a01784
--- /dev/null
+++ b/toolkit/components/downloads/DownloadCore.sys.mjs
@@ -0,0 +1,3082 @@
+/* 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/. */
+
+/**
+ * Main implementation of the Downloads API objects. Consumers should get
+ * references to these objects through the "Downloads.sys.mjs" module.
+ */
+
+import { Integration } from "resource://gre/modules/Integration.sys.mjs";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  DownloadHistory: "resource://gre/modules/DownloadHistory.sys.mjs",
+  DownloadPaths: "resource://gre/modules/DownloadPaths.sys.mjs",
+  E10SUtils: "resource://gre/modules/E10SUtils.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+  NetUtil: "resource://gre/modules/NetUtil.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gExternalAppLauncher",
+  "@mozilla.org/uriloader/external-helper-app-service;1",
+  Ci.nsPIExternalAppLauncher
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gExternalHelperAppService",
+  "@mozilla.org/uriloader/external-helper-app-service;1",
+  Ci.nsIExternalHelperAppService
+);
+
+Integration.downloads.defineESModuleGetter(
+  lazy,
+  "DownloadIntegration",
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+const BackgroundFileSaverStreamListener = Components.Constructor(
+  "@mozilla.org/network/background-file-saver;1?mode=streamlistener",
+  "nsIBackgroundFileSaver"
+);
+
+/**
+ * Returns true if the given value is a primitive string or a String object.
+ */
+function isString(aValue) {
+  // We cannot use the "instanceof" operator reliably across module boundaries.
+  return (
+    typeof aValue == "string" ||
+    (typeof aValue == "object" && "charAt" in aValue)
+  );
+}
+
+/**
+ * Serialize the unknown properties of aObject into aSerializable.
+ */
+function serializeUnknownProperties(aObject, aSerializable) {
+  if (aObject._unknownProperties) {
+    for (let property in aObject._unknownProperties) {
+      aSerializable[property] = aObject._unknownProperties[property];
+    }
+  }
+}
+
+/**
+ * Check for any unknown properties in aSerializable and preserve those in the
+ * _unknownProperties field of aObject. aFilterFn is called for each property
+ * name of aObject and should return true only for unknown properties.
+ */
+function deserializeUnknownProperties(aObject, aSerializable, aFilterFn) {
+  for (let property in aSerializable) {
+    if (aFilterFn(property)) {
+      if (!aObject._unknownProperties) {
+        aObject._unknownProperties = {};
+      }
+
+      aObject._unknownProperties[property] = aSerializable[property];
+    }
+  }
+}
+
+/**
+ * Check if the file is a placeholder.
+ *
+ * @return {Promise}
+ * @resolves {boolean}
+ * @rejects Never.
+ */
+async function isPlaceholder(path) {
+  try {
+    if ((await IOUtils.stat(path)).size == 0) {
+      return true;
+    }
+  } catch (ex) {
+    // Canceling the download may have removed the placeholder already.
+    if (ex.name != "NotFoundError") {
+      console.error(ex);
+    }
+  }
+  return false;
+}
+
+/**
+ * This determines the minimum time interval between updates to the number of
+ * bytes transferred, and is a limiting factor to the sequence of readings used
+ * in calculating the speed of the download.
+ */
+const kProgressUpdateIntervalMs = 400;
+
+/**
+ * Represents a single download, with associated state and actions.  This object
+ * is transient, though it can be included in a DownloadList so that it can be
+ * managed by the user interface and persisted across sessions.
+ */
+export var Download = function () {
+  this._deferSucceeded = Promise.withResolvers();
+};
+
+Download.prototype = {
+  /**
+   * DownloadSource object associated with this download.
+   */
+  source: null,
+
+  /**
+   * DownloadTarget object associated with this download.
+   */
+  target: null,
+
+  /**
+   * DownloadSaver object associated with this download.
+   */
+  saver: null,
+
+  /**
+   * Indicates that the download never started, has been completed successfully,
+   * failed, or has been canceled.  This property becomes false when a download
+   * is started for the first time, or when a failed or canceled download is
+   * restarted.
+   */
+  stopped: true,
+
+  /**
+   * Indicates that the download has been completed successfully.
+   */
+  succeeded: false,
+
+  /**
+   * Indicates that the download has been canceled.  This property can become
+   * true, then it can be reset to false when a canceled download is restarted.
+   *
+   * This property becomes true as soon as the "cancel" method is called, though
+   * the "stopped" property might remain false until the cancellation request
+   * has been processed.  Temporary files or part files may still exist even if
+   * they are expected to be deleted, until the "stopped" property becomes true.
+   */
+  canceled: false,
+
+  /**
+   * Downloaded files can be deleted from within Firefox, e.g. via the context
+   * menu. Currently Firefox does not track file moves (see bug 1746386), so if
+   * a download's target file stops existing we have to assume it's "moved or
+   * missing." To distinguish files intentionally deleted within Firefox from
+   * files that are moved/missing, we mark them as "deleted" with this property.
+   */
+  deleted: false,
+
+  /**
+   * When the download fails, this is set to a DownloadError instance indicating
+   * the cause of the failure.  If the download has been completed successfully
+   * or has been canceled, this property is null.  This property is reset to
+   * null when a failed download is restarted.
+   */
+  error: null,
+
+  /**
+   * Indicates the start time of the download.  When the download starts,
+   * this property is set to a valid Date object.  The default value is null
+   * before the download starts.
+   */
+  startTime: null,
+
+  /**
+   * Indicates whether this download's "progress" property is able to report
+   * partial progress while the download proceeds, and whether the value in
+   * totalBytes is relevant.  This depends on the saver and the download source.
+   */
+  hasProgress: false,
+
+  /**
+   * Progress percent, from 0 to 100.  Intermediate values are reported only if
+   * hasProgress is true.
+   *
+   * @note You shouldn't rely on this property being equal to 100 to determine
+   *       whether the download is completed.  You should use the individual
+   *       state properties instead.
+   */
+  progress: 0,
+
+  /**
+   * When hasProgress is true, indicates the total number of bytes to be
+   * transferred before the download finishes, that can be zero for empty files.
+   *
+   * When hasProgress is false, this property is always zero.
+   *
+   * @note This property may be different than the final file size on disk for
+   *       downloads that are encoded during the network transfer.  You can use
+   *       the "size" property of the DownloadTarget object to get the actual
+   *       size on disk once the download succeeds.
+   */
+  totalBytes: 0,
+
+  /**
+   * Number of bytes currently transferred.  This value starts at zero, and may
+   * be updated regardless of the value of hasProgress.
+   *
+   * @note You shouldn't rely on this property being equal to totalBytes to
+   *       determine whether the download is completed.  You should use the
+   *       individual state properties instead.  This property may not be
+   *       updated during the last part of the download.
+   */
+  currentBytes: 0,
+
+  /**
+   * Fractional number representing the speed of the download, in bytes per
+   * second.  This value is zero when the download is stopped, and may be
+   * updated regardless of the value of hasProgress.
+   */
+  speed: 0,
+
+  /**
+   * Indicates whether, at this time, there is any partially downloaded data
+   * that can be used when restarting a failed or canceled download.
+   *
+   * Even if the download has partial data on disk, hasPartialData will be false
+   * if that data cannot be used to restart the download. In order to determine
+   * if a part file is being used which contains partial data the
+   * Download.target.partFilePath should be checked.
+   *
+   * This property is relevant while the download is in progress, and also if it
+   * failed or has been canceled.  If the download has been completed
+   * successfully, this property is always false.
+   *
+   * Whether partial data can actually be retained depends on the saver and the
+   * download source, and may not be known before the download is started.
+   */
+  hasPartialData: false,
+
+  /**
+   * Indicates whether, at this time, there is any data that has been blocked.
+   * Since reputation blocking takes place after the download has fully
+   * completed a value of true also indicates 100% of the data is present.
+   */
+  hasBlockedData: false,
+
+  /**
+   * This can be set to a function that is called after other properties change.
+   */
+  onchange: null,
+
+  /**
+   * This tells if the user has chosen to open/run the downloaded file after
+   * download has completed.
+   */
+  launchWhenSucceeded: false,
+
+  /**
+   * When a download starts, we typically want to automatically open the
+   * downloads panel if the pref browser.download.alwaysOpenPanel is enabled.
+   * However, there are conditions where we want to prevent this. For example, a
+   * false value can prevent the downloads panel from opening when an add-on
+   * creates a download without user input as part of some background operation.
+   */
+  openDownloadsListOnStart: true,
+
+  /**
+   * This represents the MIME type of the download.
+   */
+  contentType: null,
+
+  /**
+   * This indicates the path of the application to be used to launch the file,
+   * or null if the file should be launched with the default application.
+   */
+  launcherPath: null,
+
+  /**
+   * Raises the onchange notification.
+   */
+  _notifyChange: function D_notifyChange() {
+    try {
+      if (this.onchange) {
+        this.onchange();
+      }
+    } catch (ex) {
+      console.error(ex);
+    }
+  },
+
+  /**
+   * The download may be stopped and restarted multiple times before it
+   * completes successfully. This may happen if any of the download attempts is
+   * canceled or fails.
+   *
+   * This property contains a promise that is linked to the current attempt, or
+   * null if the download is either stopped or in the process of being canceled.
+   * If the download restarts, this property is replaced with a new promise.
+   *
+   * The promise is resolved if the attempt it represents finishes successfully,
+   * and rejected if the attempt fails.
+   */
+  _currentAttempt: null,
+
+  /**
+   * The download was launched to open from the Downloads Panel.
+   */
+  _launchedFromPanel: false,
+
+  /**
+   * Starts the download for the first time, or restarts a download that failed
+   * or has been canceled.
+   *
+   * Calling this method when the download has been completed successfully has
+   * no effect, and the method returns a resolved promise.  If the download is
+   * in progress, the method returns the same promise as the previous call.
+   *
+   * If the "cancel" method was called but the cancellation process has not
+   * finished yet, this method waits for the cancellation to finish, then
+   * restarts the download immediately.
+   *
+   * @note If you need to start a new download from the same source, rather than
+   *       restarting a failed or canceled one, you should create a separate
+   *       Download object with the same source as the current one.
+   *
+   * @return {Promise}
+   * @resolves When the download has finished successfully.
+   * @rejects JavaScript exception if the download failed.
+   */
+  start: function D_start() {
+    // If the download succeeded, it's the final state, we have nothing to do.
+    if (this.succeeded) {
+      return Promise.resolve();
+    }
+
+    // If the download already started and hasn't failed or hasn't been
+    // canceled, return the same promise as the previous call, allowing the
+    // caller to wait for the current attempt to finish.
+    if (this._currentAttempt) {
+      return this._currentAttempt;
+    }
+
+    // While shutting down or disposing of this object, we prevent the download
+    // from returning to be in progress.
+    if (this._finalized) {
+      return Promise.reject(
+        new DownloadError({
+          message: "Cannot start after finalization.",
+        })
+      );
+    }
+
+    if (this.error && this.error.becauseBlockedByReputationCheck) {
+      return Promise.reject(
+        new DownloadError({
+          message: "Cannot start after being blocked by a reputation check.",
+        })
+      );
+    }
+
+    // Initialize all the status properties for a new or restarted download.
+    this.stopped = false;
+    this.canceled = false;
+    this.error = null;
+    // Avoid serializing the previous error, or it would be restored on the next
+    // startup, even if the download was restarted.
+    delete this._unknownProperties?.errorObj;
+    this.hasProgress = false;
+    this.hasBlockedData = false;
+    this.progress = 0;
+    this.totalBytes = 0;
+    this.currentBytes = 0;
+    this.startTime = new Date();
+
+    // Create a new deferred object and an associated promise before starting
+    // the actual download.  We store it on the download as the current attempt.
+    let deferAttempt = Promise.withResolvers();
+    let currentAttempt = deferAttempt.promise;
+    this._currentAttempt = currentAttempt;
+
+    // Restart the progress and speed calculations from scratch.
+    this._lastProgressTimeMs = 0;
+
+    // This function propagates progress from the DownloadSaver object, unless
+    // it comes in late from a download attempt that was replaced by a new one.
+    // If the cancellation process for the download has started, then the update
+    // is ignored.
+    function DS_setProgressBytes(aCurrentBytes, aTotalBytes, aHasPartialData) {
+      if (this._currentAttempt == currentAttempt) {
+        this._setBytes(aCurrentBytes, aTotalBytes, aHasPartialData);
+      }
+    }
+
+    // This function propagates download properties from the DownloadSaver
+    // object, unless it comes in late from a download attempt that was
+    // replaced by a new one.  If the cancellation process for the download has
+    // started, then the update is ignored.
+    function DS_setProperties(aOptions) {
+      if (this._currentAttempt != currentAttempt) {
+        return;
+      }
+
+      let changeMade = false;
+
+      for (let property of [
+        "contentType",
+        "progress",
+        "hasPartialData",
+        "hasBlockedData",
+      ]) {
+        if (property in aOptions && this[property] != aOptions[property]) {
+          this[property] = aOptions[property];
+          changeMade = true;
+        }
+      }
+
+      if (changeMade) {
+        this._notifyChange();
+      }
+    }
+
+    // Now that we stored the promise in the download object, we can start the
+    // task that will actually execute the download.
+    deferAttempt.resolve(
+      (async () => {
+        // Wait upon any pending operation before restarting.
+        if (this._promiseCanceled) {
+          await this._promiseCanceled;
+        }
+        if (this._promiseRemovePartialData) {
+          try {
+            await this._promiseRemovePartialData;
+          } catch (ex) {
+            // Ignore any errors, which are already reported by the original
+            // caller of the removePartialData method.
+          }
+        }
+
+        // In case the download was restarted while cancellation was in progress,
+        // but the previous attempt actually succeeded before cancellation could
+        // be processed, it is possible that the download has already finished.
+        if (this.succeeded) {
+          return;
+        }
+
+        try {
+          if (this.downloadingToSameFile()) {
+            throw new DownloadError({
+              message: "Can't overwrite the source file.",
+              becauseTargetFailed: true,
+            });
+          }
+
+          // Disallow download if parental controls service restricts it.
+          if (
+            await lazy.DownloadIntegration.shouldBlockForParentalControls(this)
+          ) {
+            throw new DownloadError({ becauseBlockedByParentalControls: true });
+          }
+
+          // We should check if we have been canceled in the meantime, after all
+          // the previous asynchronous operations have been executed and just
+          // before we call the "execute" method of the saver.
+          if (this._promiseCanceled) {
+            // The exception will become a cancellation in the "catch" block.
+            throw new Error(undefined);
+          }
+
+          // Execute the actual download through the saver object.
+          this._saverExecuting = true;
+          try {
+            await this.saver.execute(
+              DS_setProgressBytes.bind(this),
+              DS_setProperties.bind(this)
+            );
+          } catch (ex) {
+            // Remove the target file placeholder and all partial data when
+            // needed, independently of which code path failed. In some cases, the
+            // component executing the download may have already removed the file.
+            if (!this.hasPartialData && !this.hasBlockedData) {
+              await this.saver.removeData(true);
+            }
+            throw ex;
+          }
+
+          // Now that the actual saving finished, read the actual file size on
+          // disk, that may be different from the amount of data transferred.
+          await this.target.refresh();
+
+          // Check for the last time if the download has been canceled. This must
+          // be done right before setting the "stopped" property of the download,
+          // without any asynchronous operations in the middle, so that another
+          // cancellation request cannot start in the meantime and stay unhandled.
+          if (this._promiseCanceled) {
+            // To keep the internal state of the Download object consistent, we
+            // just delete the target and effectively cancel the download. Since
+            // the DownloadSaver succeeded, we already renamed the ".part" file to
+            // the final name, and this results in all the data being deleted.
+            await this.saver.removeData(true);
+
+            // Cancellation exceptions will be changed in the catch block below.
+            throw new DownloadError();
+          }
+
+          // Update the status properties for a successful download.
+          this.progress = 100;
+          this.succeeded = true;
+          this.hasPartialData = false;
+        } catch (originalEx) {
+          // We may choose a different exception to propagate in the code below,
+          // or wrap the original one. We do this mutation in a different variable
+          // because of the "no-ex-assign" ESLint rule.
+          let ex = originalEx;
+
+          // Fail with a generic status code on cancellation, so that the caller
+          // is forced to actually check the status properties to see if the
+          // download was canceled or failed because of other reasons.
+          if (this._promiseCanceled) {
+            throw new DownloadError({ message: "Download canceled." });
+          }
+
+          // An HTTP 450 error code is used by Windows to indicate that a uri is
+          // blocked by parental controls. This will prevent the download from
+          // occuring, so an error needs to be raised. This is not performed
+          // during the parental controls check above as it requires the request
+          // to start.
+          if (this._blockedByParentalControls) {
+            ex = new DownloadError({ becauseBlockedByParentalControls: true });
+          }
+
+          // Update the download error, unless a new attempt already started. The
+          // change in the status property is notified in the finally block.
+          if (this._currentAttempt == currentAttempt || !this._currentAttempt) {
+            if (!(ex instanceof DownloadError)) {
+              let properties = { innerException: ex };
+
+              if (ex.message) {
+                properties.message = ex.message;
+              }
+
+              ex = new DownloadError(properties);
+            }
+            // Don't store an error if it's an abort caused by shutdown, so the
+            // download can be retried automatically at the next startup.
+            if (
+              originalEx.result != Cr.NS_ERROR_ABORT ||
+              !Services.startup.isInOrBeyondShutdownPhase(
+                Ci.nsIAppStartup.SHUTDOWN_PHASE_APPSHUTDOWNCONFIRMED
+              )
+            ) {
+              this.error = ex;
+            }
+          }
+          throw ex;
+        } finally {
+          // Any cancellation request has now been processed.
+          this._saverExecuting = false;
+          this._promiseCanceled = null;
+
+          // Update the status properties, unless a new attempt already started.
+          if (this._currentAttempt == currentAttempt || !this._currentAttempt) {
+            this._currentAttempt = null;
+            this.stopped = true;
+            this.speed = 0;
+            this._notifyChange();
+            if (this.succeeded) {
+              await this._succeed();
+            }
+          }
+        }
+      })()
+    );
+
+    // Notify the new download state before returning.
+    this._notifyChange();
+    return currentAttempt;
+  },
+
+  /**
+   * Perform the actions necessary when a Download succeeds.
+   *
+   * @return {Promise}
+   * @resolves When the steps to take after success have completed.
+   * @rejects  JavaScript exception if any of the operations failed.
+   */
+  async _succeed() {
+    await lazy.DownloadIntegration.downloadDone(this);
+
+    this._deferSucceeded.resolve();
+
+    if (this.launchWhenSucceeded) {
+      this.launch().catch(console.error);
+
+      // Always schedule files to be deleted at the end of the private browsing
+      // mode, regardless of the value of the pref.
+      if (this.source.isPrivate) {
+        lazy.gExternalAppLauncher.deleteTemporaryPrivateFileWhenPossible(
+          new lazy.FileUtils.File(this.target.path)
+        );
+      } else if (
+        Services.prefs.getBoolPref("browser.helperApps.deleteTempFileOnExit") &&
+        Services.prefs.getBoolPref(
+          "browser.download.start_downloads_in_tmp_dir",
+          false
+        )
+      ) {
+        lazy.gExternalAppLauncher.deleteTemporaryFileOnExit(
+          new lazy.FileUtils.File(this.target.path)
+        );
+      }
+    }
+  },
+
+  /**
+   * When a request to unblock the download is received, contains a promise
+   * that will be resolved when the unblock request is completed. This property
+   * will then continue to hold the promise indefinitely.
+   */
+  _promiseUnblock: null,
+
+  /**
+   * When a request to confirm the block of the download is received, contains
+   * a promise that will be resolved when cleaning up the download has
+   * completed. This property will then continue to hold the promise
+   * indefinitely.
+   */
+  _promiseConfirmBlock: null,
+
+  /**
+   * Unblocks a download which had been blocked by reputation.
+   *
+   * The file will be moved out of quarantine and the download will be
+   * marked as succeeded.
+   *
+   * @return {Promise}
+   * @resolves When the Download has been unblocked and succeeded.
+   * @rejects  JavaScript exception if any of the operations failed.
+   */
+  unblock() {
+    if (this._promiseUnblock) {
+      return this._promiseUnblock;
+    }
+
+    if (this._promiseConfirmBlock) {
+      return Promise.reject(
+        new Error("Download block has been confirmed, cannot unblock.")
+      );
+    }
+
+    if (this.error?.becauseBlockedByReputationCheck) {
+      Services.telemetry
+        .getKeyedHistogramById("DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD")
+        .add(this.error.reputationCheckVerdict, 2); // unblock
+    }
+
+    if (
+      this.error?.reputationCheckVerdict == DownloadError.BLOCK_VERDICT_INSECURE
+    ) {
+      // In this Error case, the download was actually canceled before it was
+      // passed to the Download UI. So we need to start the download here.
+      this.error = null;
+      this.succeeded = false;
+      this.hasBlockedData = false;
+      // This ensures the verdict will not get set again after the browser
+      // restarts and the download gets serialized and de-serialized again.
+      delete this._unknownProperties?.errorObj;
+      this.start()
+        .catch(err => {
+          if (err.becauseTargetFailed) {
+            // In case we cannot write to the target file
+            // retry with a new unique name
+            let uniquePath = lazy.DownloadPaths.createNiceUniqueFile(
+              new lazy.FileUtils.File(this.target.path)
+            ).path;
+            this.target.path = uniquePath;
+            return this.start();
+          }
+          return Promise.reject(err);
+        })
+        .catch(err => {
+          if (!this.canceled) {
+            console.error(err);
+          }
+          this._notifyChange();
+        });
+      this._notifyChange();
+      this._promiseUnblock = lazy.DownloadIntegration.downloadDone(this);
+      return this._promiseUnblock;
+    }
+
+    if (!this.hasBlockedData) {
+      return Promise.reject(
+        new Error("unblock may only be called on Downloads with blocked data.")
+      );
+    }
+
+    this._promiseUnblock = (async () => {
+      try {
+        await IOUtils.move(this.target.partFilePath, this.target.path);
+        await this.target.refresh();
+      } catch (ex) {
+        await this.refresh();
+        this._promiseUnblock = null;
+        throw ex;
+      }
+
+      this.succeeded = true;
+      this.hasBlockedData = false;
+      this._notifyChange();
+      await this._succeed();
+    })();
+
+    return this._promiseUnblock;
+  },
+
+  /**
+   * Confirms that a blocked download should be cleaned up.
+   *
+   * If a download was blocked but retained on disk this method can be used
+   * to remove the file.
+   *
+   * @return {Promise}
+   * @resolves When the Download's data has been removed.
+   * @rejects  JavaScript exception if any of the operations failed.
+   */
+  confirmBlock() {
+    if (this._promiseConfirmBlock) {
+      return this._promiseConfirmBlock;
+    }
+
+    if (this._promiseUnblock) {
+      return Promise.reject(
+        new Error("Download is being unblocked, cannot confirmBlock.")
+      );
+    }
+
+    if (this.error?.becauseBlockedByReputationCheck) {
+      // We have to record the telemetry in both DownloadsCommon.deleteDownload
+      // and confirmBlock here. The former is for cases where users click
+      // "Remove file" in the download panel and the latter is when
+      // users click "X" button in about:downloads.
+      Services.telemetry
+        .getKeyedHistogramById("DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD")
+        .add(this.error.reputationCheckVerdict, 1); // confirm block
+    }
+
+    if (!this.hasBlockedData) {
+      return Promise.reject(
+        new Error(
+          "confirmBlock may only be called on Downloads with blocked data."
+        )
+      );
+    }
+
+    this._promiseConfirmBlock = (async () => {
+      // This call never throws exceptions. If the removal fails, the blocked
+      // data remains stored on disk in the ".part" file.
+      await this.saver.removeData();
+
+      this.hasBlockedData = false;
+      this._notifyChange();
+    })();
+
+    return this._promiseConfirmBlock;
+  },
+
+  /*
+   * Launches the file after download has completed. This can open
+   * the file with the default application for the target MIME type
+   * or file extension, or with a custom application if launcherPath
+   * is set.
+   *
+   * @param options.openWhere  Optional string indicating how to open when handling
+   *                           download by opening the target file URI.
+   *                           One of "window", "tab", "tabshifted"
+   * @param options.useSystemDefault
+   *                           Optional value indicating how to handle launching this download,
+   *                           this time only. Will override the associated mimeInfo.preferredAction
+   * @return {Promise}
+   * @resolves When the instruction to launch the file has been
+   *           successfully given to the operating system. Note that
+   *           the OS might still take a while until the file is actually
+   *           launched.
+   * @rejects  JavaScript exception if there was an error trying to launch
+   *           the file.
+   */
+  launch(options = {}) {
+    if (!this.succeeded) {
+      return Promise.reject(
+        new Error("launch can only be called if the download succeeded")
+      );
+    }
+
+    if (this._launchedFromPanel) {
+      Services.telemetry.scalarAdd("downloads.file_opened", 1);
+    }
+
+    return lazy.DownloadIntegration.launchDownload(this, options);
+  },
+
+  /*
+   * Shows the folder containing the target file, or where the target file
+   * will be saved. This may be called at any time, even if the download
+   * failed or is currently in progress.
+   *
+   * @return {Promise}
+   * @resolves When the instruction to open the containing folder has been
+   *           successfully given to the operating system. Note that
+   *           the OS might still take a while until the folder is actually
+   *           opened.
+   * @rejects  JavaScript exception if there was an error trying to open
+   *           the containing folder.
+   */
+  showContainingDirectory: function D_showContainingDirectory() {
+    return lazy.DownloadIntegration.showContainingDirectory(this.target.path);
+  },
+
+  /**
+   * When a request to cancel the download is received, contains a promise that
+   * will be resolved when the cancellation request is processed.  When the
+   * request is processed, this property becomes null again.
+   */
+  _promiseCanceled: null,
+
+  /**
+   * True between the call to the "execute" method of the saver and the
+   * completion of the current download attempt.
+   */
+  _saverExecuting: false,
+
+  /**
+   * Cancels the download.
+   *
+   * The cancellation request is asynchronous.  Until the cancellation process
+   * finishes, temporary files or part files may still exist even if they are
+   * expected to be deleted.
+   *
+   * In case the download completes successfully before the cancellation request
+   * could be processed, this method has no effect, and it returns a resolved
+   * promise.  You should check the properties of the download at the time the
+   * returned promise is resolved to determine if the download was cancelled.
+   *
+   * Calling this method when the download has been completed successfully,
+   * failed, or has been canceled has no effect, and the method returns a
+   * resolved promise.  This behavior is designed for the case where the call
+   * to "cancel" happens asynchronously, and is consistent with the case where
+   * the cancellation request could not be processed in time.
+   *
+   * @return {Promise}
+   * @resolves When the cancellation process has finished.
+   * @rejects Never.
+   */
+  cancel: function D_cancel() {
+    // If the download is currently stopped, we have nothing to do.
+    if (this.stopped) {
+      return Promise.resolve();
+    }
+
+    if (!this._promiseCanceled) {
+      // Start a new cancellation request.
+      this._promiseCanceled = new Promise(resolve => {
+        this._currentAttempt.then(resolve, resolve);
+      });
+
+      // The download can already be restarted.
+      this._currentAttempt = null;
+
+      // Notify that the cancellation request was received.
+      this.canceled = true;
+      this._notifyChange();
+
+      // Execute the actual cancellation through the saver object, in case it
+      // has already started.  Otherwise, the cancellation will be handled just
+      // before the saver is started.
+      if (this._saverExecuting) {
+        this.saver.cancel();
+      }
+    }
+
+    return this._promiseCanceled;
+  },
+
+  /**
+   * Indicates whether any partially downloaded data should be retained, to use
+   * when restarting a failed or canceled download.  The default is false.
+   *
+   * Whether partial data can actually be retained depends on the saver and the
+   * download source, and may not be known before the download is started.
+   *
+   * To have any effect, this property must be set before starting the download.
+   * Resetting this property to false after the download has already started
+   * will not remove any partial data.
+   *
+   * If this property is set to true, care should be taken that partial data is
+   * removed before the reference to the download is discarded.  This can be
+   * done using the removePartialData or the "finalize" methods.
+   */
+  tryToKeepPartialData: false,
+
+  /**
+   * When a request to remove partially downloaded data is received, contains a
+   * promise that will be resolved when the removal request is processed.  When
+   * the request is processed, this property becomes null again.
+   */
+  _promiseRemovePartialData: null,
+
+  /**
+   * Removes any partial data kept as part of a canceled or failed download.
+   *
+   * If the download is not canceled or failed, this method has no effect, and
+   * it returns a resolved promise.  If the "cancel" method was called but the
+   * cancellation process has not finished yet, this method waits for the
+   * cancellation to finish, then removes the partial data.
+   *
+   * After this method has been called, if the tryToKeepPartialData property is
+   * still true when the download is restarted, partial data will be retained
+   * during the new download attempt.
+   *
+   * @return {Promise}
+   * @resolves When the partial data has been successfully removed.
+   * @rejects JavaScript exception if the operation could not be completed.
+   */
+  removePartialData() {
+    if (!this.canceled && !this.error) {
+      return Promise.resolve();
+    }
+
+    if (!this._promiseRemovePartialData) {
+      this._promiseRemovePartialData = (async () => {
+        try {
+          // Wait upon any pending cancellation request.
+          if (this._promiseCanceled) {
+            await this._promiseCanceled;
+          }
+          // Ask the saver object to remove any partial data.
+          await this.saver.removeData();
+          // For completeness, clear the number of bytes transferred.
+          if (this.currentBytes != 0 || this.hasPartialData) {
+            this.currentBytes = 0;
+            this.hasPartialData = false;
+            this.target.refreshPartFileState();
+            this._notifyChange();
+          }
+        } finally {
+          this._promiseRemovePartialData = null;
+        }
+      })();
+    }
+
+    return this._promiseRemovePartialData;
+  },
+
+  /**
+   * Returns true if the download source is the same as the target file.
+   */
+  downloadingToSameFile() {
+    if (!this.source.url || !this.source.url.startsWith("file:")) {
+      return false;
+    }
+
+    try {
+      let sourceUri = lazy.NetUtil.newURI(this.source.url);
+      let targetUri = lazy.NetUtil.newURI(
+        new lazy.FileUtils.File(this.target.path)
+      );
+      return sourceUri.equals(targetUri);
+    } catch (ex) {
+      return false;
+    }
+  },
+
+  /**
+   * This deferred object contains a promise that is resolved as soon as this
+   * download finishes successfully, and is never rejected.  This property is
+   * initialized when the download is created, and never changes.
+   */
+  _deferSucceeded: null,
+
+  /**
+   * Returns a promise that is resolved as soon as this download finishes
+   * successfully, even if the download was stopped and restarted meanwhile.
+   *
+   * You can use this property for scheduling download completion actions in the
+   * current session, for downloads that are controlled interactively.  If the
+   * download is not controlled interactively, you should use the promise
+   * returned by the "start" method instead, to check for success or failure.
+   *
+   * @return {Promise}
+   * @resolves When the download has finished successfully.
+   * @rejects Never.
+   */
+  whenSucceeded: function D_whenSucceeded() {
+    return this._deferSucceeded.promise;
+  },
+
+  /**
+   * Updates the state of a finished, failed, or canceled download based on the
+   * current state in the file system.  If the download is in progress or it has
+   * been finalized, this method has no effect, and it returns a resolved
+   * promise.
+   *
+   * This allows the properties of the download to be updated in case the user
+   * moved or deleted the target file or its associated ".part" file.
+   *
+   * @return {Promise}
+   * @resolves When the operation has completed.
+   * @rejects Never.
+   */
+  refresh() {
+    return (async () => {
+      if (!this.stopped || this._finalized) {
+        return;
+      }
+
+      if (this.succeeded) {
+        let oldExists = this.target.exists;
+        let oldSize = this.target.size;
+        await this.target.refresh();
+        if (oldExists != this.target.exists || oldSize != this.target.size) {
+          this._notifyChange();
+        }
+        return;
+      }
+
+      // Update the current progress from disk if we retained partial data.
+      if (
+        (this.hasPartialData || this.hasBlockedData) &&
+        this.target.partFilePath
+      ) {
+        try {
+          let stat = await IOUtils.stat(this.target.partFilePath);
+
+          // Ignore the result if the state has changed meanwhile.
+          if (!this.stopped || this._finalized) {
+            return;
+          }
+
+          // Update the bytes transferred and the related progress properties.
+          this.currentBytes = stat.size;
+          if (this.totalBytes > 0) {
+            this.hasProgress = true;
+            this.progress = Math.floor(
+              (this.currentBytes / this.totalBytes) * 100
+            );
+          }
+        } catch (ex) {
+          if (ex.name != "NotFoundError") {
+            throw ex;
+          }
+          // Ignore the result if the state has changed meanwhile.
+          if (!this.stopped || this._finalized) {
+            return;
+          }
+          // In case we've blocked the Download becasue its
+          // insecure, we should not set hasBlockedData to
+          // false as its required to show the Unblock option.
+          if (
+            this.error.reputationCheckVerdict ==
+            DownloadError.BLOCK_VERDICT_INSECURE
+          ) {
+            return;
+          }
+
+          this.hasBlockedData = false;
+          this.hasPartialData = false;
+        }
+
+        this._notifyChange();
+      }
+    })().catch(console.error);
+  },
+
+  /**
+   * True if the "finalize" method has been called.  This prevents the download
+   * from starting again after having been stopped.
+   */
+  _finalized: false,
+
+  /**
+   * True if the "finalize" has been called and fully finished it's execution.
+   */
+  _finalizeExecuted: false,
+
+  /**
+   * Ensures that the download is stopped, and optionally removes any partial
+   * data kept as part of a canceled or failed download.  After this method has
+   * been called, the download cannot be started again.
+   *
+   * This method should be used in place of "cancel" and removePartialData while
+   * shutting down or disposing of the download object, to prevent other callers
+   * from interfering with the operation.  This is required because cancellation
+   * and other operations are asynchronous.
+   *
+   * @param aRemovePartialData
+   *        Whether any partially downloaded data should be removed after the
+   *        download has been stopped.
+   *
+   * @return {Promise}
+   * @resolves When the operation has finished successfully.
+   * @rejects JavaScript exception if an error occurred while removing the
+   *          partially downloaded data.
+   */
+  finalize(aRemovePartialData) {
+    // Prevents the download from starting again after having been stopped.
+    this._finalized = true;
+    let promise;
+
+    if (aRemovePartialData) {
+      // Cancel the download, in case it is currently in progress, then remove
+      // any partially downloaded data.  The removal operation waits for
+      // cancellation to be completed before resolving the promise it returns.
+      this.cancel();
+      promise = this.removePartialData();
+    } else {
+      // Just cancel the download, in case it is currently in progress.
+      promise = this.cancel();
+    }
+    promise.then(() => {
+      // At this point, either removing data / just cancelling the download should be done.
+      this._finalizeExecuted = true;
+    });
+
+    return promise;
+  },
+
+  /**
+   * Deletes all file data associated with a download, preserving the download
+   * object itself and updating it for download views.
+   */
+  async manuallyRemoveData() {
+    let { path } = this.target;
+    if (this.succeeded) {
+      // Temp files are made "read-only" by DownloadIntegration.downloadDone, so
+      // reset the permission bits to read/write. This won't be necessary after
+      // bug 1733587 since Downloads won't ever be temporary.
+      await IOUtils.setPermissions(path, 0o660);
+      await IOUtils.remove(path, { ignoreAbsent: true });
+    }
+    this.deleted = true;
+    await this.cancel();
+    await this.removePartialData();
+    // We need to guarantee that the UI is refreshed irrespective of what state
+    // the download is in when this is called, to ensure the download doesn't
+    // wind up stuck displaying as if it exists when it actually doesn't. And
+    // that means updating this.target.partFileExists no matter what.
+    await this.target.refreshPartFileState();
+    await this.refresh();
+    // The above methods will sometimes call _notifyChange, but not always. It
+    // depends on whether the download is `succeeded`, `stopped`, `canceled`,
+    // etc. Since this method needs to update the UI and can be invoked on any
+    // download as long as its target has some file on the system, we need to
+    // call _notifyChange no matter what state the download is in.
+    this._notifyChange();
+  },
+
+  /**
+   * Indicates the time of the last progress notification, expressed as the
+   * number of milliseconds since January 1, 1970, 00:00:00 UTC.  This is zero
+   * until some bytes have actually been transferred.
+   */
+  _lastProgressTimeMs: 0,
+
+  /**
+   * Updates progress notifications based on the number of bytes transferred.
+   *
+   * The number of bytes transferred is not updated unless enough time passed
+   * since this function was last called.  This limits the computation load, in
+   * particular when the listeners update the user interface in response.
+   *
+   * @param aCurrentBytes
+   *        Number of bytes transferred until now.
+   * @param aTotalBytes
+   *        Total number of bytes to be transferred, or -1 if unknown.
+   * @param aHasPartialData
+   *        Indicates whether the partially downloaded data can be used when
+   *        restarting the download if it fails or is canceled.
+   */
+  _setBytes: function D_setBytes(aCurrentBytes, aTotalBytes, aHasPartialData) {
+    let changeMade = this.hasPartialData != aHasPartialData;
+    this.hasPartialData = aHasPartialData;
+
+    // Unless aTotalBytes is -1, we can report partial download progress.  In
+    // this case, notify when the related properties changed since last time.
+    if (
+      aTotalBytes != -1 &&
+      (!this.hasProgress || this.totalBytes != aTotalBytes)
+    ) {
+      this.hasProgress = true;
+      this.totalBytes = aTotalBytes;
+      changeMade = true;
+    }
+
+    // Updating the progress and computing the speed require that enough time
+    // passed since the last update, or that we haven't started throttling yet.
+    let currentTimeMs = Date.now();
+    let intervalMs = currentTimeMs - this._lastProgressTimeMs;
+    if (intervalMs >= kProgressUpdateIntervalMs) {
+      // Don't compute the speed unless we started throttling notifications.
+      if (this._lastProgressTimeMs != 0) {
+        // Calculate the speed in bytes per second.
+        let rawSpeed =
+          ((aCurrentBytes - this.currentBytes) / intervalMs) * 1000;
+        if (this.speed == 0) {
+          // When the previous speed is exactly zero instead of a fractional
+          // number, this can be considered the first element of the series.
+          this.speed = rawSpeed;
+        } else {
+          // Apply exponential smoothing, with a smoothing factor of 0.1.
+          this.speed = rawSpeed * 0.1 + this.speed * 0.9;
+        }
+      }
+
+      // Start throttling notifications only when we have actually received some
+      // bytes for the first time.  The timing of the first part of the download
+      // is not reliable, due to possible latency in the initial notifications.
+      // This also allows automated tests to receive and verify the number of
+      // bytes initially transferred.
+      if (aCurrentBytes > 0) {
+        this._lastProgressTimeMs = currentTimeMs;
+
+        // Update the progress now that we don't need its previous value.
+        this.currentBytes = aCurrentBytes;
+        if (this.totalBytes > 0) {
+          this.progress = Math.floor(
+            (this.currentBytes / this.totalBytes) * 100
+          );
+        }
+        changeMade = true;
+      }
+
+      if (this.hasProgress && this.target && !this.target.partFileExists) {
+        this.target.refreshPartFileState();
+      }
+    }
+
+    if (changeMade) {
+      this._notifyChange();
+    }
+  },
+
+  /**
+   * Returns a static representation of the current object state.
+   *
+   * @return A JavaScript object that can be serialized to JSON.
+   */
+  toSerializable() {
+    let serializable = {
+      source: this.source.toSerializable(),
+      target: this.target.toSerializable(),
+    };
+
+    let saver = this.saver.toSerializable();
+    if (!serializable.source || !saver) {
+      // If we are unable to serialize either the source or the saver,
+      // we won't persist the download.
+      return null;
+    }
+
+    // Simplify the representation for the most common saver type.  If the saver
+    // is an object instead of a simple string, we can't simplify it because we
+    // need to persist all its properties, not only "type".  This may happen for
+    // savers of type "copy" as well as other types.
+    if (saver !== "copy") {
+      serializable.saver = saver;
+    }
+
+    if (this.error) {
+      serializable.errorObj = this.error.toSerializable();
+    }
+
+    if (this.startTime) {
+      serializable.startTime = this.startTime.toJSON();
+    }
+
+    // These are serialized unless they are false, null, or empty strings.
+    for (let property of kPlainSerializableDownloadProperties) {
+      if (this[property]) {
+        serializable[property] = this[property];
+      }
+    }
+
+    serializeUnknownProperties(this, serializable);
+
+    return serializable;
+  },
+
+  /**
+   * Returns a value that changes only when one of the properties of a Download
+   * object that should be saved into a file also change.  This excludes
+   * properties whose value doesn't usually change during the download lifetime.
+   *
+   * This function is used to determine whether the download should be
+   * serialized after a property change notification has been received.
+   *
+   * @return String representing the relevant download state.
+   */
+  getSerializationHash() {
+    // The "succeeded", "canceled", "error", and startTime properties are not
+    // taken into account because they all change before the "stopped" property
+    // changes, and are not altered in other cases.
+    return (
+      this.stopped +
+      "," +
+      this.totalBytes +
+      "," +
+      this.hasPartialData +
+      "," +
+      this.contentType
+    );
+  },
+};
+
+/**
+ * Defines which properties of the Download object are serializable.
+ */
+const kPlainSerializableDownloadProperties = [
+  "succeeded",
+  "canceled",
+  "totalBytes",
+  "hasPartialData",
+  "hasBlockedData",
+  "tryToKeepPartialData",
+  "launcherPath",
+  "launchWhenSucceeded",
+  "contentType",
+  "handleInternally",
+  "openDownloadsListOnStart",
+];
+
+/**
+ * Creates a new Download object from a serializable representation.  This
+ * function is used by the createDownload method of Downloads.sys.mjs when a new
+ * Download object is requested, thus some properties may refer to live objects
+ * in place of their serializable representations.
+ *
+ * @param aSerializable
+ *        An object with the following fields:
+ *        {
+ *          source: DownloadSource object, or its serializable representation.
+ *                  See DownloadSource.fromSerializable for details.
+ *          target: DownloadTarget object, or its serializable representation.
+ *                  See DownloadTarget.fromSerializable for details.
+ *          saver: Serializable representation of a DownloadSaver object.  See
+ *                 DownloadSaver.fromSerializable for details.  If omitted,
+ *                 defaults to "copy".
+ *        }
+ *
+ * @return The newly created Download object.
+ */
+Download.fromSerializable = function (aSerializable) {
+  let download = new Download();
+  if (aSerializable.source instanceof DownloadSource) {
+    download.source = aSerializable.source;
+  } else {
+    download.source = DownloadSource.fromSerializable(aSerializable.source);
+  }
+  if (aSerializable.target instanceof DownloadTarget) {
+    download.target = aSerializable.target;
+  } else {
+    download.target = DownloadTarget.fromSerializable(aSerializable.target);
+  }
+  if ("saver" in aSerializable) {
+    download.saver = DownloadSaver.fromSerializable(aSerializable.saver);
+  } else {
+    download.saver = DownloadSaver.fromSerializable("copy");
+  }
+  download.saver.download = download;
+
+  if ("startTime" in aSerializable) {
+    let time = aSerializable.startTime.getTime
+      ? aSerializable.startTime.getTime()
+      : aSerializable.startTime;
+    download.startTime = new Date(time);
+  }
+
+  // If 'errorObj' is present it will take precedence over the 'error' property.
+  // 'error' is a legacy property only containing message, which is insufficient
+  // to represent all of the error information.
+  //
+  // Instead of just replacing 'error' we use a new 'errorObj' so that previous
+  // versions will keep it as an unknown property.
+  if ("errorObj" in aSerializable) {
+    download.error = DownloadError.fromSerializable(aSerializable.errorObj);
+  } else if ("error" in aSerializable) {
+    download.error = aSerializable.error;
+  }
+
+  for (let property of kPlainSerializableDownloadProperties) {
+    if (property in aSerializable) {
+      download[property] = aSerializable[property];
+    }
+  }
+
+  deserializeUnknownProperties(
+    download,
+    aSerializable,
+    property =>
+      !kPlainSerializableDownloadProperties.includes(property) &&
+      property != "startTime" &&
+      property != "source" &&
+      property != "target" &&
+      property != "error" &&
+      property != "saver"
+  );
+
+  return download;
+};
+
+/**
+ * Represents the source of a download, for example a document or an URI.
+ */
+export var DownloadSource = function () {};
+
+DownloadSource.prototype = {
+  /**
+   * String containing the URI for the download source.
+   */
+  url: null,
+
+  /**
+   * String containing the original URL for the download source.
+   */
+  originalUrl: null,
+
+  /**
+   * Indicates whether the download originated from a private window.  This
+   * determines the context of the network request that is made to retrieve the
+   * resource.
+   */
+  isPrivate: false,
+
+  /**
+   * Represents the referrerInfo of the download source, could be null for
+   * example if the download source is not HTTP.
+   */
+  referrerInfo: null,
+
+  /**
+   * For downloads handled by the (default) DownloadCopySaver, this function
+   * can adjust the network channel before it is opened, for example to change
+   * the HTTP headers or to upload a stream as POST data.
+   *
+   * @note If this is defined this object will not be serializable, thus the
+   *       Download object will not be persisted across sessions.
+   *
+   * @param aChannel
+   *        The nsIChannel to be adjusted.
+   *
+   * @return {Promise}
+   * @resolves When the channel has been adjusted and can be opened.
+   * @rejects JavaScript exception that will cause the download to fail.
+   */
+  adjustChannel: null,
+
+  /**
+   * For downloads handled by the (default) DownloadCopySaver, this function
+   * will determine, if provided, if a download can progress or has to be
+   * cancelled based on the HTTP status code of the network channel.
+   *
+   * @note If this is defined this object will not be serializable, thus the
+   *       Download object will not be persisted across sessions.
+   *
+   * @param aDownload
+   *        The download asking.
+   * @param aStatus
+   *        The HTTP status in question
+   *
+   * @return {Boolean} Download can progress
+   */
+  allowHttpStatus: null,
+
+  /**
+   * Represents the loadingPrincipal of the download source,
+   * could be null, in which case the system principal is used instead.
+   */
+  loadingPrincipal: null,
+
+  /**
+   * Represents the cookieJarSettings of the download source, could be null if
+   * the download source is not from a document.
+   */
+  cookieJarSettings: null,
+
+  /**
+   * Represents the authentication header of the download source, could be null if
+   * the download source had no authentication header.
+   */
+  authHeader: null,
+  /**
+   * Returns a static representation of the current object state.
+   *
+   * @return A JavaScript object that can be serialized to JSON.
+   */
+  toSerializable() {
+    if (this.adjustChannel) {
+      // If the callback was used, we can't reproduce this across sessions.
+      return null;
+    }
+
+    if (this.allowHttpStatus) {
+      // If the callback was used, we can't reproduce this across sessions.
+      return null;
+    }
+
+    let serializable = { url: this.url };
+    if (this.isPrivate) {
+      serializable.isPrivate = true;
+    }
+
+    if (this.referrerInfo && isString(this.referrerInfo)) {
+      serializable.referrerInfo = this.referrerInfo;
+    } else if (this.referrerInfo) {
+      serializable.referrerInfo = lazy.E10SUtils.serializeReferrerInfo(
+        this.referrerInfo
+      );
+    }
+
+    if (this.loadingPrincipal) {
+      serializable.loadingPrincipal = isString(this.loadingPrincipal)
+        ? this.loadingPrincipal
+        : lazy.E10SUtils.serializePrincipal(this.loadingPrincipal);
+    }
+
+    if (this.cookieJarSettings) {
+      serializable.cookieJarSettings = isString(this.cookieJarSettings)
+        ? this.cookieJarSettings
+        : lazy.E10SUtils.serializeCookieJarSettings(this.cookieJarSettings);
+    }
+
+    serializeUnknownProperties(this, serializable);
+
+    // Simplify the representation if we don't have other details.
+    if (Object.keys(serializable).length === 1) {
+      // serializable's only key is "url", just return the URL as a string.
+      return this.url;
+    }
+    return serializable;
+  },
+};
+
+/**
+ * Creates a new DownloadSource object from its serializable representation.
+ *
+ * @param aSerializable
+ *        Serializable representation of a DownloadSource object.  This may be a
+ *        string containing the URI for the download source, an nsIURI, or an
+ *        object with the following properties:
+ *        {
+ *          url: String containing the URI for the download source.
+ *          isPrivate: Indicates whether the download originated from a private
+ *                     window.  If omitted, the download is public.
+ *          referrerInfo: represents the referrerInfo of the download source.
+ *                        Can be omitted or null for example if the download
+ *                        source is not HTTP.
+ *          cookieJarSettings: represents the cookieJarSettings of the download
+ *                             source. Can be omitted or null if the download
+ *                             source is not from a document.
+ *          adjustChannel: For downloads handled by (default) DownloadCopySaver,
+ *                         this function can adjust the network channel before
+ *                         it is opened, for example to change the HTTP headers
+ *                         or to upload a stream as POST data.  Optional.
+ *          allowHttpStatus: For downloads handled by the (default)
+ *                           DownloadCopySaver, this function will determine, if
+ *                           provided, if a download can progress or has to be
+ *                           cancelled based on the HTTP status code of the
+ *                           network channel.
+ *        }
+ *
+ * @return The newly created DownloadSource object.
+ */
+DownloadSource.fromSerializable = function (aSerializable) {
+  let source = new DownloadSource();
+  if (isString(aSerializable)) {
+    // Convert String objects to primitive strings at this point.
+    source.url = aSerializable.toString();
+  } else if (aSerializable instanceof Ci.nsIURI) {
+    source.url = aSerializable.spec;
+  } else {
+    // Convert String objects to primitive strings at this point.
+    source.url = aSerializable.url.toString();
+    for (let propName of ["isPrivate", "userContextId", "browsingContextId"]) {
+      if (propName in aSerializable) {
+        source[propName] = aSerializable[propName];
+      }
+    }
+    if ("originalUrl" in aSerializable) {
+      source.originalUrl = aSerializable.originalUrl;
+    }
+    if ("referrerInfo" in aSerializable) {
+      // Quick pass, pass directly nsIReferrerInfo, we don't need to serialize
+      // and deserialize
+      if (aSerializable.referrerInfo instanceof Ci.nsIReferrerInfo) {
+        source.referrerInfo = aSerializable.referrerInfo;
+      } else {
+        source.referrerInfo = lazy.E10SUtils.deserializeReferrerInfo(
+          aSerializable.referrerInfo
+        );
+      }
+    }
+    if ("loadingPrincipal" in aSerializable) {
+      // Quick pass, pass directly nsIPrincipal, we don't need to serialize
+      // and deserialize
+      if (aSerializable.loadingPrincipal instanceof Ci.nsIPrincipal) {
+        source.loadingPrincipal = aSerializable.loadingPrincipal;
+      } else {
+        source.loadingPrincipal = lazy.E10SUtils.deserializePrincipal(
+          aSerializable.loadingPrincipal
+        );
+      }
+    }
+    if ("adjustChannel" in aSerializable) {
+      source.adjustChannel = aSerializable.adjustChannel;
+    }
+
+    if ("allowHttpStatus" in aSerializable) {
+      source.allowHttpStatus = aSerializable.allowHttpStatus;
+    }
+
+    if ("cookieJarSettings" in aSerializable) {
+      if (aSerializable.cookieJarSettings instanceof Ci.nsICookieJarSettings) {
+        source.cookieJarSettings = aSerializable.cookieJarSettings;
+      } else {
+        source.cookieJarSettings = lazy.E10SUtils.deserializeCookieJarSettings(
+          aSerializable.cookieJarSettings
+        );
+      }
+    }
+
+    if ("authHeader" in aSerializable) {
+      source.authHeader = aSerializable.authHeader;
+    }
+
+    deserializeUnknownProperties(
+      source,
+      aSerializable,
+      property =>
+        property != "url" &&
+        property != "originalUrl" &&
+        property != "isPrivate" &&
+        property != "referrerInfo" &&
+        property != "cookieJarSettings" &&
+        property != "authHeader"
+    );
+  }
+
+  return source;
+};
+
+/**
+ * Represents the target of a download, for example a file in the global
+ * downloads directory, or a file in the system temporary directory.
+ */
+export var DownloadTarget = function () {};
+
+DownloadTarget.prototype = {
+  /**
+   * String containing the path of the target file.
+   */
+  path: null,
+
+  /**
+   * String containing the path of the ".part" file containing the data
+   * downloaded so far, or null to disable the use of a ".part" file to keep
+   * partially downloaded data.
+   */
+  partFilePath: null,
+
+  /**
+   * Indicates whether the target file exists.
+   *
+   * This is a dynamic property updated when the download finishes or when the
+   * "refresh" method of the Download object is called. It can be used by the
+   * front-end to reduce I/O compared to checking the target file directly.
+   */
+  exists: false,
+
+  /**
+   * Indicates whether the part file exists. Like `exists`, this is updated
+   * dynamically to reduce I/O compared to checking the target file directly.
+   */
+  partFileExists: false,
+
+  /**
+   * Size in bytes of the target file, or zero if the download has not finished.
+   *
+   * Even if the target file does not exist anymore, this property may still
+   * have a value taken from the download metadata. If the metadata has never
+   * been available in this session and the size cannot be obtained from the
+   * file because it has already been deleted, this property will be zero.
+   *
+   * For single-file downloads, this property will always match the actual file
+   * size on disk, while the totalBytes property of the Download object, when
+   * available, may represent the size of the encoded data instead.
+   *
+   * For downloads involving multiple files, like complete web pages saved to
+   * disk, the meaning of this value is undefined. It currently matches the size
+   * of the main file only rather than the sum of all the written data.
+   *
+   * This is a dynamic property updated when the download finishes or when the
+   * "refresh" method of the Download object is called. It can be used by the
+   * front-end to reduce I/O compared to checking the target file directly.
+   */
+  size: 0,
+
+  /**
+   * Sets the "exists" and "size" properties based on the actual file on disk.
+   *
+   * @return {Promise}
+   * @resolves When the operation has finished successfully.
+   * @rejects JavaScript exception.
+   */
+  async refresh() {
+    try {
+      this.size = (await IOUtils.stat(this.path)).size;
+      this.exists = true;
+    } catch (ex) {
+      // Report any error not caused by the file not being there. In any case,
+      // the size of the download is not updated and the known value is kept.
+      if (ex.name != "NotFoundError") {
+        console.error(ex);
+      }
+      this.exists = false;
+    }
+    this.refreshPartFileState();
+  },
+
+  async refreshPartFileState() {
+    if (!this.partFilePath) {
+      this.partFileExists = false;
+      return;
+    }
+    try {
+      this.partFileExists = (await IOUtils.stat(this.partFilePath)).size > 0;
+    } catch (ex) {
+      if (ex.name != "NotFoundError") {
+        console.error(ex);
+      }
+      this.partFileExists = false;
+    }
+  },
+
+  /**
+   * Returns a static representation of the current object state.
+   *
+   * @return A JavaScript object that can be serialized to JSON.
+   */
+  toSerializable() {
+    // Simplify the representation if we don't have other details.
+    if (!this.partFilePath && !this._unknownProperties) {
+      return this.path;
+    }
+
+    let serializable = { path: this.path, partFilePath: this.partFilePath };
+    serializeUnknownProperties(this, serializable);
+    return serializable;
+  },
+};
+
+/**
+ * Creates a new DownloadTarget object from its serializable representation.
+ *
+ * @param aSerializable
+ *        Serializable representation of a DownloadTarget object.  This may be a
+ *        string containing the path of the target file, an nsIFile, or an
+ *        object with the following properties:
+ *        {
+ *          path: String containing the path of the target file.
+ *          partFilePath: optional string containing the part file path.
+ *        }
+ *
+ * @return The newly created DownloadTarget object.
+ */
+DownloadTarget.fromSerializable = function (aSerializable) {
+  let target = new DownloadTarget();
+  if (isString(aSerializable)) {
+    // Convert String objects to primitive strings at this point.
+    target.path = aSerializable.toString();
+  } else if (aSerializable instanceof Ci.nsIFile) {
+    // Read the "path" property of nsIFile after checking the object type.
+    target.path = aSerializable.path;
+  } else {
+    // Read the "path" property of the serializable DownloadTarget
+    // representation, converting String objects to primitive strings.
+    target.path = aSerializable.path.toString();
+    if ("partFilePath" in aSerializable) {
+      target.partFilePath = aSerializable.partFilePath;
+    }
+
+    deserializeUnknownProperties(
+      target,
+      aSerializable,
+      property => property != "path" && property != "partFilePath"
+    );
+  }
+  return target;
+};
+
+/**
+ * Provides detailed information about a download failure.
+ *
+ * @param aProperties
+ *        Object which may contain any of the following properties:
+ *          {
+ *            result: Result error code, defaulting to Cr.NS_ERROR_FAILURE
+ *            message: String error message to be displayed, or null to use the
+ *                     message associated with the result code.
+ *            inferCause: If true, attempts to determine if the cause of the
+ *                        download is a network failure or a local file failure,
+ *                        based on a set of known values of the result code.
+ *                        This is useful when the error is received by a
+ *                        component that handles both aspects of the download.
+ *          }
+ *        The properties object may also contain any of the DownloadError's
+ *        because properties, which will be set accordingly in the error object.
+ */
+export var DownloadError = function (aProperties) {
+  const NS_ERROR_MODULE_BASE_OFFSET = 0x45;
+  const NS_ERROR_MODULE_NETWORK = 6;
+  const NS_ERROR_MODULE_FILES = 13;
+
+  // Set the error name used by the Error object prototype first.
+  this.name = "DownloadError";
+  this.result = aProperties.result || Cr.NS_ERROR_FAILURE;
+  if (aProperties.message) {
+    this.message = aProperties.message;
+  } else if (
+    aProperties.becauseBlocked ||
+    aProperties.becauseBlockedByParentalControls ||
+    aProperties.becauseBlockedByReputationCheck
+  ) {
+    this.message = "Download blocked.";
+  } else {
+    let exception = new Components.Exception("", this.result);
+    this.message = exception.toString();
+  }
+  if (aProperties.inferCause) {
+    let module =
+      ((this.result & 0x7fff0000) >> 16) - NS_ERROR_MODULE_BASE_OFFSET;
+    this.becauseSourceFailed = module == NS_ERROR_MODULE_NETWORK;
+    this.becauseTargetFailed = module == NS_ERROR_MODULE_FILES;
+  } else {
+    if (aProperties.becauseSourceFailed) {
+      this.becauseSourceFailed = true;
+    }
+    if (aProperties.becauseTargetFailed) {
+      this.becauseTargetFailed = true;
+    }
+  }
+
+  if (aProperties.becauseBlockedByParentalControls) {
+    this.becauseBlocked = true;
+    this.becauseBlockedByParentalControls = true;
+  } else if (aProperties.becauseBlockedByReputationCheck) {
+    this.becauseBlocked = true;
+    this.becauseBlockedByReputationCheck = true;
+    this.reputationCheckVerdict = aProperties.reputationCheckVerdict || "";
+  } else if (aProperties.becauseBlocked) {
+    this.becauseBlocked = true;
+  }
+
+  if (aProperties.innerException) {
+    this.innerException = aProperties.innerException;
+  }
+
+  this.stack = new Error().stack;
+};
+
+/**
+ * These constants are used by the reputationCheckVerdict property and indicate
+ * the detailed reason why a download is blocked.
+ *
+ * @note These values should not be changed because they can be serialized.
+ */
+DownloadError.BLOCK_VERDICT_MALWARE = "Malware";
+DownloadError.BLOCK_VERDICT_POTENTIALLY_UNWANTED = "PotentiallyUnwanted";
+DownloadError.BLOCK_VERDICT_INSECURE = "Insecure";
+DownloadError.BLOCK_VERDICT_UNCOMMON = "Uncommon";
+DownloadError.BLOCK_VERDICT_DOWNLOAD_SPAM = "DownloadSpam";
+
+DownloadError.prototype = {
+  /**
+   * The result code associated with this error.
+   */
+  result: false,
+
+  /**
+   * Indicates an error occurred while reading from the remote location.
+   */
+  becauseSourceFailed: false,
+
+  /**
+   * Indicates an error occurred while writing to the local target.
+   */
+  becauseTargetFailed: false,
+
+  /**
+   * Indicates the download failed because it was blocked.  If the reason for
+   * blocking is known, the corresponding property will be also set.
+   */
+  becauseBlocked: false,
+
+  /**
+   * Indicates the download was blocked because downloads are globally
+   * disallowed by the Parental Controls or Family Safety features on Windows.
+   */
+  becauseBlockedByParentalControls: false,
+
+  /**
+   * Indicates the download was blocked because it failed the reputation check
+   * and may be malware.
+   */
+  becauseBlockedByReputationCheck: false,
+
+  /**
+   * If becauseBlockedByReputationCheck is true, indicates the detailed reason
+   * why the download was blocked, according to the "BLOCK_VERDICT_" constants.
+   *
+   * If the download was not blocked or the reason for the block is unknown,
+   * this will be an empty string.
+   */
+  reputationCheckVerdict: "",
+
+  /**
+   * If this DownloadError was caused by an exception this property will
+   * contain the original exception. This will not be serialized when saving
+   * to the store.
+   */
+  innerException: null,
+
+  /**
+   * Returns a static representation of the current object state.
+   *
+   * @return A JavaScript object that can be serialized to JSON.
+   */
+  toSerializable() {
+    let serializable = {
+      result: this.result,
+      message: this.message,
+      becauseSourceFailed: this.becauseSourceFailed,
+      becauseTargetFailed: this.becauseTargetFailed,
+      becauseBlocked: this.becauseBlocked,
+      becauseBlockedByParentalControls: this.becauseBlockedByParentalControls,
+      becauseBlockedByReputationCheck: this.becauseBlockedByReputationCheck,
+      reputationCheckVerdict: this.reputationCheckVerdict,
+    };
+
+    serializeUnknownProperties(this, serializable);
+    return serializable;
+  },
+};
+Object.setPrototypeOf(DownloadError.prototype, Error.prototype);
+
+/**
+ * Creates a new DownloadError object from its serializable representation.
+ *
+ * @param aSerializable
+ *        Serializable representation of a DownloadError object.
+ *
+ * @return The newly created DownloadError object.
+ */
+DownloadError.fromSerializable = function (aSerializable) {
+  let e = new DownloadError(aSerializable);
+  deserializeUnknownProperties(
+    e,
+    aSerializable,
+    property =>
+      property != "result" &&
+      property != "message" &&
+      property != "becauseSourceFailed" &&
+      property != "becauseTargetFailed" &&
+      property != "becauseBlocked" &&
+      property != "becauseBlockedByParentalControls" &&
+      property != "becauseBlockedByReputationCheck" &&
+      property != "reputationCheckVerdict"
+  );
+
+  return e;
+};
+
+/**
+ * Template for an object that actually transfers the data for the download.
+ */
+export var DownloadSaver = function () {};
+
+DownloadSaver.prototype = {
+  /**
+   * Download object for raising notifications and reading properties.
+   *
+   * If the tryToKeepPartialData property of the download object is false, the
+   * saver should never try to keep partially downloaded data if the download
+   * fails.
+   */
+  download: null,
+
+  /**
+   * Executes the download.
+   *
+   * @param aSetProgressBytesFn
+   *        This function may be called by the saver to report progress. It
+   *        takes three arguments: the first is the number of bytes transferred
+   *        until now, the second is the total number of bytes to be
+   *        transferred (or -1 if unknown), the third indicates whether the
+   *        partially downloaded data can be used when restarting the download
+   *        if it fails or is canceled.
+   * @param aSetPropertiesFn
+   *        This function may be called by the saver to report information
+   *        about new download properties discovered by the saver during the
+   *        download process. It takes an object where the keys represents
+   *        the names of the properties to set, and the value represents the
+   *        value to set.
+   *
+   * @return {Promise}
+   * @resolves When the download has finished successfully.
+   * @rejects JavaScript exception if the download failed.
+   */
+  async execute(aSetProgressBytesFn, aSetPropertiesFn) {
+    throw new Error("Not implemented.");
+  },
+
+  /**
+   * Cancels the download.
+   */
+  cancel: function DS_cancel() {
+    throw new Error("Not implemented.");
+  },
+
+  /**
+   * Removes any target file placeholder and any partial data kept as part of a
+   * canceled, failed, or temporarily blocked download.
+   *
+   * This method is never called until the promise returned by "execute" is
+   * either resolved or rejected, and the "execute" method is not called again
+   * until the promise returned by this method is resolved or rejected.
+   *
+   * @param canRemoveFinalTarget
+   *        True if can remove target file regardless of it being a placeholder.
+   * @return {Promise}
+   * @resolves When the operation has finished successfully.
+   * @rejects Never.
+   */
+  async removeData(canRemoveFinalTarget) {},
+
+  /**
+   * This can be called by the saver implementation when the download is already
+   * started, to add it to the browsing history.  This method has no effect if
+   * the download is private.
+   */
+  addToHistory() {
+    if (AppConstants.MOZ_PLACES) {
+      lazy.DownloadHistory.addDownloadToHistory(this.download).catch(
+        console.error
+      );
+    }
+  },
+
+  /**
+   * Returns a static representation of the current object state.
+   *
+   * @return A JavaScript object that can be serialized to JSON.
+   */
+  toSerializable() {
+    throw new Error("Not implemented.");
+  },
+
+  /**
+   * Returns the SHA-256 hash of the downloaded file, if it exists.
+   */
+  getSha256Hash() {
+    throw new Error("Not implemented.");
+  },
+
+  getSignatureInfo() {
+    throw new Error("Not implemented.");
+  },
+}; // DownloadSaver
+
+/**
+ * Creates a new DownloadSaver object from its serializable representation.
+ *
+ * @param aSerializable
+ *        Serializable representation of a DownloadSaver object.  If no initial
+ *        state information for the saver object is needed, can be a string
+ *        representing the class of the download operation, for example "copy".
+ *
+ * @return The newly created DownloadSaver object.
+ */
+DownloadSaver.fromSerializable = function (aSerializable) {
+  let serializable = isString(aSerializable)
+    ? { type: aSerializable }
+    : aSerializable;
+  let saver;
+  switch (serializable.type) {
+    case "copy":
+      saver = DownloadCopySaver.fromSerializable(serializable);
+      break;
+    case "legacy":
+      saver = DownloadLegacySaver.fromSerializable(serializable);
+      break;
+    default:
+      throw new Error("Unrecoginzed download saver type.");
+  }
+  return saver;
+};
+
+/**
+ * Saver object that simply copies the entire source file to the target.
+ */
+export var DownloadCopySaver = function () {};
+
+DownloadCopySaver.prototype = {
+  /**
+   * BackgroundFileSaver object currently handling the download.
+   */
+  _backgroundFileSaver: null,
+
+  /**
+   * Indicates whether the "cancel" method has been called.  This is used to
+   * prevent the request from starting in case the operation is canceled before
+   * the BackgroundFileSaver instance has been created.
+   */
+  _canceled: false,
+
+  /**
+   * Save the SHA-256 hash in raw bytes of the downloaded file. This is null
+   * unless BackgroundFileSaver has successfully completed saving the file.
+   */
+  _sha256Hash: null,
+
+  /**
+   * Save the signature info as an Array of Array of raw bytes of nsIX509Cert
+   * if the file is signed. This is empty if the file is unsigned, and null
+   * unless BackgroundFileSaver has successfully completed saving the file.
+   */
+  _signatureInfo: null,
+
+  /**
+   * Save the redirects chain as an nsIArray of nsIPrincipal.
+   */
+  _redirects: null,
+
+  /**
+   * True if the associated download has already been added to browsing history.
+   */
+  alreadyAddedToHistory: false,
+
+  /**
+   * String corresponding to the entityID property of the nsIResumableChannel
+   * used to execute the download, or null if the channel was not resumable or
+   * the saver was instructed not to keep partially downloaded data.
+   */
+  entityID: null,
+
+  /**
+   * Implements "DownloadSaver.execute".
+   */
+  async execute(aSetProgressBytesFn, aSetPropertiesFn) {
+    this._canceled = false;
+
+    let download = this.download;
+    let targetPath = download.target.path;
+    let partFilePath = download.target.partFilePath;
+    let keepPartialData = download.tryToKeepPartialData;
+
+    // Add the download to history the first time it is started in this
+    // session.  If the download is restarted in a different session, a new
+    // history visit will be added.  We do this just to avoid the complexity
+    // of serializing this state between sessions, since adding a new visit
+    // does not have any noticeable side effect.
+    if (!this.alreadyAddedToHistory) {
+      this.addToHistory();
+      this.alreadyAddedToHistory = true;
+    }
+
+    // To reduce the chance that other downloads reuse the same final target
+    // file name, we should create a placeholder as soon as possible, before
+    // starting the network request.  The placeholder is also required in case
+    // we are using a ".part" file instead of the final target while the
+    // download is in progress.
+    try {
+      // If the file already exists, don't delete its contents yet.
+      await IOUtils.writeUTF8(targetPath, "", { mode: "appendOrCreate" });
+    } catch (ex) {
+      if (!DOMException.isInstance(ex)) {
+        throw ex;
+      }
+      // Throw a DownloadError indicating that the operation failed because of
+      // the target file.  We cannot translate this into a specific result
+      // code, but we preserve the original message.
+      let error = new DownloadError({ message: ex.message });
+      error.becauseTargetFailed = true;
+      throw error;
+    }
+
+    let deferSaveComplete = Promise.withResolvers();
+
+    if (this._canceled) {
+      // Don't create the BackgroundFileSaver object if we have been
+      // canceled meanwhile.
+      throw new DownloadError({ message: "Saver canceled." });
+    }
+
+    // Create the object that will save the file in a background thread.
+    let backgroundFileSaver = new BackgroundFileSaverStreamListener();
+    backgroundFileSaver.QueryInterface(Ci.nsIStreamListener);
+
+    try {
+      // When the operation completes, reflect the status in the promise
+      // returned by this download execution function.
+      backgroundFileSaver.observer = {
+        onTargetChange() {},
+        onSaveComplete: (aSaver, aStatus) => {
+          // Send notifications now that we can restart if needed.
+          if (Components.isSuccessCode(aStatus)) {
+            // Save the hash before freeing backgroundFileSaver.
+            this._sha256Hash = aSaver.sha256Hash;
+            this._signatureInfo = aSaver.signatureInfo;
+            this._redirects = aSaver.redirects;
+            deferSaveComplete.resolve();
+          } else {
+            // Infer the origin of the error from the failure code, because
+            // BackgroundFileSaver does not provide more specific data.
+            let properties = { result: aStatus, inferCause: true };
+            deferSaveComplete.reject(new DownloadError(properties));
+          }
+          // Free the reference cycle, to release resources earlier.
+          backgroundFileSaver.observer = null;
+          this._backgroundFileSaver = null;
+        },
+      };
+
+      // If we have data that we can use to resume the download from where
+      // it stopped, try to use it.
+      let resumeAttempted = false;
+      let resumeFromBytes = 0;
+
+      const notificationCallbacks = {
+        QueryInterface: ChromeUtils.generateQI(["nsIInterfaceRequestor"]),
+        getInterface: ChromeUtils.generateQI(["nsIProgressEventSink"]),
+        onProgress: function DCSE_onProgress(
+          aRequest,
+          aProgress,
+          aProgressMax
+        ) {
+          let currentBytes = resumeFromBytes + aProgress;
+          let totalBytes =
+            aProgressMax == -1 ? -1 : resumeFromBytes + aProgressMax;
+          aSetProgressBytesFn(
+            currentBytes,
+            totalBytes,
+            aProgress > 0 && partFilePath && keepPartialData
+          );
+        },
+        onStatus() {},
+      };
+
+      const streamListener = {
+        onStartRequest: function (aRequest) {
+          backgroundFileSaver.onStartRequest(aRequest);
+
+          if (aRequest instanceof Ci.nsIHttpChannel) {
+            // Check if the request's response has been blocked by Windows
+            // Parental Controls with an HTTP 450 error code.
+            if (aRequest.responseStatus == 450) {
+              // Set a flag that can be retrieved later when handling the
+              // cancellation so that the proper error can be thrown.
+              this.download._blockedByParentalControls = true;
+              aRequest.cancel(Cr.NS_BINDING_ABORTED);
+              return;
+            }
+
+            // Check back with the initiator if we should allow a certain
+            // HTTP code. By default, we'll just save error pages too,
+            // however a consumer down the line, such as the WebExtensions
+            // downloads API might want to handle this differently.
+            if (
+              download.source.allowHttpStatus &&
+              !download.source.allowHttpStatus(
+                download,
+                aRequest.responseStatus
+              )
+            ) {
+              aRequest.cancel(Cr.NS_BINDING_ABORTED);
+              return;
+            }
+          }
+
+          if (aRequest instanceof Ci.nsIChannel) {
+            aSetPropertiesFn({ contentType: aRequest.contentType });
+
+            // Ensure we report the value of "Content-Length", if available,
+            // even if the download doesn't generate any progress events
+            // later.
+            if (aRequest.contentLength >= 0) {
+              aSetProgressBytesFn(0, aRequest.contentLength);
+            }
+          }
+
+          // If the URL we are downloading from includes a file extension
+          // that matches the "Content-Encoding" header, for example ".gz"
+          // with a "gzip" encoding, we should save the file in its encoded
+          // form.  In all other cases, we decode the body while saving.
+          if (
+            aRequest instanceof Ci.nsIEncodedChannel &&
+            aRequest.contentEncodings
+          ) {
+            let uri = aRequest.URI;
+            if (uri instanceof Ci.nsIURL && uri.fileExtension) {
+              // Only the first, outermost encoding is considered.
+              let encoding = aRequest.contentEncodings.getNext();
+              if (encoding) {
+                aRequest.applyConversion =
+                  lazy.gExternalHelperAppService.applyDecodingForExtension(
+                    uri.fileExtension,
+                    encoding
+                  );
+              }
+            }
+          }
+
+          if (keepPartialData) {
+            // If the source is not resumable, don't keep partial data even
+            // if we were asked to try and do it.
+            if (aRequest instanceof Ci.nsIResumableChannel) {
+              try {
+                // If reading the ID succeeds, the source is resumable.
+                this.entityID = aRequest.entityID;
+              } catch (ex) {
+                if (
+                  !(ex instanceof Components.Exception) ||
+                  ex.result != Cr.NS_ERROR_NOT_RESUMABLE
+                ) {
+                  throw ex;
+                }
+                keepPartialData = false;
+              }
+            } else {
+              keepPartialData = false;
+            }
+          }
+
+          // Enable hashing and signature verification before setting the
+          // target.
+          backgroundFileSaver.enableSha256();
+          backgroundFileSaver.enableSignatureInfo();
+          if (partFilePath) {
+            // If we actually resumed a request, append to the partial data.
+            if (resumeAttempted) {
+              // TODO: Handle Cr.NS_ERROR_ENTITY_CHANGED
+              backgroundFileSaver.enableAppend();
+            }
+
+            // Use a part file, determining if we should keep it on failure.
+            backgroundFileSaver.setTarget(
+              new lazy.FileUtils.File(partFilePath),
+              keepPartialData
+            );
+          } else {
+            // Set the final target file, and delete it on failure.
+            backgroundFileSaver.setTarget(
+              new lazy.FileUtils.File(targetPath),
+              false
+            );
+          }
+        }.bind(this),
+
+        onStopRequest(aRequest, aStatusCode) {
+          try {
+            backgroundFileSaver.onStopRequest(aRequest, aStatusCode);
+          } finally {
+            // If the data transfer completed successfully, indicate to the
+            // background file saver that the operation can finish.  If the
+            // data transfer failed, the saver has been already stopped.
+            if (Components.isSuccessCode(aStatusCode)) {
+              backgroundFileSaver.finish(Cr.NS_OK);
+            }
+          }
+        },
+
+        onDataAvailable: (aRequest, aInputStream, aOffset, aCount) => {
+          // Check if the download have been canceled in the mean time,
+          // and close the channel and return earlier, BackgroundFileSaver
+          // methods shouldn't be called anymore after `finish` was called
+          // on download cancellation.
+          if (this._canceled) {
+            aRequest.cancel(Cr.NS_BINDING_ABORTED);
+            return;
+          }
+          backgroundFileSaver.onDataAvailable(
+            aRequest,
+            aInputStream,
+            aOffset,
+            aCount
+          );
+        },
+      };
+
+      // Wrap the channel creation, to prevent the listener code from
+      // accidentally using the wrong channel.
+      // The channel that is created here is not necessarily the same channel
+      // that will eventually perform the actual download.
+      // When a HTTP redirect happens, the http backend will create a new
+      // channel, this initial channel will be abandoned, and its properties
+      // will either return incorrect data, or worse, will throw exceptions
+      // upon access.
+      const open = async () => {
+        // Create a channel from the source, and listen to progress
+        // notifications.
+        let channel;
+        if (download.source.loadingPrincipal) {
+          channel = lazy.NetUtil.newChannel({
+            uri: download.source.url,
+            contentPolicyType: Ci.nsIContentPolicy.TYPE_SAVEAS_DOWNLOAD,
+            loadingPrincipal: download.source.loadingPrincipal,
+            // triggeringPrincipal must be the system principal to prevent the
+            // request from being mistaken as a third-party request.
+            triggeringPrincipal:
+              Services.scriptSecurityManager.getSystemPrincipal(),
+            securityFlags:
+              Ci.nsILoadInfo.SEC_ALLOW_CROSS_ORIGIN_SEC_CONTEXT_IS_NULL,
+          });
+        } else {
+          channel = lazy.NetUtil.newChannel({
+            uri: download.source.url,
+            contentPolicyType: Ci.nsIContentPolicy.TYPE_SAVEAS_DOWNLOAD,
+            loadUsingSystemPrincipal: true,
+          });
+        }
+        if (channel instanceof Ci.nsIPrivateBrowsingChannel) {
+          channel.setPrivate(download.source.isPrivate);
+        }
+        if (
+          channel instanceof Ci.nsIHttpChannel &&
+          download.source.referrerInfo
+        ) {
+          channel.referrerInfo = download.source.referrerInfo;
+          // Stored computed referrerInfo;
+          download.source.referrerInfo = channel.referrerInfo;
+        }
+        if (
+          channel instanceof Ci.nsIHttpChannel &&
+          download.source.cookieJarSettings
+        ) {
+          channel.loadInfo.cookieJarSettings =
+            download.source.cookieJarSettings;
+        }
+        if (
+          channel instanceof Ci.nsIHttpChannel &&
+          download.source.authHeader
+        ) {
+          try {
+            channel.setRequestHeader(
+              "Authorization",
+              download.source.authHeader,
+              true
+            );
+          } catch (e) {}
+        }
+
+        if (download.source.userContextId) {
+          // Getters and setters only exist on originAttributes,
+          // so it has to be cloned, changed, and re-set
+          channel.loadInfo.originAttributes = {
+            ...channel.loadInfo.originAttributes,
+            userContextId: download.source.userContextId,
+          };
+        }
+
+        // This makes the channel be corretly throttled during page loads
+        // and also prevents its caching.
+        if (channel instanceof Ci.nsIHttpChannelInternal) {
+          channel.channelIsForDownload = true;
+
+          // Include cookies even if cookieBehavior is BEHAVIOR_REJECT_FOREIGN.
+          channel.forceAllowThirdPartyCookie = true;
+        }
+
+        if (
+          channel instanceof Ci.nsIResumableChannel &&
+          this.entityID &&
+          partFilePath &&
+          keepPartialData
+        ) {
+          try {
+            let stat = await IOUtils.stat(partFilePath);
+            channel.resumeAt(stat.size, this.entityID);
+            resumeAttempted = true;
+            resumeFromBytes = stat.size;
+          } catch (ex) {
+            if (ex.name != "NotFoundError") {
+              throw ex;
+            }
+          }
+        }
+
+        channel.notificationCallbacks = notificationCallbacks;
+
+        // If the callback was set, handle it now before opening the channel.
+        if (download.source.adjustChannel) {
+          await download.source.adjustChannel(channel);
+        }
+        channel.asyncOpen(streamListener);
+      };
+
+      // Kick off the download, creating and opening the channel.
+      await open();
+
+      // We should check if we have been canceled in the meantime, after
+      // all the previous asynchronous operations have been executed and
+      // just before we set the _backgroundFileSaver property.
+      if (this._canceled) {
+        throw new DownloadError({ message: "Saver canceled." });
+      }
+
+      // If the operation succeeded, store the object to allow cancellation.
+      this._backgroundFileSaver = backgroundFileSaver;
+    } catch (ex) {
+      // In case an error occurs while setting up the chain of objects for
+      // the download, ensure that we release the resources of the saver.
+      backgroundFileSaver.finish(Cr.NS_ERROR_FAILURE);
+      // Since we're not going to handle deferSaveComplete.promise below,
+      // we need to make sure that the rejection is handled.
+      deferSaveComplete.promise.catch(() => {});
+      throw ex;
+    }
+
+    // We will wait on this promise in case no error occurred while setting
+    // up the chain of objects for the download.
+    await deferSaveComplete.promise;
+
+    await this._checkReputationAndMove(aSetPropertiesFn);
+  },
+
+  /**
+   * Perform the reputation check and cleanup the downloaded data if required.
+   * If the download passes the reputation check and is using a part file we
+   * will move it to the target path since reputation checking is the final
+   * step in the saver.
+   *
+   * @param aSetPropertiesFn
+   *        Function provided to the "execute" method.
+   *
+   * @return {Promise}
+   * @resolves When the reputation check and cleanup is complete.
+   * @rejects DownloadError if the download should be blocked.
+   */
+  async _checkReputationAndMove(aSetPropertiesFn) {
+    let download = this.download;
+    let targetPath = this.download.target.path;
+    let partFilePath = this.download.target.partFilePath;
+
+    let { shouldBlock, verdict } =
+      await lazy.DownloadIntegration.shouldBlockForReputationCheck(download);
+    if (shouldBlock) {
+      Services.telemetry
+        .getKeyedHistogramById("DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD")
+        .add(verdict, 0);
+
+      let newProperties = { progress: 100, hasPartialData: false };
+
+      // We will remove the potentially dangerous file if instructed by
+      // DownloadIntegration. We will always remove the file when the
+      // download did not use a partial file path, meaning it
+      // currently has its final filename.
+      if (!lazy.DownloadIntegration.shouldKeepBlockedData() || !partFilePath) {
+        await this.removeData(!partFilePath);
+      } else {
+        newProperties.hasBlockedData = true;
+      }
+
+      aSetPropertiesFn(newProperties);
+
+      throw new DownloadError({
+        becauseBlockedByReputationCheck: true,
+        reputationCheckVerdict: verdict,
+      });
+    }
+
+    if (partFilePath) {
+      try {
+        await IOUtils.move(partFilePath, targetPath);
+      } catch (e) {
+        if (e.name === "NotAllowedError") {
+          // In case we cannot write to the target file
+          // retry with a new unique name
+          let uniquePath = lazy.DownloadPaths.createNiceUniqueFile(
+            new lazy.FileUtils.File(targetPath)
+          ).path;
+          await IOUtils.move(partFilePath, uniquePath);
+          this.download.target.path = uniquePath;
+        } else {
+          throw e;
+        }
+      }
+    }
+  },
+
+  /**
+   * Implements "DownloadSaver.cancel".
+   */
+  cancel: function DCS_cancel() {
+    this._canceled = true;
+    if (this._backgroundFileSaver) {
+      this._backgroundFileSaver.finish(Cr.NS_ERROR_FAILURE);
+      this._backgroundFileSaver = null;
+    }
+  },
+
+  /**
+   * Implements "DownloadSaver.removeData".
+   */
+  async removeData(canRemoveFinalTarget = false) {
+    // Defined inline so removeData can be shared with DownloadLegacySaver.
+    async function _tryToRemoveFile(path) {
+      try {
+        await IOUtils.remove(path);
+      } catch (ex) {
+        // On Windows we may get an access denied error instead of a no such
+        // file error if the file existed before, and was recently deleted. This
+        // is likely to happen when the component that executed the download has
+        // just deleted the target file itself.
+        if (!["NotFoundError", "NotAllowedError"].includes(ex.name)) {
+          console.error(ex);
+        }
+      }
+    }
+
+    if (this.download.target.partFilePath) {
+      await _tryToRemoveFile(this.download.target.partFilePath);
+    }
+
+    if (this.download.target.path) {
+      if (
+        canRemoveFinalTarget ||
+        (await isPlaceholder(this.download.target.path))
+      ) {
+        await _tryToRemoveFile(this.download.target.path);
+      }
+      this.download.target.exists = false;
+      this.download.target.size = 0;
+    }
+  },
+
+  /**
+   * Implements "DownloadSaver.toSerializable".
+   */
+  toSerializable() {
+    // Simplify the representation if we don't have other details.
+    if (!this.entityID && !this._unknownProperties) {
+      return "copy";
+    }
+
+    let serializable = { type: "copy", entityID: this.entityID };
+    serializeUnknownProperties(this, serializable);
+    return serializable;
+  },
+
+  /**
+   * Implements "DownloadSaver.getSha256Hash"
+   */
+  getSha256Hash() {
+    return this._sha256Hash;
+  },
+
+  /*
+   * Implements DownloadSaver.getSignatureInfo.
+   */
+  getSignatureInfo() {
+    return this._signatureInfo;
+  },
+
+  /*
+   * Implements DownloadSaver.getRedirects.
+   */
+  getRedirects() {
+    return this._redirects;
+  },
+};
+Object.setPrototypeOf(DownloadCopySaver.prototype, DownloadSaver.prototype);
+
+/**
+ * Creates a new DownloadCopySaver object, with its initial state derived from
+ * its serializable representation.
+ *
+ * @param aSerializable
+ *        Serializable representation of a DownloadCopySaver object.
+ *
+ * @return The newly created DownloadCopySaver object.
+ */
+DownloadCopySaver.fromSerializable = function (aSerializable) {
+  let saver = new DownloadCopySaver();
+  if ("entityID" in aSerializable) {
+    saver.entityID = aSerializable.entityID;
+  }
+
+  deserializeUnknownProperties(
+    saver,
+    aSerializable,
+    property => property != "entityID" && property != "type"
+  );
+
+  return saver;
+};
+
+/**
+ * Saver object that integrates with the legacy nsITransfer interface.
+ *
+ * For more background on the process, see the DownloadLegacyTransfer object.
+ */
+export var DownloadLegacySaver = function () {
+  this.deferExecuted = Promise.withResolvers();
+  this.deferCanceled = Promise.withResolvers();
+};
+
+DownloadLegacySaver.prototype = {
+  /**
+   * Save the SHA-256 hash in raw bytes of the downloaded file. This may be
+   * null when nsExternalHelperAppService (and thus BackgroundFileSaver) is not
+   * invoked.
+   */
+  _sha256Hash: null,
+
+  /**
+   * Save the signature info as an Array of Array of raw bytes of nsIX509Cert
+   * if the file is signed. This is empty if the file is unsigned, and null
+   * unless BackgroundFileSaver has successfully completed saving the file.
+   */
+  _signatureInfo: null,
+
+  /**
+   * Save the redirect chain as an nsIArray of nsIPrincipal.
+   */
+  _redirects: null,
+
+  /**
+   * nsIRequest object associated to the status and progress updates we
+   * received.  This object is null before we receive the first status and
+   * progress update, and is also reset to null when the download is stopped.
+   */
+  request: null,
+
+  /**
+   * This deferred object contains a promise that is resolved as soon as this
+   * download finishes successfully, and is rejected in case the download is
+   * canceled or receives a failure notification through nsITransfer.
+   */
+  deferExecuted: null,
+
+  /**
+   * This deferred object contains a promise that is resolved if the download
+   * receives a cancellation request through the "cancel" method, and is never
+   * rejected.  The nsITransfer implementation will register a handler that
+   * actually causes the download cancellation.
+   */
+  deferCanceled: null,
+
+  /**
+   * This is populated with the value of the aSetProgressBytesFn argument of the
+   * "execute" method, and is null before the method is called.
+   */
+  setProgressBytesFn: null,
+
+  /**
+   * Called by the nsITransfer implementation while the download progresses.
+   *
+   * @param aCurrentBytes
+   *        Number of bytes transferred until now.
+   * @param aTotalBytes
+   *        Total number of bytes to be transferred, or -1 if unknown.
+   */
+  onProgressBytes: function DLS_onProgressBytes(aCurrentBytes, aTotalBytes) {
+    this.progressWasNotified = true;
+
+    // Ignore progress notifications until we are ready to process them.
+    if (!this.setProgressBytesFn) {
+      // Keep the data from the last progress notification that was received.
+      this.currentBytes = aCurrentBytes;
+      this.totalBytes = aTotalBytes;
+      return;
+    }
+
+    let hasPartFile = !!this.download.target.partFilePath;
+
+    this.setProgressBytesFn(
+      aCurrentBytes,
+      aTotalBytes,
+      aCurrentBytes > 0 && hasPartFile
+    );
+  },
+
+  /**
+   * Whether the onProgressBytes function has been called at least once.
+   */
+  progressWasNotified: false,
+
+  /**
+   * Called by the nsITransfer implementation when the request has started.
+   *
+   * @param aRequest
+   *        nsIRequest associated to the status update.
+   */
+  onTransferStarted(aRequest) {
+    // Store a reference to the request, used in some cases when handling
+    // completion, and also checked during the download by unit tests.
+    this.request = aRequest;
+
+    // Store the entity ID to use for resuming if required.
+    if (
+      this.download.tryToKeepPartialData &&
+      aRequest instanceof Ci.nsIResumableChannel
+    ) {
+      try {
+        // If reading the ID succeeds, the source is resumable.
+        this.entityID = aRequest.entityID;
+      } catch (ex) {
+        if (
+          !(ex instanceof Components.Exception) ||
+          ex.result != Cr.NS_ERROR_NOT_RESUMABLE
+        ) {
+          throw ex;
+        }
+      }
+    }
+
+    // For legacy downloads, we must update the referrerInfo at this time.
+    if (aRequest instanceof Ci.nsIHttpChannel) {
+      this.download.source.referrerInfo = aRequest.referrerInfo;
+    }
+
+    // Don't open the download panel when the user initiated to save a
+    // link or document.
+    if (
+      aRequest instanceof Ci.nsIChannel &&
+      aRequest.loadInfo.isUserTriggeredSave
+    ) {
+      this.download.openDownloadsListOnStart = false;
+    }
+
+    this.addToHistory();
+  },
+
+  /**
+   * Called by the nsITransfer implementation when the request has finished.
+   *
+   * @param aStatus
+   *        Status code received by the nsITransfer implementation.
+   */
+  onTransferFinished: function DLS_onTransferFinished(aStatus) {
+    if (Components.isSuccessCode(aStatus)) {
+      this.deferExecuted.resolve();
+    } else {
+      // Infer the origin of the error from the failure code, because more
+      // specific data is not available through the nsITransfer implementation.
+      let properties = { result: aStatus, inferCause: true };
+      this.deferExecuted.reject(new DownloadError(properties));
+    }
+  },
+
+  /**
+   * When the first execution of the download finished, it can be restarted by
+   * using a DownloadCopySaver object instead of the original legacy component
+   * that executed the download.
+   */
+  firstExecutionFinished: false,
+
+  /**
+   * In case the download is restarted after the first execution finished, this
+   * property contains a reference to the DownloadCopySaver that is executing
+   * the new download attempt.
+   */
+  copySaver: null,
+
+  /**
+   * String corresponding to the entityID property of the nsIResumableChannel
+   * used to execute the download, or null if the channel was not resumable or
+   * the saver was instructed not to keep partially downloaded data.
+   */
+  entityID: null,
+
+  /**
+   * Implements "DownloadSaver.execute".
+   */
+  async execute(aSetProgressBytesFn, aSetPropertiesFn) {
+    // Check if this is not the first execution of the download.  The Download
+    // object guarantees that this function is not re-entered during execution.
+    if (this.firstExecutionFinished) {
+      if (!this.copySaver) {
+        this.copySaver = new DownloadCopySaver();
+        this.copySaver.download = this.download;
+        this.copySaver.entityID = this.entityID;
+        this.copySaver.alreadyAddedToHistory = true;
+      }
+      await this.copySaver.execute.apply(this.copySaver, arguments);
+      return;
+    }
+
+    this.setProgressBytesFn = aSetProgressBytesFn;
+    if (this.progressWasNotified) {
+      this.onProgressBytes(this.currentBytes, this.totalBytes);
+    }
+
+    try {
+      // Wait for the component that executes the download to finish.
+      await this.deferExecuted.promise;
+
+      // At this point, the "request" property has been populated.  Ensure we
+      // report the value of "Content-Length", if available, even if the
+      // download didn't generate any progress events.
+      if (
+        !this.progressWasNotified &&
+        this.request instanceof Ci.nsIChannel &&
+        this.request.contentLength >= 0
+      ) {
+        aSetProgressBytesFn(0, this.request.contentLength);
+      }
+
+      // If the component executing the download provides the path of a
+      // ".part" file, it means that it expects the listener to move the file
+      // to its final target path when the download succeeds.  In this case,
+      // an empty ".part" file is created even if no data was received from
+      // the source.
+      //
+      // When no ".part" file path is provided the download implementation may
+      // not have created the target file (if no data was received from the
+      // source).  In this case, ensure that an empty file is created as
+      // expected.
+      if (!this.download.target.partFilePath) {
+        try {
+          // This atomic operation is more efficient than an existence check.
+          await IOUtils.writeUTF8(this.download.target.path, "", {
+            mode: "create",
+          });
+        } catch (ex) {
+          if (
+            !DOMException.isInstance(ex) ||
+            ex.name !== "NoModificationAllowedError"
+          ) {
+            throw ex;
+          }
+        }
+      }
+
+      await this._checkReputationAndMove(aSetPropertiesFn);
+    } catch (ex) {
+      // In case the operation failed, ensure we stop downloading data.  Since
+      // we never re-enter this function, deferCanceled is always available.
+      this.deferCanceled.resolve();
+      throw ex;
+    } finally {
+      // We don't need the reference to the request anymore.  We must also set
+      // deferCanceled to null in order to free any indirect references it
+      // may hold to the request.
+      this.request = null;
+      this.deferCanceled = null;
+      // Allow the download to restart through a DownloadCopySaver.
+      this.firstExecutionFinished = true;
+    }
+  },
+
+  _checkReputationAndMove() {
+    return DownloadCopySaver.prototype._checkReputationAndMove.apply(
+      this,
+      arguments
+    );
+  },
+
+  /**
+   * Implements "DownloadSaver.cancel".
+   */
+  cancel: function DLS_cancel() {
+    // We may be using a DownloadCopySaver to handle resuming.
+    if (this.copySaver) {
+      return this.copySaver.cancel.apply(this.copySaver, arguments);
+    }
+
+    // If the download hasn't stopped already, resolve deferCanceled so that the
+    // operation is canceled as soon as a cancellation handler is registered.
+    // Note that the handler might not have been registered yet.
+    if (this.deferCanceled) {
+      this.deferCanceled.resolve();
+    }
+  },
+
+  /**
+   * Implements "DownloadSaver.removeData".
+   */
+  removeData(canRemoveFinalTarget) {
+    // DownloadCopySaver and DownloadLegacySaver use the same logic for removing
+    // partially downloaded data, though this implementation isn't shared by
+    // other saver types, thus it isn't found on their shared prototype.
+    return DownloadCopySaver.prototype.removeData.call(
+      this,
+      canRemoveFinalTarget
+    );
+  },
+
+  /**
+   * Implements "DownloadSaver.toSerializable".
+   */
+  toSerializable() {
+    // This object depends on legacy components that are created externally,
+    // thus it cannot be rebuilt during deserialization.  To support resuming
+    // across different browser sessions, this object is transformed into a
+    // DownloadCopySaver for the purpose of serialization.
+    return DownloadCopySaver.prototype.toSerializable.call(this);
+  },
+
+  /**
+   * Implements "DownloadSaver.getSha256Hash".
+   */
+  getSha256Hash() {
+    if (this.copySaver) {
+      return this.copySaver.getSha256Hash();
+    }
+    return this._sha256Hash;
+  },
+
+  /**
+   * Called by the nsITransfer implementation when the hash is available.
+   */
+  setSha256Hash(hash) {
+    this._sha256Hash = hash;
+  },
+
+  /**
+   * Implements "DownloadSaver.getSignatureInfo".
+   */
+  getSignatureInfo() {
+    if (this.copySaver) {
+      return this.copySaver.getSignatureInfo();
+    }
+    return this._signatureInfo;
+  },
+
+  /**
+   * Called by the nsITransfer implementation when the hash is available.
+   */
+  setSignatureInfo(signatureInfo) {
+    this._signatureInfo = signatureInfo;
+  },
+
+  /**
+   * Implements "DownloadSaver.getRedirects".
+   */
+  getRedirects() {
+    if (this.copySaver) {
+      return this.copySaver.getRedirects();
+    }
+    return this._redirects;
+  },
+
+  /**
+   * Called by the nsITransfer implementation when the redirect chain is
+   * available.
+   */
+  setRedirects(redirects) {
+    this._redirects = redirects;
+  },
+};
+Object.setPrototypeOf(DownloadLegacySaver.prototype, DownloadSaver.prototype);
+
+/**
+ * Returns a new DownloadLegacySaver object.  This saver type has a
+ * deserializable form only when creating a new object in memory, because it
+ * cannot be serialized to disk.
+ */
+DownloadLegacySaver.fromSerializable = function () {
+  return new DownloadLegacySaver();
+};
diff --git a/toolkit/components/downloads/DownloadHistory.sys.mjs b/toolkit/components/downloads/DownloadHistory.sys.mjs
new file mode 100644
index 0000000000..0077601d84
--- /dev/null
+++ b/toolkit/components/downloads/DownloadHistory.sys.mjs
@@ -0,0 +1,863 @@
+/* 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/. */
+
+/**
+ * Provides access to downloads from previous sessions on platforms that store
+ * them in a different location than session downloads.
+ *
+ * This module works with objects that are compatible with Download, while using
+ * the Places interfaces internally. Some of the Places objects may also be
+ * exposed to allow the consumers to integrate with history view commands.
+ */
+
+import { DownloadList } from "resource://gre/modules/DownloadList.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Downloads: "resource://gre/modules/Downloads.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+});
+
+// Places query used to retrieve all history downloads for the related list.
+const HISTORY_PLACES_QUERY = `place:transition=${Ci.nsINavHistoryService.TRANSITION_DOWNLOAD}&sort=${Ci.nsINavHistoryQueryOptions.SORT_BY_DATE_DESCENDING}`;
+const DESTINATIONFILEURI_ANNO = "downloads/destinationFileURI";
+const METADATA_ANNO = "downloads/metaData";
+
+const METADATA_STATE_FINISHED = 1;
+const METADATA_STATE_FAILED = 2;
+const METADATA_STATE_CANCELED = 3;
+const METADATA_STATE_PAUSED = 4;
+const METADATA_STATE_BLOCKED_PARENTAL = 6;
+const METADATA_STATE_DIRTY = 8;
+
+/**
+ * Provides methods to retrieve downloads from previous sessions and store
+ * downloads for future sessions.
+ */
+export let DownloadHistory = {
+  /**
+   * Retrieves the main DownloadHistoryList object which provides a unified view
+   * on downloads from both previous browsing sessions and this session.
+   *
+   * @param type
+   *        Determines which type of downloads from this session should be
+   *        included in the list. This is Downloads.PUBLIC by default, but can
+   *        also be Downloads.PRIVATE or Downloads.ALL.
+   * @param maxHistoryResults
+   *        Optional number that limits the amount of results the history query
+   *        may return.
+   *
+   * @return {Promise}
+   * @resolves The requested DownloadHistoryList object.
+   * @rejects JavaScript exception.
+   */
+  async getList({ type = lazy.Downloads.PUBLIC, maxHistoryResults } = {}) {
+    await DownloadCache.ensureInitialized();
+
+    let key = `${type}|${maxHistoryResults ? maxHistoryResults : -1}`;
+    if (!this._listPromises[key]) {
+      this._listPromises[key] = lazy.Downloads.getList(type).then(list => {
+        // When the amount of history downloads is capped, we request the list in
+        // descending order, to make sure that the list can apply the limit.
+        let query =
+          HISTORY_PLACES_QUERY +
+          (maxHistoryResults ? `&maxResults=${maxHistoryResults}` : "");
+
+        return new DownloadHistoryList(list, query);
+      });
+    }
+
+    return this._listPromises[key];
+  },
+
+  /**
+   * This object is populated with one key for each type of download list that
+   * can be returned by the getList method. The values are promises that resolve
+   * to DownloadHistoryList objects.
+   */
+  _listPromises: {},
+
+  async addDownloadToHistory(download) {
+    if (
+      download.source.isPrivate ||
+      !lazy.PlacesUtils.history.canAddURI(
+        lazy.PlacesUtils.toURI(download.source.url)
+      )
+    ) {
+      return;
+    }
+
+    await DownloadCache.addDownload(download);
+
+    await this._updateHistoryListData(download.source.url);
+  },
+
+  /**
+   * Stores new detailed metadata for the given download in history. This is
+   * normally called after a download finishes, fails, or is canceled.
+   *
+   * Failed or canceled downloads with partial data are not stored as paused,
+   * because the information from the session download is required for resuming.
+   *
+   * @param download
+   *        Download object whose metadata should be updated. If the object
+   *        represents a private download, the call has no effect.
+   */
+  async updateMetaData(download) {
+    if (
+      download.source.isPrivate ||
+      !download.stopped ||
+      !lazy.PlacesUtils.history.canAddURI(
+        lazy.PlacesUtils.toURI(download.source.url)
+      )
+    ) {
+      return;
+    }
+
+    let state = METADATA_STATE_CANCELED;
+    if (download.succeeded) {
+      state = METADATA_STATE_FINISHED;
+    } else if (download.error) {
+      if (download.error.becauseBlockedByParentalControls) {
+        state = METADATA_STATE_BLOCKED_PARENTAL;
+      } else if (download.error.becauseBlockedByReputationCheck) {
+        state = METADATA_STATE_DIRTY;
+      } else {
+        state = METADATA_STATE_FAILED;
+      }
+    }
+
+    let metaData = {
+      state,
+      deleted: download.deleted,
+      endTime: download.endTime,
+    };
+    if (download.succeeded) {
+      metaData.fileSize = download.target.size;
+    }
+
+    // The verdict may still be present even if the download succeeded.
+    if (download.error && download.error.reputationCheckVerdict) {
+      metaData.reputationCheckVerdict = download.error.reputationCheckVerdict;
+    }
+
+    // This should be executed before any async parts, to ensure the cache is
+    // updated before any notifications are activated.
+    await DownloadCache.setMetadata(download.source.url, metaData);
+
+    await this._updateHistoryListData(download.source.url);
+  },
+
+  async _updateHistoryListData(sourceUrl) {
+    for (let key of Object.getOwnPropertyNames(this._listPromises)) {
+      let downloadHistoryList = await this._listPromises[key];
+      downloadHistoryList.updateForMetaDataChange(
+        sourceUrl,
+        DownloadCache.get(sourceUrl)
+      );
+    }
+  },
+};
+
+/**
+ * This cache exists:
+ * - in order to optimize the load of DownloadsHistoryList, when Places
+ *   annotations for history downloads must be read. In fact, annotations are
+ *   stored in a single table, and reading all of them at once is much more
+ *   efficient than an individual query.
+ * - to avoid needing to do asynchronous reading of the database during download
+ *   list updates, which are designed to be synchronous (to improve UI
+ *   responsiveness).
+ *
+ * The cache is initialized the first time DownloadHistory.getList is called, or
+ * when data is added.
+ */
+let DownloadCache = {
+  _data: new Map(),
+  _initializePromise: null,
+
+  /**
+   * Initializes the cache, loading the data from the places database.
+   *
+   * @return {Promise} Returns a promise that is resolved once the
+   *                   initialization is complete.
+   */
+  ensureInitialized() {
+    if (this._initializePromise) {
+      return this._initializePromise;
+    }
+    this._initializePromise = (async () => {
+      const placesObserver = new PlacesWeakCallbackWrapper(
+        this.handlePlacesEvents.bind(this)
+      );
+      PlacesObservers.addListener(
+        ["history-cleared", "page-removed"],
+        placesObserver
+      );
+
+      let pageAnnos = await lazy.PlacesUtils.history.fetchAnnotatedPages([
+        METADATA_ANNO,
+        DESTINATIONFILEURI_ANNO,
+      ]);
+
+      let metaDataPages = pageAnnos.get(METADATA_ANNO);
+      if (metaDataPages) {
+        for (let { uri, content } of metaDataPages) {
+          try {
+            this._data.set(uri.href, JSON.parse(content));
+          } catch (ex) {
+            // Do nothing - JSON.parse could throw.
+          }
+        }
+      }
+
+      let destinationFilePages = pageAnnos.get(DESTINATIONFILEURI_ANNO);
+      if (destinationFilePages) {
+        for (let { uri, content } of destinationFilePages) {
+          let newData = this.get(uri.href);
+          newData.targetFileSpec = content;
+          this._data.set(uri.href, newData);
+        }
+      }
+    })();
+
+    return this._initializePromise;
+  },
+
+  /**
+   * This returns an object containing the meta data for the supplied URL.
+   *
+   * @param {String} url The url to get the meta data for.
+   * @return {Object|null} Returns an empty object if there is no meta data found, or
+   *                       an object containing the meta data. The meta data
+   *                       will look like:
+   *
+   * { targetFileSpec, state, deleted, endTime, fileSize, ... }
+   *
+   * The targetFileSpec property is the value of "downloads/destinationFileURI",
+   * while the other properties are taken from "downloads/metaData". Any of the
+   * properties may be missing from the object.
+   */
+  get(url) {
+    return this._data.get(url) || {};
+  },
+
+  /**
+   * Adds a download to the cache and the places database.
+   *
+   * @param {Download} download The download to add to the database and cache.
+   */
+  async addDownload(download) {
+    await this.ensureInitialized();
+
+    let targetFile = new lazy.FileUtils.File(download.target.path);
+    let targetUri = Services.io.newFileURI(targetFile);
+
+    // This should be executed before any async parts, to ensure the cache is
+    // updated before any notifications are activated.
+    // Note: this intentionally overwrites any metadata as this is
+    // the start of a new download.
+    this._data.set(download.source.url, { targetFileSpec: targetUri.spec });
+
+    let originalPageInfo = await lazy.PlacesUtils.history.fetch(
+      download.source.url
+    );
+
+    let pageInfo = await lazy.PlacesUtils.history.insert({
+      url: download.source.url,
+      // In case we are downloading a file that does not correspond to a web
+      // page for which the title is present, we populate the otherwise empty
+      // history title with the name of the destination file, to allow it to be
+      // visible and searchable in history results.
+      title:
+        (originalPageInfo && originalPageInfo.title) || targetFile.leafName,
+      visits: [
+        {
+          // The start time is always available when we reach this point.
+          date: download.startTime,
+          transition: lazy.PlacesUtils.history.TRANSITIONS.DOWNLOAD,
+          referrer: download.source.referrerInfo
+            ? download.source.referrerInfo.originalReferrer
+            : null,
+        },
+      ],
+    });
+
+    await lazy.PlacesUtils.history.update({
+      annotations: new Map([["downloads/destinationFileURI", targetUri.spec]]),
+      // XXX Bug 1479445: We shouldn't have to supply both guid and url here,
+      // but currently we do.
+      guid: pageInfo.guid,
+      url: pageInfo.url,
+    });
+  },
+
+  /**
+   * Sets the metadata for a given url. If the cache already contains meta data
+   * for the given url, it will be overwritten (note: the targetFileSpec will be
+   * maintained).
+   *
+   * @param {String} url The url to set the meta data for.
+   * @param {Object} metadata The new metaData to save in the cache.
+   */
+  async setMetadata(url, metadata) {
+    await this.ensureInitialized();
+
+    // This should be executed before any async parts, to ensure the cache is
+    // updated before any notifications are activated.
+    let existingData = this.get(url);
+    let newData = { ...metadata };
+    if ("targetFileSpec" in existingData) {
+      newData.targetFileSpec = existingData.targetFileSpec;
+    }
+    this._data.set(url, newData);
+
+    try {
+      await lazy.PlacesUtils.history.update({
+        annotations: new Map([[METADATA_ANNO, JSON.stringify(metadata)]]),
+        url,
+      });
+    } catch (ex) {
+      console.error(ex);
+    }
+  },
+
+  QueryInterface: ChromeUtils.generateQI(["nsISupportsWeakReference"]),
+
+  handlePlacesEvents(events) {
+    for (const event of events) {
+      switch (event.type) {
+        case "history-cleared": {
+          this._data.clear();
+          break;
+        }
+        case "page-removed": {
+          if (event.isRemovedFromStore) {
+            this._data.delete(event.url);
+          }
+          break;
+        }
+      }
+    }
+  },
+};
+
+/**
+ * Represents a download from the browser history. This object implements part
+ * of the interface of the Download object.
+ *
+ * While Download objects are shared between the public DownloadList and all the
+ * DownloadHistoryList instances, multiple HistoryDownload objects referring to
+ * the same item can be created for different DownloadHistoryList instances.
+ *
+ * @param placesNode
+ *        The Places node from which the history download should be initialized.
+ */
+class HistoryDownload {
+  constructor(placesNode) {
+    this.placesNode = placesNode;
+
+    // History downloads should get the referrer from Places (bug 829201).
+    this.source = {
+      url: placesNode.uri,
+      isPrivate: false,
+    };
+    this.target = {
+      path: undefined,
+      exists: false,
+      size: undefined,
+    };
+
+    // In case this download cannot obtain its end time from the Places metadata,
+    // use the time from the Places node, that is the start time of the download.
+    this.endTime = placesNode.time / 1000;
+  }
+
+  /**
+   * DownloadSlot containing this history download.
+   *
+   * @type {DownloadSlot}
+   */
+  slot = null;
+
+  /**
+   * History downloads are never in progress.
+   *
+   * @type {Boolean}
+   */
+  stopped = true;
+
+  /**
+   * No percentage indication is shown for history downloads.
+   *
+   * @type {Boolean}
+   */
+  hasProgress = false;
+
+  /**
+   * History downloads cannot be restarted using their partial data, even if
+   * they are indicated as paused in their Places metadata. The only way is to
+   * use the information from a persisted session download, that will be shown
+   * instead of the history download. In case this session download is not
+   * available, we show the history download as canceled, not paused.
+   *
+   * @type {Boolean}
+   */
+  hasPartialData = false;
+
+  /**
+   * Pushes information from Places metadata into this object.
+   */
+  updateFromMetaData(metaData) {
+    try {
+      this.target.path = Cc["@mozilla.org/network/protocol;1?name=file"]
+        .getService(Ci.nsIFileProtocolHandler)
+        .getFileFromURLSpec(metaData.targetFileSpec).path;
+    } catch (ex) {
+      this.target.path = undefined;
+    }
+
+    if ("state" in metaData) {
+      this.succeeded = metaData.state == METADATA_STATE_FINISHED;
+      this.canceled =
+        metaData.state == METADATA_STATE_CANCELED ||
+        metaData.state == METADATA_STATE_PAUSED;
+      this.endTime = metaData.endTime;
+      this.deleted = metaData.deleted;
+
+      // Recreate partial error information from the state saved in history.
+      if (metaData.state == METADATA_STATE_FAILED) {
+        this.error = { message: "History download failed." };
+      } else if (metaData.state == METADATA_STATE_BLOCKED_PARENTAL) {
+        this.error = { becauseBlockedByParentalControls: true };
+      } else if (metaData.state == METADATA_STATE_DIRTY) {
+        this.error = {
+          becauseBlockedByReputationCheck: true,
+          reputationCheckVerdict: metaData.reputationCheckVerdict || "",
+        };
+      } else {
+        this.error = null;
+      }
+
+      // Normal history downloads are assumed to exist until the user interface
+      // is refreshed, at which point these values may be updated.
+      this.target.exists = true;
+      this.target.size = metaData.fileSize;
+    } else {
+      // Metadata might be missing from a download that has started but hasn't
+      // stopped already. Normally, this state is overridden with the one from
+      // the corresponding in-progress session download. But if the browser is
+      // terminated abruptly and additionally the file with information about
+      // in-progress downloads is lost, we may end up using this state. We use
+      // the failed state to allow the download to be restarted.
+      //
+      // On the other hand, if the download is missing the target file
+      // annotation as well, it is just a very old one, and we can assume it
+      // succeeded.
+      this.succeeded = !this.target.path;
+      this.error = this.target.path ? { message: "Unstarted download." } : null;
+      this.canceled = false;
+      this.deleted = false;
+
+      // These properties may be updated if the user interface is refreshed.
+      this.target.exists = false;
+      this.target.size = undefined;
+    }
+  }
+
+  /**
+   * This method may be called when deleting a history download.
+   */
+  async finalize() {}
+
+  /**
+   * This method mimicks the "refresh" method of session downloads.
+   */
+  async refresh() {
+    try {
+      this.target.size = (await IOUtils.stat(this.target.path)).size;
+      this.target.exists = true;
+    } catch (ex) {
+      // We keep the known file size from the metadata, if any.
+      this.target.exists = false;
+    }
+
+    this.slot.list._notifyAllViews("onDownloadChanged", this);
+  }
+
+  /**
+   * This method mimicks the "manuallyRemoveData" method of session downloads.
+   */
+  async manuallyRemoveData() {
+    let { path } = this.target;
+    if (this.target.path && this.succeeded) {
+      // Temp files are made "read-only" by DownloadIntegration.downloadDone, so
+      // reset the permission bits to read/write. This won't be necessary after
+      // bug 1733587 since Downloads won't ever be temporary.
+      await IOUtils.setPermissions(path, 0o660);
+      await IOUtils.remove(path, { ignoreAbsent: true });
+    }
+    this.deleted = true;
+    await this.refresh();
+  }
+}
+
+/**
+ * Represents one item in the list of public session and history downloads.
+ *
+ * The object may contain a session download, a history download, or both. When
+ * both a history and a session download are present, the session download gets
+ * priority and its information is accessed.
+ *
+ * @param list
+ *        The DownloadHistoryList that owns this DownloadSlot object.
+ */
+class DownloadSlot {
+  constructor(list) {
+    this.list = list;
+  }
+
+  /**
+   * Download object representing the session download contained in this slot.
+   */
+  sessionDownload = null;
+  _historyDownload = null;
+
+  /**
+   * HistoryDownload object contained in this slot.
+   */
+  get historyDownload() {
+    return this._historyDownload;
+  }
+
+  set historyDownload(historyDownload) {
+    this._historyDownload = historyDownload;
+    if (historyDownload) {
+      historyDownload.slot = this;
+    }
+  }
+
+  /**
+   * Returns the Download or HistoryDownload object for displaying information
+   * and executing commands in the user interface.
+   */
+  get download() {
+    return this.sessionDownload || this.historyDownload;
+  }
+}
+
+/**
+ * Represents an ordered collection of DownloadSlot objects containing a merged
+ * view on session downloads and history downloads. Views on this list will
+ * receive notifications for changes to both types of downloads.
+ *
+ * Downloads in this list are sorted from oldest to newest, with all session
+ * downloads after all the history downloads. When a new history download is
+ * added and the list also contains session downloads, the insertBefore option
+ * of the onDownloadAdded notification refers to the first session download.
+ *
+ * The list of downloads cannot be modified using the DownloadList methods.
+ *
+ * @param publicList
+ *        Underlying DownloadList containing public downloads.
+ * @param place
+ *        Places query used to retrieve history downloads.
+ */
+class DownloadHistoryList extends DownloadList {
+  constructor(publicList, place) {
+    super();
+
+    // While "this._slots" contains all the data in order, the other properties
+    // provide fast access for the most common operations.
+    this._slots = [];
+    this._slotsForUrl = new Map();
+    this._slotForDownload = new WeakMap();
+
+    // Start the asynchronous queries to retrieve history and session downloads.
+    publicList.addView(this).catch(console.error);
+    let query = {},
+      options = {};
+    lazy.PlacesUtils.history.queryStringToQuery(place, query, options);
+
+    // NB: The addObserver call sets our nsINavHistoryResultObserver.result.
+    let result = lazy.PlacesUtils.history.executeQuery(
+      query.value,
+      options.value
+    );
+    result.addObserver(this);
+
+    // Our history result observer is long lived for fast shared views, so free
+    // the reference on shutdown to prevent leaks.
+    Services.obs.addObserver(() => {
+      this.result = null;
+    }, "quit-application-granted");
+  }
+
+  /**
+   * This is set when executing the Places query.
+   */
+  _result = null;
+
+  /**
+   * Index of the first slot that contains a session download. This is equal to
+   * the length of the list when there are no session downloads.
+   *
+   * @type {Number}
+   */
+  _firstSessionSlotIndex = 0;
+
+  get result() {
+    return this._result;
+  }
+
+  set result(result) {
+    if (this._result == result) {
+      return;
+    }
+
+    if (this._result) {
+      this._result.removeObserver(this);
+      this._result.root.containerOpen = false;
+    }
+
+    this._result = result;
+
+    if (this._result) {
+      this._result.root.containerOpen = true;
+    }
+  }
+
+  /**
+   * Updates the download history item when the meta data or destination file
+   * changes.
+   *
+   * @param {String} sourceUrl The sourceUrl which was updated.
+   * @param {Object} metaData The new meta data for the sourceUrl.
+   */
+  updateForMetaDataChange(sourceUrl, metaData) {
+    let slotsForUrl = this._slotsForUrl.get(sourceUrl);
+    if (!slotsForUrl) {
+      return;
+    }
+
+    for (let slot of slotsForUrl) {
+      if (slot.sessionDownload) {
+        // The visible data doesn't change, so we don't have to notify views.
+        return;
+      }
+      slot.historyDownload.updateFromMetaData(metaData);
+      this._notifyAllViews("onDownloadChanged", slot.download);
+    }
+  }
+
+  _insertSlot({ slot, index, slotsForUrl }) {
+    // Add the slot to the ordered array.
+    this._slots.splice(index, 0, slot);
+    this._downloads.splice(index, 0, slot.download);
+    if (!slot.sessionDownload) {
+      this._firstSessionSlotIndex++;
+    }
+
+    // Add the slot to the fast access maps.
+    slotsForUrl.add(slot);
+    this._slotsForUrl.set(slot.download.source.url, slotsForUrl);
+
+    // Add the associated view items.
+    this._notifyAllViews("onDownloadAdded", slot.download, {
+      insertBefore: this._downloads[index + 1],
+    });
+  }
+
+  _removeSlot({ slot, slotsForUrl }) {
+    // Remove the slot from the ordered array.
+    let index = this._slots.indexOf(slot);
+    this._slots.splice(index, 1);
+    this._downloads.splice(index, 1);
+    if (this._firstSessionSlotIndex > index) {
+      this._firstSessionSlotIndex--;
+    }
+
+    // Remove the slot from the fast access maps.
+    slotsForUrl.delete(slot);
+    if (slotsForUrl.size == 0) {
+      this._slotsForUrl.delete(slot.download.source.url);
+    }
+
+    // Remove the associated view items.
+    this._notifyAllViews("onDownloadRemoved", slot.download);
+  }
+
+  /**
+   * Ensures that the information about a history download is stored in at least
+   * one slot, adding a new one at the end of the list if necessary.
+   *
+   * A reference to the same Places node will be stored in the HistoryDownload
+   * object for all the DownloadSlot objects associated with the source URL.
+   *
+   * @param placesNode
+   *        The Places node that represents the history download.
+   */
+  _insertPlacesNode(placesNode) {
+    let slotsForUrl = this._slotsForUrl.get(placesNode.uri) || new Set();
+
+    // If there are existing slots associated with this URL, we only have to
+    // ensure that the Places node reference is kept updated in case the more
+    // recent Places notification contained a different node object.
+    if (slotsForUrl.size > 0) {
+      for (let slot of slotsForUrl) {
+        if (!slot.historyDownload) {
+          slot.historyDownload = new HistoryDownload(placesNode);
+        } else {
+          slot.historyDownload.placesNode = placesNode;
+        }
+      }
+      return;
+    }
+
+    // If there are no existing slots for this URL, we have to create a new one.
+    // Since the history download is visible in the slot, we also have to update
+    // the object using the Places metadata.
+    let historyDownload = new HistoryDownload(placesNode);
+    historyDownload.updateFromMetaData(DownloadCache.get(placesNode.uri));
+    let slot = new DownloadSlot(this);
+    slot.historyDownload = historyDownload;
+    this._insertSlot({ slot, slotsForUrl, index: this._firstSessionSlotIndex });
+  }
+
+  // nsINavHistoryResultObserver
+  containerStateChanged(node, oldState, newState) {
+    this.invalidateContainer(node);
+  }
+
+  // nsINavHistoryResultObserver
+  invalidateContainer(container) {
+    this._notifyAllViews("onDownloadBatchStarting");
+
+    // Remove all the current slots containing only history downloads.
+    for (let index = this._slots.length - 1; index >= 0; index--) {
+      let slot = this._slots[index];
+      if (slot.sessionDownload) {
+        // The visible data doesn't change, so we don't have to notify views.
+        slot.historyDownload = null;
+      } else {
+        let slotsForUrl = this._slotsForUrl.get(slot.download.source.url);
+        this._removeSlot({ slot, slotsForUrl });
+      }
+    }
+
+    // Add new slots or reuse existing ones for history downloads.
+    for (let index = container.childCount - 1; index >= 0; --index) {
+      try {
+        this._insertPlacesNode(container.getChild(index));
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+
+    this._notifyAllViews("onDownloadBatchEnded");
+  }
+
+  // nsINavHistoryResultObserver
+  nodeInserted(parent, placesNode) {
+    this._insertPlacesNode(placesNode);
+  }
+
+  // nsINavHistoryResultObserver
+  nodeRemoved(parent, placesNode, aOldIndex) {
+    let slotsForUrl = this._slotsForUrl.get(placesNode.uri);
+    for (let slot of slotsForUrl) {
+      if (slot.sessionDownload) {
+        // The visible data doesn't change, so we don't have to notify views.
+        slot.historyDownload = null;
+      } else {
+        this._removeSlot({ slot, slotsForUrl });
+      }
+    }
+  }
+
+  // nsINavHistoryResultObserver
+  nodeIconChanged() {}
+  nodeTitleChanged() {}
+  nodeKeywordChanged() {}
+  nodeDateAddedChanged() {}
+  nodeLastModifiedChanged() {}
+  nodeHistoryDetailsChanged() {}
+  nodeTagsChanged() {}
+  sortingChanged() {}
+  nodeMoved() {}
+  nodeURIChanged() {}
+  batching() {}
+
+  // DownloadList callback
+  onDownloadAdded(download) {
+    let url = download.source.url;
+    let slotsForUrl = this._slotsForUrl.get(url) || new Set();
+
+    // For every source URL, there can be at most one slot containing a history
+    // download without an associated session download. If we find one, then we
+    // can reuse it for the current session download, although we have to move
+    // it together with the other session downloads.
+    let slot = [...slotsForUrl][0];
+    if (slot && !slot.sessionDownload) {
+      // Remove the slot because we have to change its position.
+      this._removeSlot({ slot, slotsForUrl });
+    } else {
+      slot = new DownloadSlot(this);
+    }
+    slot.sessionDownload = download;
+    this._insertSlot({ slot, slotsForUrl, index: this._slots.length });
+    this._slotForDownload.set(download, slot);
+  }
+
+  // DownloadList callback
+  onDownloadChanged(download) {
+    let slot = this._slotForDownload.get(download);
+    this._notifyAllViews("onDownloadChanged", slot.download);
+  }
+
+  // DownloadList callback
+  onDownloadRemoved(download) {
+    let url = download.source.url;
+    let slotsForUrl = this._slotsForUrl.get(url);
+    let slot = this._slotForDownload.get(download);
+    this._removeSlot({ slot, slotsForUrl });
+
+    this._slotForDownload.delete(download);
+
+    // If there was only one slot for this source URL and it also contained a
+    // history download, we should resurrect it in the correct area of the list.
+    if (slotsForUrl.size == 0 && slot.historyDownload) {
+      // We have one download slot containing both a session download and a
+      // history download, and we are now removing the session download.
+      // Previously, we did not use the Places metadata because it was obscured
+      // by the session download. Since this is no longer the case, we have to
+      // read the latest metadata before resurrecting the history download.
+      slot.historyDownload.updateFromMetaData(DownloadCache.get(url));
+      slot.sessionDownload = null;
+      // Place the resurrected history slot after all the session slots.
+      this._insertSlot({
+        slot,
+        slotsForUrl,
+        index: this._firstSessionSlotIndex,
+      });
+    }
+  }
+
+  // DownloadList
+  add() {
+    throw new Error("Not implemented.");
+  }
+
+  // DownloadList
+  remove() {
+    throw new Error("Not implemented.");
+  }
+
+  // DownloadList
+  removeFinished() {
+    throw new Error("Not implemented.");
+  }
+}
diff --git a/toolkit/components/downloads/DownloadIntegration.sys.mjs b/toolkit/components/downloads/DownloadIntegration.sys.mjs
new file mode 100644
index 0000000000..5762f95cc2
--- /dev/null
+++ b/toolkit/components/downloads/DownloadIntegration.sys.mjs
@@ -0,0 +1,1327 @@
+/* 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/. */
+
+/**
+ * Provides functions to integrate with the host application, handling for
+ * example the global prompts on shutdown.
+ */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+
+import { Downloads } from "resource://gre/modules/Downloads.sys.mjs";
+import { Integration } from "resource://gre/modules/Integration.sys.mjs";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AsyncShutdown: "resource://gre/modules/AsyncShutdown.sys.mjs",
+  DeferredTask: "resource://gre/modules/DeferredTask.sys.mjs",
+  DownloadSpamProtection: "resource:///modules/DownloadSpamProtection.sys.mjs",
+  DownloadStore: "resource://gre/modules/DownloadStore.sys.mjs",
+  DownloadUIHelper: "resource://gre/modules/DownloadUIHelper.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+  NetUtil: "resource://gre/modules/NetUtil.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gDownloadPlatform",
+  "@mozilla.org/toolkit/download-platform;1",
+  "mozIDownloadPlatform"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gMIMEService",
+  "@mozilla.org/mime;1",
+  "nsIMIMEService"
+);
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gExternalProtocolService",
+  "@mozilla.org/uriloader/external-protocol-service;1",
+  "nsIExternalProtocolService"
+);
+
+ChromeUtils.defineLazyGetter(lazy, "gParentalControlsService", function () {
+  if ("@mozilla.org/parental-controls-service;1" in Cc) {
+    return Cc["@mozilla.org/parental-controls-service;1"].createInstance(
+      Ci.nsIParentalControlsService
+    );
+  }
+  return null;
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gApplicationReputationService",
+  "@mozilla.org/reputationservice/application-reputation-service;1",
+  Ci.nsIApplicationReputationService
+);
+
+Integration.downloads.defineESModuleGetter(
+  lazy,
+  "DownloadIntegration",
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+ChromeUtils.defineLazyGetter(lazy, "gCombinedDownloadIntegration", () => {
+  return lazy.DownloadIntegration;
+});
+
+ChromeUtils.defineLazyGetter(lazy, "stringBundle", () =>
+  Services.strings.createBundle(
+    "chrome://mozapps/locale/downloads/downloads.properties"
+  )
+);
+
+const Timer = Components.Constructor(
+  "@mozilla.org/timer;1",
+  "nsITimer",
+  "initWithCallback"
+);
+
+/**
+ * Indicates the delay between a change to the downloads data and the related
+ * save operation.
+ *
+ * For best efficiency, this value should be high enough that the input/output
+ * for opening or closing the target file does not overlap with the one for
+ * saving the list of downloads.
+ */
+const kSaveDelayMs = 1500;
+
+/**
+ * List of observers to listen against
+ */
+const kObserverTopics = [
+  "quit-application-requested",
+  "offline-requested",
+  "last-pb-context-exiting",
+  "last-pb-context-exited",
+  "sleep_notification",
+  "suspend_process_notification",
+  "wake_notification",
+  "resume_process_notification",
+  "network:offline-about-to-go-offline",
+  "network:offline-status-changed",
+  "xpcom-will-shutdown",
+  "blocked-automatic-download",
+];
+
+/**
+ * Maps nsIApplicationReputationService verdicts with the DownloadError ones.
+ */
+const kVerdictMap = {
+  [Ci.nsIApplicationReputationService.VERDICT_DANGEROUS]:
+    Downloads.Error.BLOCK_VERDICT_MALWARE,
+  [Ci.nsIApplicationReputationService.VERDICT_UNCOMMON]:
+    Downloads.Error.BLOCK_VERDICT_UNCOMMON,
+  [Ci.nsIApplicationReputationService.VERDICT_POTENTIALLY_UNWANTED]:
+    Downloads.Error.BLOCK_VERDICT_POTENTIALLY_UNWANTED,
+  [Ci.nsIApplicationReputationService.VERDICT_DANGEROUS_HOST]:
+    Downloads.Error.BLOCK_VERDICT_MALWARE,
+};
+
+/**
+ * Provides functions to integrate with the host application, handling for
+ * example the global prompts on shutdown.
+ */
+export var DownloadIntegration = {
+  /**
+   * Main DownloadStore object for loading and saving the list of persistent
+   * downloads, or null if the download list was never requested and thus it
+   * doesn't need to be persisted.
+   */
+  _store: null,
+
+  /**
+   * Returns whether data for blocked downloads should be kept on disk.
+   * Implementations which support unblocking downloads may return true to
+   * keep the blocked download on disk until its fate is decided.
+   *
+   * If a download is blocked and the partial data is kept the Download's
+   * 'hasBlockedData' property will be true. In this state Download.unblock()
+   * or Download.confirmBlock() may be used to either unblock the download or
+   * remove the downloaded data respectively.
+   *
+   * Even if shouldKeepBlockedData returns true, if the download did not use a
+   * partFile the blocked data will be removed - preventing the complete
+   * download from existing on disk with its final filename.
+   *
+   * @return boolean True if data should be kept.
+   */
+  shouldKeepBlockedData() {
+    const FIREFOX_ID = "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}";
+    return Services.appinfo.ID == FIREFOX_ID;
+  },
+
+  /**
+   * Performs initialization of the list of persistent downloads, before its
+   * first use by the host application.  This function may be called only once
+   * during the entire lifetime of the application.
+   *
+   * @param list
+   *        DownloadList object to be initialized.
+   *
+   * @return {Promise}
+   * @resolves When the list has been initialized.
+   * @rejects JavaScript exception.
+   */
+  async initializePublicDownloadList(list) {
+    try {
+      await this.loadPublicDownloadListFromStore(list);
+    } catch (ex) {
+      console.error(ex);
+    }
+
+    if (AppConstants.MOZ_PLACES) {
+      // After the list of persistent downloads has been loaded, we can add the
+      // history observers, even if the load operation failed. This object is kept
+      // alive by the history service.
+      new DownloadHistoryObserver(list);
+    }
+  },
+
+  /**
+   * Called by initializePublicDownloadList to load the list of persistent
+   * downloads, before its first use by the host application.  This function may
+   * be called only once during the entire lifetime of the application.
+   *
+   * @param list
+   *        DownloadList object to be populated with the download objects
+   *        serialized from the previous session.  This list will be persisted
+   *        to disk during the session lifetime.
+   *
+   * @return {Promise}
+   * @resolves When the list has been populated.
+   * @rejects JavaScript exception.
+   */
+  async loadPublicDownloadListFromStore(list) {
+    if (this._store) {
+      throw new Error("Initialization may be performed only once.");
+    }
+
+    this._store = new lazy.DownloadStore(
+      list,
+      PathUtils.join(PathUtils.profileDir, "downloads.json")
+    );
+    this._store.onsaveitem = this.shouldPersistDownload.bind(this);
+
+    try {
+      await this._store.load();
+    } catch (ex) {
+      console.error(ex);
+    }
+
+    // Add the view used for detecting changes to downloads to be persisted.
+    // We must do this after the list of persistent downloads has been loaded,
+    // even if the load operation failed. We wait for a complete initialization
+    // so other callers cannot modify the list without being detected. The
+    // DownloadAutoSaveView is kept alive by the underlying DownloadList.
+    await new DownloadAutoSaveView(list, this._store).initialize();
+  },
+
+  /**
+   * Determines if a Download object from the list of persistent downloads
+   * should be saved into a file, so that it can be restored across sessions.
+   *
+   * This function allows filtering out downloads that the host application is
+   * not interested in persisting across sessions, for example downloads that
+   * finished successfully.
+   *
+   * @param aDownload
+   *        The Download object to be inspected.  This is originally taken from
+   *        the global DownloadList object for downloads that were not started
+   *        from a private browsing window.  The item may have been removed
+   *        from the list since the save operation started, though in this case
+   *        the save operation will be repeated later.
+   *
+   * @return True to save the download, false otherwise.
+   */
+  shouldPersistDownload(aDownload) {
+    // On all platforms, we save all the downloads currently in progress, as
+    // well as stopped downloads for which we retained partially downloaded
+    // data or we have blocked data.
+    // On Android we store all history; on Desktop, stopped downloads for which
+    // we don't need to track the presence of a ".part" file are only retained
+    // in the browser history.
+    return (
+      !aDownload.stopped ||
+      aDownload.hasPartialData ||
+      aDownload.hasBlockedData ||
+      AppConstants.platform == "android"
+    );
+  },
+
+  /**
+   * Returns the system downloads directory asynchronously.
+   *
+   * @return {Promise}
+   * @resolves The downloads directory string path.
+   */
+  async getSystemDownloadsDirectory() {
+    if (this._downloadsDirectory) {
+      return this._downloadsDirectory;
+    }
+
+    if (AppConstants.platform == "android") {
+      // Android doesn't have a $HOME directory, and by default we only have
+      // write access to /data/data/org.mozilla.{$APP} and /sdcard
+      this._downloadsDirectory = Services.env.get("DOWNLOADS_DIRECTORY");
+      if (!this._downloadsDirectory) {
+        throw new Components.Exception(
+          "DOWNLOADS_DIRECTORY is not set.",
+          Cr.NS_ERROR_FILE_UNRECOGNIZED_PATH
+        );
+      }
+    } else {
+      try {
+        this._downloadsDirectory = this._getDirectory("DfltDwnld");
+      } catch (e) {
+        this._downloadsDirectory = await this._createDownloadsDirectory("Home");
+      }
+    }
+
+    return this._downloadsDirectory;
+  },
+  _downloadsDirectory: null,
+
+  /**
+   * Returns the user downloads directory asynchronously.
+   *
+   * @return {Promise}
+   * @resolves The downloads directory string path.
+   */
+  async getPreferredDownloadsDirectory() {
+    let directoryPath = null;
+    let prefValue = Services.prefs.getIntPref("browser.download.folderList", 1);
+
+    switch (prefValue) {
+      case 0: // Desktop
+        directoryPath = this._getDirectory("Desk");
+        break;
+      case 1: // Downloads
+        directoryPath = await this.getSystemDownloadsDirectory();
+        break;
+      case 2: // Custom
+        try {
+          let directory = Services.prefs.getComplexValue(
+            "browser.download.dir",
+            Ci.nsIFile
+          );
+          directoryPath = directory.path;
+          await IOUtils.makeDirectory(directoryPath, {
+            createAncestors: false,
+          });
+        } catch (ex) {
+          console.error(ex);
+          // Either the preference isn't set or the directory cannot be created.
+          directoryPath = await this.getSystemDownloadsDirectory();
+        }
+        break;
+      default:
+        directoryPath = await this.getSystemDownloadsDirectory();
+    }
+    return directoryPath;
+  },
+
+  /**
+   * Returns the temporary downloads directory asynchronously.
+   *
+   * @return {Promise}
+   * @resolves The downloads directory string path.
+   */
+  async getTemporaryDownloadsDirectory() {
+    let directoryPath = null;
+    if (AppConstants.platform == "macosx") {
+      directoryPath = await this.getPreferredDownloadsDirectory();
+    } else if (AppConstants.platform == "android") {
+      directoryPath = await this.getSystemDownloadsDirectory();
+    } else {
+      directoryPath = this._getDirectory("TmpD");
+    }
+    return directoryPath;
+  },
+
+  /**
+   * Checks to determine whether to block downloads for parental controls.
+   *
+   * aParam aDownload
+   *        The download object.
+   *
+   * @return {Promise}
+   * @resolves The boolean indicates to block downloads or not.
+   */
+  shouldBlockForParentalControls(aDownload) {
+    let isEnabled =
+      lazy.gParentalControlsService &&
+      lazy.gParentalControlsService.parentalControlsEnabled;
+    let shouldBlock =
+      isEnabled && lazy.gParentalControlsService.blockFileDownloadsEnabled;
+
+    // Log the event if required by parental controls settings.
+    if (isEnabled && lazy.gParentalControlsService.loggingEnabled) {
+      lazy.gParentalControlsService.log(
+        lazy.gParentalControlsService.ePCLog_FileDownload,
+        shouldBlock,
+        lazy.NetUtil.newURI(aDownload.source.url),
+        null
+      );
+    }
+
+    return Promise.resolve(shouldBlock);
+  },
+
+  /**
+   * Checks to determine whether to block downloads because they might be
+   * malware, based on application reputation checks.
+   *
+   * aParam aDownload
+   *        The download object.
+   *
+   * @return {Promise}
+   * @resolves Object with the following properties:
+   *           {
+   *             shouldBlock: Whether the download should be blocked.
+   *             verdict: Detailed reason for the block, according to the
+   *                      "Downloads.Error.BLOCK_VERDICT_" constants, or empty
+   *                      string if the reason is unknown.
+   *           }
+   */
+  shouldBlockForReputationCheck(aDownload) {
+    let hash;
+    let sigInfo;
+    let channelRedirects;
+    try {
+      hash = aDownload.saver.getSha256Hash();
+      sigInfo = aDownload.saver.getSignatureInfo();
+      channelRedirects = aDownload.saver.getRedirects();
+    } catch (ex) {
+      // Bail if DownloadSaver doesn't have a hash or signature info.
+      return Promise.resolve({
+        shouldBlock: false,
+        verdict: "",
+      });
+    }
+    if (!hash || !sigInfo) {
+      return Promise.resolve({
+        shouldBlock: false,
+        verdict: "",
+      });
+    }
+    return new Promise(resolve => {
+      lazy.gApplicationReputationService.queryReputation(
+        {
+          sourceURI: lazy.NetUtil.newURI(aDownload.source.url),
+          referrerInfo: aDownload.source.referrerInfo,
+          fileSize: aDownload.currentBytes,
+          sha256Hash: hash,
+          suggestedFileName: PathUtils.filename(aDownload.target.path),
+          signatureInfo: sigInfo,
+          redirects: channelRedirects,
+        },
+        function onComplete(aShouldBlock, aRv, aVerdict) {
+          resolve({
+            shouldBlock: aShouldBlock,
+            verdict: (aShouldBlock && kVerdictMap[aVerdict]) || "",
+          });
+        }
+      );
+    });
+  },
+
+  /**
+   * Checks whether downloaded files should be marked as coming from
+   * Internet Zone.
+   *
+   * @return true if files should be marked
+   */
+  _shouldSaveZoneInformation() {
+    let key = Cc["@mozilla.org/windows-registry-key;1"].createInstance(
+      Ci.nsIWindowsRegKey
+    );
+    try {
+      key.open(
+        Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+        "Software\\Microsoft\\Windows\\CurrentVersion\\Policies\\Attachments",
+        Ci.nsIWindowsRegKey.ACCESS_QUERY_VALUE
+      );
+      try {
+        return key.readIntValue("SaveZoneInformation") != 1;
+      } finally {
+        key.close();
+      }
+    } catch (ex) {
+      // If the key is not present, files should be marked by default.
+      return true;
+    }
+  },
+
+  /**
+   * Builds a key and URL value pair for the "Zone.Identifier" Alternate Data
+   * Stream.
+   *
+   * @param aKey
+   *        String to write before the "=" sign. This is not validated.
+   * @param aUrl
+   *        URL string to write after the "=" sign. Only the "http(s)" and
+   *        "ftp" schemes are allowed, and usernames and passwords are
+   *        stripped.
+   * @param [optional] aFallback
+   *        Value to place after the "=" sign in case the URL scheme is not
+   *        allowed. If unspecified, an empty string is returned when the
+   *        scheme is not allowed.
+   *
+   * @return Line to add to the stream, including the final CRLF, or an empty
+   *         string if the validation failed.
+   */
+  _zoneIdKey(aKey, aUrl, aFallback) {
+    try {
+      let url;
+      const uri = lazy.NetUtil.newURI(aUrl);
+      if (["http", "https", "ftp"].includes(uri.scheme)) {
+        url = uri.mutate().setUserPass("").finalize().spec;
+      } else if (aFallback) {
+        url = aFallback;
+      } else {
+        return "";
+      }
+      return aKey + "=" + url + "\r\n";
+    } catch (e) {
+      return "";
+    }
+  },
+
+  /**
+   * Performs platform-specific operations when a download is done.
+   *
+   * aParam aDownload
+   *        The Download object.
+   *
+   * @return {Promise}
+   * @resolves When all the operations completed successfully.
+   * @rejects JavaScript exception if any of the operations failed.
+   */
+  async downloadDone(aDownload) {
+    // On Windows, we mark any file saved to the NTFS file system as coming
+    // from the Internet security zone unless Group Policy disables the
+    // feature.  We do this by writing to the "Zone.Identifier" Alternate
+    // Data Stream directly, because the Save method of the
+    // IAttachmentExecute interface would trigger operations that may cause
+    // the application to hang, or other performance issues.
+    // The stream created in this way is forward-compatible with all the
+    // current and future versions of Windows.
+    if (AppConstants.platform == "win" && this._shouldSaveZoneInformation()) {
+      let zone;
+      try {
+        zone = lazy.gDownloadPlatform.mapUrlToZone(aDownload.source.url);
+      } catch (e) {
+        // Default to Internet Zone if mapUrlToZone failed for
+        // whatever reason.
+        zone = Ci.mozIDownloadPlatform.ZONE_INTERNET;
+      }
+      // Don't write zone IDs for Local, Intranet, or Trusted sites
+      // to match Windows behavior.
+      if (zone >= Ci.mozIDownloadPlatform.ZONE_INTERNET) {
+        let path = aDownload.target.path + ":Zone.Identifier";
+        try {
+          let zoneId = "[ZoneTransfer]\r\nZoneId=" + zone + "\r\n";
+          let { url, isPrivate, referrerInfo } = aDownload.source;
+          if (!isPrivate) {
+            let referrer = referrerInfo
+              ? referrerInfo.computedReferrerSpec
+              : "";
+            zoneId +=
+              this._zoneIdKey("ReferrerUrl", referrer) +
+              this._zoneIdKey("HostUrl", url, "about:internet");
+          }
+          await IOUtils.writeUTF8(
+            PathUtils.toExtendedWindowsPath(path),
+            zoneId
+          );
+        } catch (ex) {
+          // If writing to the file fails, we ignore the error and continue.
+          if (!DOMException.isInstance(ex)) {
+            console.error(ex);
+          }
+        }
+      }
+    }
+
+    // The file with the partially downloaded data has restrictive permissions
+    // that don't allow other users on the system to access it.  Now that the
+    // download is completed, we need to adjust permissions based on whether
+    // this is a permanently downloaded file or a temporary download to be
+    // opened read-only with an external application.
+    try {
+      let isTemporaryDownload =
+        aDownload.launchWhenSucceeded && aDownload.source.isPrivate;
+      // Permanently downloaded files are made accessible by other users on
+      // this system, while temporary downloads are marked as read-only.
+      let unixMode;
+      if (isTemporaryDownload) {
+        unixMode = 0o400;
+      } else {
+        unixMode = 0o666;
+      }
+      // On Unix, the umask of the process is respected.
+      await IOUtils.setPermissions(aDownload.target.path, unixMode);
+    } catch (ex) {
+      // We should report errors with making the permissions less restrictive
+      // or marking the file as read-only on Unix and Mac, but this should not
+      // prevent the download from completing.
+      if (!DOMException.isInstance(ex)) {
+        console.error(ex);
+      }
+    }
+
+    let aReferrer = null;
+    if (aDownload.source.referrerInfo) {
+      aReferrer = aDownload.source.referrerInfo.originalReferrer;
+    }
+
+    await lazy.gDownloadPlatform.downloadDone(
+      lazy.NetUtil.newURI(aDownload.source.url),
+      aReferrer,
+      new lazy.FileUtils.File(aDownload.target.path),
+      aDownload.contentType,
+      aDownload.source.isPrivate
+    );
+  },
+
+  /**
+   * Decide whether a download of this type, opened from the downloads
+   * list, should open internally.
+   *
+   * @param aMimeType
+   *        The MIME type of the file, as a string
+   * @param [optional] aExtension
+   *        The file extension, which can match instead of the MIME type.
+   */
+  shouldViewDownloadInternally(aMimeType, aExtension) {
+    // Refuse all files by default, this is meant to be replaced with a check
+    // for specific types via Integration.downloads.register().
+    return false;
+  },
+
+  /**
+   * Launches a file represented by the target of a download. This can
+   * open the file with the default application for the target MIME type
+   * or file extension, or with a custom application if
+   * aDownload.launcherPath is set.
+   *
+   * @param    aDownload
+   *           A Download object that contains the necessary information
+   *           to launch the file. The relevant properties are: the target
+   *           file, the contentType and the custom application chosen
+   *           to launch it.
+   * @param options.openWhere     Optional string indicating how to open when handling
+   *                              download by opening the target file URI.
+   *                              One of "window", "tab", "tabshifted"
+   * @param options.useSystemDefault
+   *                              Optional value indicating how to handle launching this download,
+   *                              this time only. Will override the associated mimeInfo.preferredAction
+   *
+   * @return {Promise}
+   * @resolves When the instruction to launch the file has been
+   *           successfully given to the operating system. Note that
+   *           the OS might still take a while until the file is actually
+   *           launched.
+   * @rejects  JavaScript exception if there was an error trying to launch
+   *           the file.
+   */
+  async launchDownload(aDownload, { openWhere, useSystemDefault = null }) {
+    let file = new lazy.FileUtils.File(aDownload.target.path);
+
+    // In case of a double extension, like ".tar.gz", we only
+    // consider the last one, because the MIME service cannot
+    // handle multiple extensions.
+    let fileExtension = null,
+      mimeInfo = null;
+    let match = file.leafName.match(/\.([^.]+)$/);
+    if (match) {
+      fileExtension = match[1];
+    }
+
+    let isWindowsExe =
+      AppConstants.platform == "win" &&
+      fileExtension &&
+      fileExtension.toLowerCase() == "exe";
+
+    let isExemptExecutableExtension =
+      Services.policies.isExemptExecutableExtension(
+        aDownload.source.url,
+        fileExtension
+      );
+
+    // Ask for confirmation if the file is executable, except for .exe on
+    // Windows where the operating system will show the prompt based on the
+    // security zone.  We do this here, instead of letting the caller handle
+    // the prompt separately in the user interface layer, for two reasons.  The
+    // first is because of its security nature, so that add-ons cannot forget
+    // to do this check.  The second is that the system-level security prompt
+    // would be displayed at launch time in any case.
+    // We allow policy to override this behavior for file extensions on specific domains.
+    if (
+      file.isExecutable() &&
+      !isWindowsExe &&
+      !isExemptExecutableExtension &&
+      !(await this.confirmLaunchExecutable(file.path))
+    ) {
+      return;
+    }
+
+    try {
+      // The MIME service might throw if contentType == "" and it can't find
+      // a MIME type for the given extension, so we'll treat this case as
+      // an unknown mimetype.
+      mimeInfo = lazy.gMIMEService.getFromTypeAndExtension(
+        aDownload.contentType,
+        fileExtension
+      );
+    } catch (e) {}
+
+    if (aDownload.launcherPath) {
+      if (!mimeInfo) {
+        // This should not happen on normal circumstances because launcherPath
+        // is only set when we had an instance of nsIMIMEInfo to retrieve
+        // the custom application chosen by the user.
+        throw new Error(
+          "Unable to create nsIMIMEInfo to launch a custom application"
+        );
+      }
+
+      // Custom application chosen
+      let localHandlerApp = Cc[
+        "@mozilla.org/uriloader/local-handler-app;1"
+      ].createInstance(Ci.nsILocalHandlerApp);
+      localHandlerApp.executable = new lazy.FileUtils.File(
+        aDownload.launcherPath
+      );
+
+      mimeInfo.preferredApplicationHandler = localHandlerApp;
+      mimeInfo.preferredAction = Ci.nsIMIMEInfo.useHelperApp;
+
+      this.launchFile(file, mimeInfo);
+      // After an attempt has been made to launch the download, clear the
+      // launchWhenSucceeded bit so future attempts to open the download can go
+      // through Firefox when possible.
+      aDownload.launchWhenSucceeded = false;
+      return;
+    }
+
+    if (!useSystemDefault && mimeInfo) {
+      useSystemDefault = mimeInfo.preferredAction == mimeInfo.useSystemDefault;
+    }
+    if (!useSystemDefault) {
+      // No explicit instruction was passed to launch this download using the default system viewer.
+      if (
+        aDownload.handleInternally ||
+        (mimeInfo &&
+          this.shouldViewDownloadInternally(mimeInfo.type, fileExtension) &&
+          !mimeInfo.alwaysAskBeforeHandling &&
+          (mimeInfo.preferredAction === Ci.nsIHandlerInfo.handleInternally ||
+            (["image/svg+xml", "text/xml", "application/xml"].includes(
+              mimeInfo.type
+            ) &&
+              mimeInfo.preferredAction === Ci.nsIHandlerInfo.saveToDisk)) &&
+          !aDownload.launchWhenSucceeded)
+      ) {
+        lazy.DownloadUIHelper.loadFileIn(file, {
+          browsingContextId: aDownload.source.browsingContextId,
+          isPrivate: aDownload.source.isPrivate,
+          openWhere,
+          userContextId: aDownload.source.userContextId,
+        });
+        return;
+      }
+    }
+
+    // An attempt will now be made to launch the download, clear the
+    // launchWhenSucceeded bit so future attempts to open the download can go
+    // through Firefox when possible.
+    aDownload.launchWhenSucceeded = false;
+
+    // When a file has no extension, and there's an executable file with the
+    // same name in the same folder, Windows shell can get confused.
+    // For this reason we show the file in the containing folder instead of
+    // trying to open it.
+    // We also don't trust mimeinfo, it could be a type we can forward to a
+    // system handler, but it could also be an executable type, and we
+    // don't have an exhaustive list with all of them.
+    if (!fileExtension && AppConstants.platform == "win") {
+      // We can't check for the existance of a same-name file with every
+      // possible executable extension, so this is a catch-all.
+      this.showContainingDirectory(aDownload.target.path);
+      return;
+    }
+
+    // No custom application chosen, let's launch the file with the default
+    // handler. First, let's try to launch it through the MIME service.
+    if (mimeInfo) {
+      mimeInfo.preferredAction = Ci.nsIMIMEInfo.useSystemDefault;
+      try {
+        this.launchFile(file, mimeInfo);
+        return;
+      } catch (ex) {}
+    }
+
+    // If it didn't work or if there was no MIME info available,
+    // let's try to directly launch the file.
+    try {
+      this.launchFile(file);
+      return;
+    } catch (ex) {}
+
+    // If our previous attempts failed, try sending it through
+    // the system's external "file:" URL handler.
+    lazy.gExternalProtocolService.loadURI(
+      lazy.NetUtil.newURI(file),
+      Services.scriptSecurityManager.getSystemPrincipal()
+    );
+  },
+
+  /**
+   * Asks for confirmation for launching the specified executable file. This
+   * can be overridden by regression tests to avoid the interactive prompt.
+   */
+  async confirmLaunchExecutable(path) {
+    // We don't anchor the prompt to a specific window intentionally, not
+    // only because this is the same behavior as the system-level prompt,
+    // but also because the most recently active window is the right choice
+    // in basically all cases.
+    return lazy.DownloadUIHelper.getPrompter().confirmLaunchExecutable(path);
+  },
+
+  /**
+   * Launches the specified file, unless overridden by regression tests.
+   * @note Always use launchDownload() from the outside of this module, it is
+   *       both more powerful and safer.
+   */
+  launchFile(file, mimeInfo) {
+    if (mimeInfo) {
+      mimeInfo.launchWithFile(file);
+    } else {
+      file.launch();
+    }
+  },
+
+  /**
+   * Shows the containing folder of a file.
+   *
+   * @param aFilePath
+   *        The path to the file.
+   *
+   * @return {Promise}
+   * @resolves When the instruction to open the containing folder has been
+   *           successfully given to the operating system. Note that
+   *           the OS might still take a while until the folder is actually
+   *           opened.
+   * @rejects  JavaScript exception if there was an error trying to open
+   *           the containing folder.
+   */
+  async showContainingDirectory(aFilePath) {
+    let file = new lazy.FileUtils.File(aFilePath);
+
+    try {
+      // Show the directory containing the file and select the file.
+      file.reveal();
+      return;
+    } catch (ex) {}
+
+    // If reveal fails for some reason (e.g., it's not implemented on unix
+    // or the file doesn't exist), try using the parent if we have it.
+    let parent = file.parent;
+    if (!parent) {
+      throw new Error(
+        "Unexpected reference to a top-level directory instead of a file"
+      );
+    }
+
+    try {
+      // Open the parent directory to show where the file should be.
+      parent.launch();
+      return;
+    } catch (ex) {}
+
+    // If launch also fails (probably because it's not implemented), let
+    // the OS handler try to open the parent.
+    lazy.gExternalProtocolService.loadURI(
+      lazy.NetUtil.newURI(parent),
+      Services.scriptSecurityManager.getSystemPrincipal()
+    );
+  },
+
+  /**
+   * Calls the directory service, create a downloads directory and returns an
+   * nsIFile for the downloads directory.
+   *
+   * @return {Promise}
+   * @resolves The directory string path.
+   */
+  _createDownloadsDirectory(aName) {
+    // We read the name of the directory from the list of translated strings
+    // that is kept by the UI helper module, even if this string is not strictly
+    // displayed in the user interface.
+    let directoryPath = PathUtils.join(
+      this._getDirectory(aName),
+      lazy.stringBundle.GetStringFromName("downloadsFolder")
+    );
+
+    // Create the Downloads folder and ignore if it already exists.
+    return IOUtils.makeDirectory(directoryPath, {
+      createAncestors: false,
+    }).then(() => directoryPath);
+  },
+
+  /**
+   * Returns the string path for the given directory service location name. This
+   * can be overridden by regression tests to return the path of the system
+   * temporary directory in all cases.
+   */
+  _getDirectory(name) {
+    return Services.dirsvc.get(name, Ci.nsIFile).path;
+  },
+
+  /**
+   * Initializes the DownloadSpamProtection instance.
+   * This is used to observe and group multiple automatic downloads.
+   */
+  _initializeDownloadSpamProtection() {
+    if (!this.downloadSpamProtection) {
+      this.downloadSpamProtection = new lazy.DownloadSpamProtection();
+    }
+  },
+
+  /**
+   * Register the downloads interruption observers.
+   *
+   * @param aList
+   *        The public or private downloads list.
+   * @param aIsPrivate
+   *        True if the list is private, false otherwise.
+   *
+   * @return {Promise}
+   * @resolves When the views and observers are added.
+   */
+  addListObservers(aList, aIsPrivate) {
+    DownloadObserver.registerView(aList, aIsPrivate);
+    if (!DownloadObserver.observersAdded) {
+      DownloadObserver.observersAdded = true;
+      for (let topic of kObserverTopics) {
+        Services.obs.addObserver(DownloadObserver, topic);
+      }
+    }
+    return Promise.resolve();
+  },
+
+  /**
+   * Force a save on _store if it exists. Used to ensure downloads do not
+   * persist after being sanitized on Android.
+   *
+   * @return {Promise}
+   * @resolves When _store.save() completes.
+   */
+  forceSave() {
+    if (this._store) {
+      return this._store.save();
+    }
+    return Promise.resolve();
+  },
+};
+
+var DownloadObserver = {
+  /**
+   * Flag to determine if the observers have been added previously.
+   */
+  observersAdded: false,
+
+  /**
+   * Timer used to delay restarting canceled downloads upon waking and returning
+   * online.
+   */
+  _wakeTimer: null,
+
+  /**
+   * Set that contains the in progress publics downloads.
+   * It's kept updated when a public download is added, removed or changes its
+   * properties.
+   */
+  _publicInProgressDownloads: new Set(),
+
+  /**
+   * Set that contains the in progress private downloads.
+   * It's kept updated when a private download is added, removed or changes its
+   * properties.
+   */
+  _privateInProgressDownloads: new Set(),
+
+  /**
+   * Set that contains the downloads that have been canceled when going offline
+   * or to sleep. These are started again when returning online or waking. This
+   * list is not persisted so when exiting and restarting, the downloads will not
+   * be started again.
+   */
+  _canceledOfflineDownloads: new Set(),
+
+  /**
+   * Registers a view that updates the corresponding downloads state set, based
+   * on the aIsPrivate argument. The set is updated when a download is added,
+   * removed or changes its properties.
+   *
+   * @param aList
+   *        The public or private downloads list.
+   * @param aIsPrivate
+   *        True if the list is private, false otherwise.
+   */
+  registerView: function DO_registerView(aList, aIsPrivate) {
+    let downloadsSet = aIsPrivate
+      ? this._privateInProgressDownloads
+      : this._publicInProgressDownloads;
+    let downloadsView = {
+      onDownloadAdded: aDownload => {
+        if (!aDownload.stopped) {
+          downloadsSet.add(aDownload);
+        }
+      },
+      onDownloadChanged: aDownload => {
+        if (aDownload.stopped) {
+          downloadsSet.delete(aDownload);
+        } else {
+          downloadsSet.add(aDownload);
+        }
+      },
+      onDownloadRemoved: aDownload => {
+        downloadsSet.delete(aDownload);
+        // The download must also be removed from the canceled when offline set.
+        this._canceledOfflineDownloads.delete(aDownload);
+      },
+    };
+
+    // We register the view asynchronously.
+    aList.addView(downloadsView).catch(console.error);
+  },
+
+  /**
+   * Wrapper that handles the test mode before calling the prompt that display
+   * a warning message box that informs that there are active downloads,
+   * and asks whether the user wants to cancel them or not.
+   *
+   * @param aCancel
+   *        The observer notification subject.
+   * @param aDownloadsCount
+   *        The current downloads count.
+   * @param aPrompter
+   *        The prompter object that shows the confirm dialog.
+   * @param aPromptType
+   *        The type of prompt notification depending on the observer.
+   */
+  _confirmCancelDownloads: function DO_confirmCancelDownload(
+    aCancel,
+    aDownloadsCount,
+    aPromptType
+  ) {
+    // Handle test mode
+    if (lazy.gCombinedDownloadIntegration._testPromptDownloads) {
+      lazy.gCombinedDownloadIntegration._testPromptDownloads = aDownloadsCount;
+      return;
+    }
+
+    if (!aDownloadsCount) {
+      return;
+    }
+
+    // If user has already dismissed the request, then do nothing.
+    if (aCancel instanceof Ci.nsISupportsPRBool && aCancel.data) {
+      return;
+    }
+
+    let prompter = lazy.DownloadUIHelper.getPrompter();
+    aCancel.data = prompter.confirmCancelDownloads(
+      aDownloadsCount,
+      prompter[aPromptType]
+    );
+  },
+
+  /**
+   * Resume all downloads that were paused when going offline, used when waking
+   * from sleep or returning from being offline.
+   */
+  _resumeOfflineDownloads: function DO_resumeOfflineDownloads() {
+    this._wakeTimer = null;
+
+    for (let download of this._canceledOfflineDownloads) {
+      download.start().catch(() => {});
+    }
+    this._canceledOfflineDownloads.clear();
+  },
+
+  // nsIObserver
+  observe: function DO_observe(aSubject, aTopic, aData) {
+    let downloadsCount;
+    switch (aTopic) {
+      case "quit-application-requested":
+        downloadsCount =
+          this._publicInProgressDownloads.size +
+          this._privateInProgressDownloads.size;
+        this._confirmCancelDownloads(aSubject, downloadsCount, "ON_QUIT");
+        break;
+      case "offline-requested":
+        downloadsCount =
+          this._publicInProgressDownloads.size +
+          this._privateInProgressDownloads.size;
+        this._confirmCancelDownloads(aSubject, downloadsCount, "ON_OFFLINE");
+        break;
+      case "last-pb-context-exiting":
+        downloadsCount = this._privateInProgressDownloads.size;
+        this._confirmCancelDownloads(
+          aSubject,
+          downloadsCount,
+          "ON_LEAVE_PRIVATE_BROWSING"
+        );
+        break;
+      case "last-pb-context-exited":
+        let promise = (async function () {
+          let list = await Downloads.getList(Downloads.PRIVATE);
+          let downloads = await list.getAll();
+
+          // We can remove the downloads and finalize them in parallel.
+          for (let download of downloads) {
+            list.remove(download).catch(console.error);
+            download.finalize(true).catch(console.error);
+          }
+        })();
+        // Handle test mode
+        if (lazy.gCombinedDownloadIntegration._testResolveClearPrivateList) {
+          lazy.gCombinedDownloadIntegration._testResolveClearPrivateList(
+            promise
+          );
+        } else {
+          promise.catch(ex => console.error(ex));
+        }
+        break;
+      case "sleep_notification":
+      case "suspend_process_notification":
+      case "network:offline-about-to-go-offline":
+        // Ignore shutdown notification so aborted downloads will be restarted
+        // on the next session.
+        if (
+          Services.startup.isInOrBeyondShutdownPhase(
+            Ci.nsIAppStartup.SHUTDOWN_PHASE_APPSHUTDOWNCONFIRMED
+          )
+        ) {
+          break;
+        }
+        for (let download of this._publicInProgressDownloads) {
+          download.cancel();
+          this._canceledOfflineDownloads.add(download);
+        }
+        for (let download of this._privateInProgressDownloads) {
+          download.cancel();
+          this._canceledOfflineDownloads.add(download);
+        }
+        break;
+      case "wake_notification":
+      case "resume_process_notification":
+        let wakeDelay = Services.prefs.getIntPref(
+          "browser.download.manager.resumeOnWakeDelay",
+          10000
+        );
+
+        if (wakeDelay >= 0) {
+          this._wakeTimer = new Timer(
+            this._resumeOfflineDownloads.bind(this),
+            wakeDelay,
+            Ci.nsITimer.TYPE_ONE_SHOT
+          );
+        }
+        break;
+      case "network:offline-status-changed":
+        if (aData == "online") {
+          this._resumeOfflineDownloads();
+        }
+        break;
+      // We need to unregister observers explicitly before we reach the
+      // "xpcom-shutdown" phase, otherwise observers may be notified when some
+      // required services are not available anymore. We can't unregister
+      // observers on "quit-application", because this module is also loaded
+      // during "make package" automation, and the quit notification is not sent
+      // in that execution environment (bug 973637).
+      case "xpcom-will-shutdown":
+        for (let topic of kObserverTopics) {
+          Services.obs.removeObserver(this, topic);
+        }
+        break;
+      case "blocked-automatic-download":
+        if (AppConstants.MOZ_BUILD_APP == "browser") {
+          DownloadIntegration._initializeDownloadSpamProtection();
+          DownloadIntegration.downloadSpamProtection.update(
+            aData,
+            aSubject.topChromeWindow
+          );
+        }
+        break;
+    }
+  },
+
+  QueryInterface: ChromeUtils.generateQI(["nsIObserver"]),
+};
+
+/**
+ * Registers a Places observer so that operations on download history are
+ * reflected on the provided list of downloads.
+ *
+ * You do not need to keep a reference to this object in order to keep it alive,
+ * because the history service already keeps a strong reference to it.
+ *
+ * @param aList
+ *        DownloadList object linked to this observer.
+ */
+var DownloadHistoryObserver = function (aList) {
+  this._list = aList;
+
+  const placesObserver = new PlacesWeakCallbackWrapper(
+    this.handlePlacesEvents.bind(this)
+  );
+  PlacesObservers.addListener(
+    ["history-cleared", "page-removed"],
+    placesObserver
+  );
+};
+
+DownloadHistoryObserver.prototype = {
+  /**
+   * DownloadList object linked to this observer.
+   */
+  _list: null,
+
+  handlePlacesEvents(events) {
+    for (const event of events) {
+      switch (event.type) {
+        case "history-cleared": {
+          this._list.removeFinished();
+          break;
+        }
+        case "page-removed": {
+          if (event.isRemovedFromStore) {
+            this._list.removeFinished(
+              download => event.url === download.source.url
+            );
+          }
+          break;
+        }
+      }
+    }
+  },
+};
+
+/**
+ * This view can be added to a DownloadList object to trigger a save operation
+ * in the given DownloadStore object when a relevant change occurs.  You should
+ * call the "initialize" method in order to register the view and load the
+ * current state from disk.
+ *
+ * You do not need to keep a reference to this object in order to keep it alive,
+ * because the DownloadList object already keeps a strong reference to it.
+ *
+ * @param aList
+ *        The DownloadList object on which the view should be registered.
+ * @param aStore
+ *        The DownloadStore object used for saving.
+ */
+var DownloadAutoSaveView = function (aList, aStore) {
+  this._list = aList;
+  this._store = aStore;
+  this._downloadsMap = new Map();
+  this._writer = new lazy.DeferredTask(() => this._store.save(), kSaveDelayMs);
+  lazy.AsyncShutdown.profileBeforeChange.addBlocker(
+    "DownloadAutoSaveView: writing data",
+    () => this._writer.finalize()
+  );
+};
+
+DownloadAutoSaveView.prototype = {
+  /**
+   * DownloadList object linked to this view.
+   */
+  _list: null,
+
+  /**
+   * The DownloadStore object used for saving.
+   */
+  _store: null,
+
+  /**
+   * True when the initial state of the downloads has been loaded.
+   */
+  _initialized: false,
+
+  /**
+   * Registers the view and loads the current state from disk.
+   *
+   * @return {Promise}
+   * @resolves When the view has been registered.
+   * @rejects JavaScript exception.
+   */
+  initialize() {
+    // We set _initialized to true after adding the view, so that
+    // onDownloadAdded doesn't cause a save to occur.
+    return this._list.addView(this).then(() => (this._initialized = true));
+  },
+
+  /**
+   * This map contains only Download objects that should be saved to disk, and
+   * associates them with the result of their getSerializationHash function, for
+   * the purpose of detecting changes to the relevant properties.
+   */
+  _downloadsMap: null,
+
+  /**
+   * DeferredTask for the save operation.
+   */
+  _writer: null,
+
+  /**
+   * Called when the list of downloads changed, this triggers the asynchronous
+   * serialization of the list of downloads.
+   */
+  saveSoon() {
+    this._writer.arm();
+  },
+
+  // DownloadList callback
+  onDownloadAdded(aDownload) {
+    if (lazy.gCombinedDownloadIntegration.shouldPersistDownload(aDownload)) {
+      this._downloadsMap.set(aDownload, aDownload.getSerializationHash());
+      if (this._initialized) {
+        this.saveSoon();
+      }
+    }
+  },
+
+  // DownloadList callback
+  onDownloadChanged(aDownload) {
+    if (!lazy.gCombinedDownloadIntegration.shouldPersistDownload(aDownload)) {
+      if (this._downloadsMap.has(aDownload)) {
+        this._downloadsMap.delete(aDownload);
+        this.saveSoon();
+      }
+      return;
+    }
+
+    let hash = aDownload.getSerializationHash();
+    if (this._downloadsMap.get(aDownload) != hash) {
+      this._downloadsMap.set(aDownload, hash);
+      this.saveSoon();
+    }
+  },
+
+  // DownloadList callback
+  onDownloadRemoved(aDownload) {
+    if (this._downloadsMap.has(aDownload)) {
+      this._downloadsMap.delete(aDownload);
+      this.saveSoon();
+    }
+  },
+};
diff --git a/toolkit/components/downloads/DownloadLegacy.sys.mjs b/toolkit/components/downloads/DownloadLegacy.sys.mjs
new file mode 100644
index 0000000000..ae38b0d26c
--- /dev/null
+++ b/toolkit/components/downloads/DownloadLegacy.sys.mjs
@@ -0,0 +1,503 @@
+/* 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/. */
+
+/**
+ * This component implements the XPCOM interfaces required for integration with
+ * the legacy download components.
+ *
+ * New code is expected to use the "Downloads.sys.mjs" module directly, without
+ * going through the interfaces implemented in this XPCOM component.  These
+ * interfaces are only maintained for backwards compatibility with components
+ * that still work synchronously on the main thread.
+ */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  DownloadError: "resource://gre/modules/DownloadCore.sys.mjs",
+  Downloads: "resource://gre/modules/Downloads.sys.mjs",
+});
+
+/**
+ * nsITransfer implementation that provides a bridge to a Download object.
+ *
+ * Legacy downloads work differently than the JavaScript implementation.  In the
+ * latter, the caller only provides the properties for the Download object and
+ * the entire process is handled by the "start" method.  In the legacy
+ * implementation, the caller must create a separate object to execute the
+ * download, and then make the download visible to the user by hooking it up to
+ * an nsITransfer instance.
+ *
+ * Since nsITransfer instances may be created before the download system is
+ * initialized, and initialization as well as other operations are asynchronous,
+ * this implementation is able to delay all progress and status notifications it
+ * receives until the associated Download object is finally created.
+ *
+ * Conversely, the DownloadLegacySaver object can also receive execution and
+ * cancellation requests asynchronously, before or after it is connected to
+ * this nsITransfer instance.  For that reason, those requests are communicated
+ * in a potentially deferred way, using promise objects.
+ *
+ * The component that executes the download implements nsICancelable to receive
+ * cancellation requests, but after cancellation it cannot be reused again.
+ *
+ * Since the components that execute the download may be different and they
+ * don't always give consistent results, this bridge takes care of enforcing the
+ * expectations, for example by ensuring the target file exists when the
+ * download is successful, even if the source has a size of zero bytes.
+ */
+export function DownloadLegacyTransfer() {
+  this._promiseDownload = new Promise(r => (this._resolveDownload = r));
+}
+
+DownloadLegacyTransfer.prototype = {
+  classID: Components.ID("{1b4c85df-cbdd-4bb6-b04e-613caece083c}"),
+
+  QueryInterface: ChromeUtils.generateQI([
+    "nsIWebProgressListener",
+    "nsIWebProgressListener2",
+    "nsITransfer",
+  ]),
+
+  // nsIWebProgressListener
+  onStateChange: function DLT_onStateChange(
+    aWebProgress,
+    aRequest,
+    aStateFlags,
+    aStatus
+  ) {
+    if (!Components.isSuccessCode(aStatus)) {
+      this._componentFailed = true;
+    }
+
+    if (
+      aStateFlags & Ci.nsIWebProgressListener.STATE_START &&
+      aStateFlags & Ci.nsIWebProgressListener.STATE_IS_NETWORK
+    ) {
+      let blockedByParentalControls = false;
+      // If it is a failed download, aRequest.responseStatus doesn't exist.
+      // (missing file on the server, network failure to download)
+      try {
+        // If the request's response has been blocked by Windows Parental Controls
+        // with an HTTP 450 error code, we must cancel the request synchronously.
+        blockedByParentalControls =
+          aRequest instanceof Ci.nsIHttpChannel &&
+          aRequest.responseStatus == 450;
+      } catch (e) {
+        if (e.result == Cr.NS_ERROR_NOT_AVAILABLE) {
+          aRequest.cancel(Cr.NS_BINDING_ABORTED);
+        }
+      }
+
+      if (blockedByParentalControls) {
+        aRequest.cancel(Cr.NS_BINDING_ABORTED);
+      }
+
+      // The main request has just started.  Wait for the associated Download
+      // object to be available before notifying.
+      this._promiseDownload
+        .then(download => {
+          // If the request was blocked, now that we have the download object we
+          // should set a flag that can be retrieved later when handling the
+          // cancellation so that the proper error can be thrown.
+          if (blockedByParentalControls) {
+            download._blockedByParentalControls = true;
+          }
+
+          download.saver.onTransferStarted(aRequest);
+
+          // To handle asynchronous cancellation properly, we should hook up the
+          // handler only after we have been notified that the main request
+          // started.  We will wait until the main request stopped before
+          // notifying that the download has been canceled.  Since the request has
+          // not completed yet, deferCanceled is guaranteed to be set.
+          return download.saver.deferCanceled.promise.then(() => {
+            // Only cancel if the object executing the download is still running.
+            if (this._cancelable && !this._componentFailed) {
+              this._cancelable.cancel(Cr.NS_ERROR_ABORT);
+            }
+          });
+        })
+        .catch(console.error);
+    } else if (
+      aStateFlags & Ci.nsIWebProgressListener.STATE_STOP &&
+      aStateFlags & Ci.nsIWebProgressListener.STATE_IS_NETWORK
+    ) {
+      // The last file has been received, or the download failed.  Wait for the
+      // associated Download object to be available before notifying.
+      this._promiseDownload
+        .then(download => {
+          // At this point, the hash has been set and we need to copy it to the
+          // DownloadSaver.
+          if (Components.isSuccessCode(aStatus)) {
+            download.saver.setSha256Hash(this._sha256Hash);
+            download.saver.setSignatureInfo(this._signatureInfo);
+            download.saver.setRedirects(this._redirects);
+          }
+          download.saver.onTransferFinished(aStatus);
+        })
+        .catch(console.error);
+
+      // Release the reference to the component executing the download.
+      this._cancelable = null;
+    }
+  },
+
+  // nsIWebProgressListener
+  onProgressChange: function DLT_onProgressChange(
+    aWebProgress,
+    aRequest,
+    aCurSelfProgress,
+    aMaxSelfProgress,
+    aCurTotalProgress,
+    aMaxTotalProgress
+  ) {
+    this.onProgressChange64(
+      aWebProgress,
+      aRequest,
+      aCurSelfProgress,
+      aMaxSelfProgress,
+      aCurTotalProgress,
+      aMaxTotalProgress
+    );
+  },
+
+  onLocationChange() {},
+
+  // nsIWebProgressListener
+  onStatusChange: function DLT_onStatusChange(
+    aWebProgress,
+    aRequest,
+    aStatus,
+    aMessage
+  ) {
+    // The status change may optionally be received in addition to the state
+    // change, but if no network request actually started, it is possible that
+    // we only receive a status change with an error status code.
+    if (!Components.isSuccessCode(aStatus)) {
+      this._componentFailed = true;
+
+      // Wait for the associated Download object to be available.
+      this._promiseDownload
+        .then(download => {
+          download.saver.onTransferFinished(aStatus);
+        })
+        .catch(console.error);
+    }
+  },
+
+  onSecurityChange() {},
+
+  onContentBlockingEvent() {},
+
+  // nsIWebProgressListener2
+  onProgressChange64: function DLT_onProgressChange64(
+    aWebProgress,
+    aRequest,
+    aCurSelfProgress,
+    aMaxSelfProgress,
+    aCurTotalProgress,
+    aMaxTotalProgress
+  ) {
+    // Since this progress function is invoked frequently, we use a slightly
+    // more complex solution that optimizes the case where we already have an
+    // associated Download object, avoiding the Promise overhead.
+    if (this._download) {
+      this._hasDelayedProgress = false;
+      this._download.saver.onProgressBytes(
+        aCurTotalProgress,
+        aMaxTotalProgress
+      );
+      return;
+    }
+
+    // If we don't have a Download object yet, store the most recent progress
+    // notification to send later. We must do this because there is no guarantee
+    // that a future notification will be sent if the download stalls.
+    this._delayedCurTotalProgress = aCurTotalProgress;
+    this._delayedMaxTotalProgress = aMaxTotalProgress;
+
+    // Do not enqueue multiple callbacks for the progress report.
+    if (this._hasDelayedProgress) {
+      return;
+    }
+    this._hasDelayedProgress = true;
+
+    this._promiseDownload
+      .then(download => {
+        // Check whether an immediate progress report has been already processed
+        // before we could send the delayed progress report.
+        if (!this._hasDelayedProgress) {
+          return;
+        }
+        download.saver.onProgressBytes(
+          this._delayedCurTotalProgress,
+          this._delayedMaxTotalProgress
+        );
+      })
+      .catch(console.error);
+  },
+  _hasDelayedProgress: false,
+  _delayedCurTotalProgress: 0,
+  _delayedMaxTotalProgress: 0,
+
+  // nsIWebProgressListener2
+  onRefreshAttempted: function DLT_onRefreshAttempted(
+    aWebProgress,
+    aRefreshURI,
+    aMillis,
+    aSameURI
+  ) {
+    // Indicate that refreshes and redirects are allowed by default.  However,
+    // note that download components don't usually call this method at all.
+    return true;
+  },
+
+  // nsITransfer
+  init: function DLT_init(
+    aSource,
+    aSourceOriginalURI,
+    aTarget,
+    aDisplayName,
+    aMIMEInfo,
+    aStartTime,
+    aTempFile,
+    aCancelable,
+    aIsPrivate,
+    aDownloadClassification,
+    aReferrerInfo,
+    aOpenDownloadsListOnStart
+  ) {
+    return this._nsITransferInitInternal(
+      aSource,
+      aSourceOriginalURI,
+      aTarget,
+      aDisplayName,
+      aMIMEInfo,
+      aStartTime,
+      aTempFile,
+      aCancelable,
+      aIsPrivate,
+      aDownloadClassification,
+      aReferrerInfo,
+      aOpenDownloadsListOnStart
+    );
+  },
+
+  // nsITransfer
+  initWithBrowsingContext(
+    aSource,
+    aTarget,
+    aDisplayName,
+    aMIMEInfo,
+    aStartTime,
+    aTempFile,
+    aCancelable,
+    aIsPrivate,
+    aDownloadClassification,
+    aReferrerInfo,
+    aOpenDownloadsListOnStart,
+    aBrowsingContext,
+    aHandleInternally,
+    aHttpChannel
+  ) {
+    let browsingContextId;
+    let userContextId;
+    if (aBrowsingContext && aBrowsingContext.currentWindowGlobal) {
+      browsingContextId = aBrowsingContext.id;
+      let windowGlobal = aBrowsingContext.currentWindowGlobal;
+      let originAttributes = windowGlobal.documentPrincipal.originAttributes;
+      userContextId = originAttributes.userContextId;
+    }
+    return this._nsITransferInitInternal(
+      aSource,
+      null,
+      aTarget,
+      aDisplayName,
+      aMIMEInfo,
+      aStartTime,
+      aTempFile,
+      aCancelable,
+      aIsPrivate,
+      aDownloadClassification,
+      aReferrerInfo,
+      aOpenDownloadsListOnStart,
+      userContextId,
+      browsingContextId,
+      aHandleInternally,
+      aHttpChannel
+    );
+  },
+
+  _nsITransferInitInternal(
+    aSource,
+    aSourceOriginalURI,
+    aTarget,
+    aDisplayName,
+    aMIMEInfo,
+    aStartTime,
+    aTempFile,
+    aCancelable,
+    isPrivate,
+    aDownloadClassification,
+    referrerInfo,
+    openDownloadsListOnStart = true,
+    userContextId = 0,
+    browsingContextId = 0,
+    handleInternally = false,
+    aHttpChannel = null
+  ) {
+    this._cancelable = aCancelable;
+    let launchWhenSucceeded = false,
+      contentType = null,
+      launcherPath = null;
+
+    if (aMIMEInfo instanceof Ci.nsIMIMEInfo) {
+      launchWhenSucceeded =
+        aMIMEInfo.preferredAction != Ci.nsIMIMEInfo.saveToDisk;
+      contentType = aMIMEInfo.type;
+
+      let appHandler = aMIMEInfo.preferredApplicationHandler;
+      if (
+        aMIMEInfo.preferredAction == Ci.nsIMIMEInfo.useHelperApp &&
+        appHandler instanceof Ci.nsILocalHandlerApp
+      ) {
+        launcherPath = appHandler.executable.path;
+      }
+    }
+    // Create a new Download object associated to a DownloadLegacySaver, and
+    // wait for it to be available.  This operation may cause the entire
+    // download system to initialize before the object is created.
+    let authHeader = null;
+    if (aHttpChannel) {
+      try {
+        authHeader = aHttpChannel.getRequestHeader("Authorization");
+      } catch (e) {}
+    }
+    let serialisedDownload = {
+      source: {
+        url: aSource.spec,
+        originalUrl: aSourceOriginalURI && aSourceOriginalURI.spec,
+        isPrivate,
+        userContextId,
+        browsingContextId,
+        referrerInfo,
+        authHeader,
+      },
+      target: {
+        path: aTarget.QueryInterface(Ci.nsIFileURL).file.path,
+        partFilePath: aTempFile && aTempFile.path,
+      },
+      saver: "legacy",
+      launchWhenSucceeded,
+      contentType,
+      launcherPath,
+      handleInternally,
+      openDownloadsListOnStart,
+    };
+
+    // In case the Download was classified as insecure/dangerous
+    // it is already canceled, so we need to generate and attach the
+    // corresponding error to the download.
+    if (aDownloadClassification == Ci.nsITransfer.DOWNLOAD_POTENTIALLY_UNSAFE) {
+      Services.telemetry
+        .getKeyedHistogramById("DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD")
+        .add(lazy.DownloadError.BLOCK_VERDICT_INSECURE, 0);
+
+      serialisedDownload.errorObj = {
+        becauseBlockedByReputationCheck: true,
+        reputationCheckVerdict: lazy.DownloadError.BLOCK_VERDICT_INSECURE,
+      };
+      // hasBlockedData needs to be true
+      // because the unblock UI is hidden if there is
+      // no data to be unblocked.
+      serialisedDownload.hasBlockedData = true;
+      // We cannot use the legacy saver here, as the original channel
+      // is already closed. A copy saver would create a new channel once
+      // start() is called.
+      serialisedDownload.saver = "copy";
+
+      // Since the download is canceled already, we do not need to keep refrences
+      this._download = null;
+      this._cancelable = null;
+    }
+
+    lazy.Downloads.createDownload(serialisedDownload)
+      .then(async aDownload => {
+        // Legacy components keep partial data when they use a ".part" file.
+        if (aTempFile) {
+          aDownload.tryToKeepPartialData = true;
+        }
+
+        // Start the download before allowing it to be controlled.  Ignore errors.
+        aDownload.start().catch(() => {});
+
+        // Start processing all the other events received through nsITransfer.
+        this._download = aDownload;
+        this._resolveDownload(aDownload);
+
+        // Add the download to the list, allowing it to be seen and canceled.
+        await (await lazy.Downloads.getList(lazy.Downloads.ALL)).add(aDownload);
+        if (serialisedDownload.errorObj) {
+          // In case we added an already canceled dummy download
+          // we need to manually trigger a change event
+          // as all the animations for finishing downloads are
+          // listening on onChange.
+          aDownload._notifyChange();
+        }
+      })
+      .catch(console.error);
+  },
+
+  setSha256Hash(hash) {
+    this._sha256Hash = hash;
+  },
+
+  setSignatureInfo(signatureInfo) {
+    this._signatureInfo = signatureInfo;
+  },
+
+  setRedirects(redirects) {
+    this._redirects = redirects;
+  },
+
+  /**
+   * Download object associated with this nsITransfer instance. This is not
+   * available immediately when the nsITransfer instance is created.
+   */
+  _download: null,
+
+  /**
+   * Promise that resolves to the Download object associated with this
+   * nsITransfer instance after the _resolveDownload method is invoked.
+   *
+   * Waiting on this promise using "then" ensures that the callbacks are invoked
+   * in the correct order even if enqueued before the object is available.
+   */
+  _promiseDownload: null,
+  _resolveDownload: null,
+
+  /**
+   * Reference to the component that is executing the download.  This component
+   * allows cancellation through its nsICancelable interface.
+   */
+  _cancelable: null,
+
+  /**
+   * Indicates that the component that executes the download has notified a
+   * failure condition.  In this case, we should never use the component methods
+   * that cancel the download.
+   */
+  _componentFailed: false,
+
+  /**
+   * Save the SHA-256 hash in raw bytes of the downloaded file.
+   */
+  _sha256Hash: null,
+
+  /**
+   * Save the signature info in a serialized protobuf of the downloaded file.
+   */
+  _signatureInfo: null,
+};
diff --git a/toolkit/components/downloads/DownloadList.sys.mjs b/toolkit/components/downloads/DownloadList.sys.mjs
new file mode 100644
index 0000000000..46f917d16b
--- /dev/null
+++ b/toolkit/components/downloads/DownloadList.sys.mjs
@@ -0,0 +1,668 @@
+/* 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/. */
+
+/**
+ * Provides collections of Download objects and aggregate views on them.
+ */
+
+const FILE_EXTENSIONS = [
+  "aac",
+  "adt",
+  "adts",
+  "accdb",
+  "accde",
+  "accdr",
+  "accdt",
+  "aif",
+  "aifc",
+  "aiff",
+  "apng",
+  "aspx",
+  "avi",
+  "avif",
+  "bat",
+  "bin",
+  "bmp",
+  "cab",
+  "cda",
+  "csv",
+  "dif",
+  "dll",
+  "doc",
+  "docm",
+  "docx",
+  "dot",
+  "dotx",
+  "eml",
+  "eps",
+  "exe",
+  "flac",
+  "flv",
+  "gif",
+  "htm",
+  "html",
+  "ico",
+  "ini",
+  "iso",
+  "jar",
+  "jfif",
+  "jpg",
+  "jpeg",
+  "json",
+  "m4a",
+  "mdb",
+  "mid",
+  "midi",
+  "mov",
+  "mp3",
+  "mp4",
+  "mpeg",
+  "mpg",
+  "msi",
+  "mui",
+  "oga",
+  "ogg",
+  "ogv",
+  "opus",
+  "pdf",
+  "pjpeg",
+  "pjp",
+  "png",
+  "pot",
+  "potm",
+  "potx",
+  "ppam",
+  "pps",
+  "ppsm",
+  "ppsx",
+  "ppt",
+  "pptm",
+  "pptx",
+  "psd",
+  "pst",
+  "pub",
+  "rar",
+  "rdf",
+  "rtf",
+  "shtml",
+  "sldm",
+  "sldx",
+  "svg",
+  "swf",
+  "sys",
+  "tif",
+  "tiff",
+  "tmp",
+  "txt",
+  "vob",
+  "vsd",
+  "vsdm",
+  "vsdx",
+  "vss",
+  "vssm",
+  "vst",
+  "vstm",
+  "vstx",
+  "wav",
+  "wbk",
+  "webm",
+  "webp",
+  "wks",
+  "wma",
+  "wmd",
+  "wmv",
+  "wmz",
+  "wms",
+  "wpd",
+  "wp5",
+  "xht",
+  "xhtml",
+  "xla",
+  "xlam",
+  "xll",
+  "xlm",
+  "xls",
+  "xlsm",
+  "xlsx",
+  "xlt",
+  "xltm",
+  "xltx",
+  "xml",
+  "zip",
+];
+
+const TELEMETRY_EVENT_CATEGORY = "downloads";
+
+/**
+ * Represents a collection of Download objects that can be viewed and managed by
+ * the user interface, and persisted across sessions.
+ */
+export class DownloadList {
+  constructor() {
+    /**
+     * Array of Download objects currently in the list.
+     */
+    this._downloads = [];
+
+    /**
+     * Set of currently registered views.
+     */
+    this._views = new Set();
+  }
+
+  /**
+   * Retrieves a snapshot of the downloads that are currently in the list.  The
+   * returned array does not change when downloads are added or removed, though
+   * the Download objects it contains are still updated in real time.
+   *
+   * @return {Promise}
+   * @resolves An array of Download objects.
+   * @rejects JavaScript exception.
+   */
+  async getAll() {
+    return Array.from(this._downloads);
+  }
+
+  /**
+   * Adds a new download to the end of the items list.
+   *
+   * @note When a download is added to the list, its "onchange" event is
+   *       registered by the list, thus it cannot be used to monitor the
+   *       download.  To receive change notifications for downloads that are
+   *       added to the list, use the addView method to register for
+   *       onDownloadChanged notifications.
+   *
+   * @param download
+   *        The Download object to add.
+   *
+   * @return {Promise}
+   * @resolves When the download has been added.
+   * @rejects JavaScript exception.
+   */
+  async add(download) {
+    this._downloads.push(download);
+    download.onchange = this._change.bind(this, download);
+    this._notifyAllViews("onDownloadAdded", download);
+  }
+
+  /**
+   * Removes a download from the list.  If the download was already removed,
+   * this method has no effect.
+   *
+   * This method does not change the state of the download, to allow adding it
+   * to another list, or control it directly.  If you want to dispose of the
+   * download object, you should cancel it afterwards, and remove any partially
+   * downloaded data if needed.
+   *
+   * @param download
+   *        The Download object to remove.
+   *
+   * @return {Promise}
+   * @resolves When the download has been removed.
+   * @rejects JavaScript exception.
+   */
+  async remove(download) {
+    let index = this._downloads.indexOf(download);
+    if (index != -1) {
+      this._downloads.splice(index, 1);
+      download.onchange = null;
+      this._notifyAllViews("onDownloadRemoved", download);
+    }
+  }
+
+  /**
+   * This function is called when "onchange" events of downloads occur.
+   *
+   * @param download
+   *        The Download object that changed.
+   */
+  _change(download) {
+    this._notifyAllViews("onDownloadChanged", download);
+  }
+
+  /**
+   * Adds a view that will be notified of changes to downloads.  The newly added
+   * view will receive onDownloadAdded notifications for all the downloads that
+   * are already in the list.
+   *
+   * @param view
+   *        The view object to add.  The following methods may be defined:
+   *        {
+   *          onDownloadAdded: function (download) {
+   *            // Called after download is added to the end of the list.
+   *          },
+   *          onDownloadChanged: function (download) {
+   *            // Called after the properties of download change.
+   *          },
+   *          onDownloadRemoved: function (download) {
+   *            // Called after download is removed from the list.
+   *          },
+   *          onDownloadBatchStarting: function () {
+   *            // Called before multiple changes are made at the same time.
+   *          },
+   *          onDownloadBatchEnded: function () {
+   *            // Called after all the changes have been made.
+   *          },
+   *        }
+   *
+   * @return {Promise}
+   * @resolves When the view has been registered and all the onDownloadAdded
+   *           notifications for the existing downloads have been sent.
+   * @rejects JavaScript exception.
+   */
+  async addView(view) {
+    this._views.add(view);
+
+    if ("onDownloadAdded" in view) {
+      this._notifyAllViews("onDownloadBatchStarting");
+      for (let download of this._downloads) {
+        try {
+          view.onDownloadAdded(download);
+        } catch (ex) {
+          console.error(ex);
+        }
+      }
+      this._notifyAllViews("onDownloadBatchEnded");
+    }
+  }
+
+  /**
+   * Removes a view that was previously added using addView.
+   *
+   * @param view
+   *        The view object to remove.
+   *
+   * @return {Promise}
+   * @resolves When the view has been removed.  At this point, the removed view
+   *           will not receive any more notifications.
+   * @rejects JavaScript exception.
+   */
+  async removeView(view) {
+    this._views.delete(view);
+  }
+
+  /**
+   * Notifies all the views of a download addition, change, removal, or other
+   * event. The additional arguments are passed to the called method.
+   *
+   * @param methodName
+   *        String containing the name of the method to call on the view.
+   */
+  _notifyAllViews(methodName, ...args) {
+    for (let view of this._views) {
+      try {
+        if (methodName in view) {
+          view[methodName](...args);
+        }
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+  }
+
+  /**
+   * Removes downloads from the list that have finished, have failed, or have
+   * been canceled without keeping partial data.  A filter function may be
+   * specified to remove only a subset of those downloads.
+   *
+   * This method finalizes each removed download, ensuring that any partially
+   * downloaded data associated with it is also removed.
+   *
+   * @param filterFn
+   *        The filter function is called with each download as its only
+   *        argument, and should return true to remove the download and false
+   *        to keep it.  This parameter may be null or omitted to have no
+   *        additional filter.
+   */
+  removeFinished(filterFn) {
+    (async () => {
+      let list = await this.getAll();
+      for (let download of list) {
+        // Remove downloads that have been canceled, even if the cancellation
+        // operation hasn't completed yet so we don't check "stopped" here.
+        // Failed downloads with partial data are also removed.
+        if (
+          download.stopped &&
+          (!download.hasPartialData || download.error) &&
+          (!filterFn || filterFn(download))
+        ) {
+          // Remove the download first, so that the views don't get the change
+          // notifications that may occur during finalization.
+          await this.remove(download);
+          // Find if a file with the same path is also downloading.
+          let sameFileIsDownloading = false;
+          for (let otherDownload of await this.getAll()) {
+            if (
+              download !== otherDownload &&
+              download.target.path == otherDownload.target.path &&
+              !otherDownload.error
+            ) {
+              sameFileIsDownloading = true;
+            }
+          }
+          // Ensure that the download is stopped and no partial data is kept.
+          // This works even if the download state has changed meanwhile.  We
+          // don't need to wait for the procedure to be complete before
+          // processing the other downloads in the list.
+          let removePartialData = !sameFileIsDownloading;
+          download.finalize(removePartialData).catch(console.error);
+        }
+      }
+    })().catch(console.error);
+  }
+}
+
+/**
+ * Provides a unified, unordered list combining public and private downloads.
+ *
+ * Download objects added to this list are also added to one of the two
+ * underlying lists, based on their "source.isPrivate" property.  Views on this
+ * list will receive notifications for both public and private downloads.
+ *
+ * @param publicList
+ *        Underlying DownloadList containing public downloads.
+ * @param privateList
+ *        Underlying DownloadList containing private downloads.
+ */
+export class DownloadCombinedList extends DownloadList {
+  constructor(publicList, privateList) {
+    super();
+
+    /**
+     * Underlying DownloadList containing public downloads.
+     */
+    this._publicList = publicList;
+
+    /**
+     * Underlying DownloadList containing private downloads.
+     */
+    this._privateList = privateList;
+
+    publicList.addView(this).catch(console.error);
+    privateList.addView(this).catch(console.error);
+  }
+
+  /**
+   * Adds a new download to the end of the items list.
+   *
+   * @note When a download is added to the list, its "onchange" event is
+   *       registered by the list, thus it cannot be used to monitor the
+   *       download.  To receive change notifications for downloads that are
+   *       added to the list, use the addView method to register for
+   *       onDownloadChanged notifications.
+   *
+   * @param download
+   *        The Download object to add.
+   *
+   * @return {Promise}
+   * @resolves When the download has been added.
+   * @rejects JavaScript exception.
+   */
+  add(download) {
+    let extension = download.target.path.split(".").pop();
+
+    if (!FILE_EXTENSIONS.includes(extension)) {
+      extension = "other";
+    }
+
+    try {
+      Services.telemetry.recordEvent(
+        TELEMETRY_EVENT_CATEGORY,
+        "added",
+        "fileExtension",
+        extension,
+        {}
+      );
+    } catch (ex) {
+      console.error(
+        "DownloadsCommon: error recording telemetry event.",
+        ex.message
+      );
+    }
+
+    if (download.source.isPrivate) {
+      return this._privateList.add(download);
+    }
+
+    return this._publicList.add(download);
+  }
+
+  /**
+   * Removes a download from the list.  If the download was already removed,
+   * this method has no effect.
+   *
+   * This method does not change the state of the download, to allow adding it
+   * to another list, or control it directly.  If you want to dispose of the
+   * download object, you should cancel it afterwards, and remove any partially
+   * downloaded data if needed.
+   *
+   * @param download
+   *        The Download object to remove.
+   *
+   * @return {Promise}
+   * @resolves When the download has been removed.
+   * @rejects JavaScript exception.
+   */
+  remove(download) {
+    if (download.source.isPrivate) {
+      return this._privateList.remove(download);
+    }
+    return this._publicList.remove(download);
+  }
+
+  // DownloadList callback
+  onDownloadAdded(download) {
+    this._downloads.push(download);
+    this._notifyAllViews("onDownloadAdded", download);
+  }
+
+  // DownloadList callback
+  onDownloadChanged(download) {
+    this._notifyAllViews("onDownloadChanged", download);
+  }
+
+  // DownloadList callback
+  onDownloadRemoved(download) {
+    let index = this._downloads.indexOf(download);
+    if (index != -1) {
+      this._downloads.splice(index, 1);
+    }
+    this._notifyAllViews("onDownloadRemoved", download);
+  }
+}
+/**
+ * Provides an aggregated view on the contents of a DownloadList.
+ */
+export class DownloadSummary {
+  constructor() {
+    /**
+     * Array of Download objects that are currently part of the summary.
+     */
+    this._downloads = [];
+
+    /**
+     * Set of currently registered views.
+     */
+    this._views = new Set();
+  }
+
+  /**
+   * Underlying DownloadList whose contents should be summarized.
+   */
+  _list = null;
+
+  /**
+   * Indicates whether all the downloads are currently stopped.
+   */
+  allHaveStopped = true;
+
+  /**
+   * Indicates whether whether all downloads have an unknown final size.
+   */
+  allUnknownSize = true;
+
+  /**
+   * Indicates the total number of bytes to be transferred before completing all
+   * the downloads that are currently in progress.
+   *
+   * For downloads that do not have a known final size, the number of bytes
+   * currently transferred is reported as part of this property.
+   *
+   * This is zero if no downloads are currently in progress.
+   */
+  progressTotalBytes = 0;
+
+  /**
+   * Number of bytes currently transferred as part of all the downloads that are
+   * currently in progress.
+   *
+   * This is zero if no downloads are currently in progress.
+   */
+  progressCurrentBytes = 0;
+
+  /**
+   * This method may be called once to bind this object to a DownloadList.
+   *
+   * Views on the summarized data can be registered before this object is bound
+   * to an actual list.  This allows the summary to be used without requiring
+   * the initialization of the DownloadList first.
+   *
+   * @param list
+   *        Underlying DownloadList whose contents should be summarized.
+   *
+   * @return {Promise}
+   * @resolves When the view on the underlying list has been registered.
+   * @rejects JavaScript exception.
+   */
+  async bindToList(list) {
+    if (this._list) {
+      throw new Error("bindToList may be called only once.");
+    }
+
+    await list.addView(this);
+    // Set the list reference only after addView has returned, so that we don't
+    // send a notification to our views for each download that is added.
+    this._list = list;
+    this._onListChanged();
+  }
+
+  /**
+   * Adds a view that will be notified of changes to the summary.  The newly
+   * added view will receive an initial onSummaryChanged notification.
+   *
+   * @param view
+   *        The view object to add.  The following methods may be defined:
+   *        {
+   *          onSummaryChanged: function () {
+   *            // Called after any property of the summary has changed.
+   *          },
+   *        }
+   *
+   * @return {Promise}
+   * @resolves When the view has been registered and the onSummaryChanged
+   *           notification has been sent.
+   * @rejects JavaScript exception.
+   */
+  async addView(view) {
+    this._views.add(view);
+
+    if ("onSummaryChanged" in view) {
+      try {
+        view.onSummaryChanged();
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+  }
+
+  /**
+   * Removes a view that was previously added using addView.
+   *
+   * @param view
+   *        The view object to remove.
+   *
+   * @return {Promise}
+   * @resolves When the view has been removed.  At this point, the removed view
+   *           will not receive any more notifications.
+   * @rejects JavaScript exception.
+   */
+  async removeView(view) {
+    this._views.delete(view);
+  }
+
+  /**
+   * This function is called when any change in the list of downloads occurs,
+   * and will recalculate the summary and notify the views in case the
+   * aggregated properties are different.
+   */
+  _onListChanged() {
+    let allHaveStopped = true;
+    let allUnknownSize = true;
+    let progressTotalBytes = 0;
+    let progressCurrentBytes = 0;
+
+    // Recalculate the aggregated state.  See the description of the individual
+    // properties for an explanation of the summarization logic.
+    for (let download of this._downloads) {
+      if (!download.stopped) {
+        allHaveStopped = false;
+        if (download.hasProgress) {
+          allUnknownSize = false;
+          progressTotalBytes += download.totalBytes;
+        } else {
+          progressTotalBytes += download.currentBytes;
+        }
+        progressCurrentBytes += download.currentBytes;
+      }
+    }
+
+    // Exit now if the properties did not change.
+    if (
+      this.allHaveStopped == allHaveStopped &&
+      this.allUnknownSize == allUnknownSize &&
+      this.progressTotalBytes == progressTotalBytes &&
+      this.progressCurrentBytes == progressCurrentBytes
+    ) {
+      return;
+    }
+
+    this.allHaveStopped = allHaveStopped;
+    this.allUnknownSize = allUnknownSize;
+    this.progressTotalBytes = progressTotalBytes;
+    this.progressCurrentBytes = progressCurrentBytes;
+
+    // Notify all the views that our properties changed.
+    for (let view of this._views) {
+      try {
+        if ("onSummaryChanged" in view) {
+          view.onSummaryChanged();
+        }
+      } catch (ex) {
+        console.error(ex);
+      }
+    }
+  }
+
+  // DownloadList callback
+  onDownloadAdded(download) {
+    this._downloads.push(download);
+    if (this._list) {
+      this._onListChanged();
+    }
+  }
+
+  // DownloadList callback
+  onDownloadChanged(download) {
+    this._onListChanged();
+  }
+
+  // DownloadList callback
+  onDownloadRemoved(download) {
+    let index = this._downloads.indexOf(download);
+    if (index != -1) {
+      this._downloads.splice(index, 1);
+    }
+    this._onListChanged();
+  }
+}
diff --git a/toolkit/components/downloads/DownloadPaths.sys.mjs b/toolkit/components/downloads/DownloadPaths.sys.mjs
new file mode 100644
index 0000000000..f1e0a080c2
--- /dev/null
+++ b/toolkit/components/downloads/DownloadPaths.sys.mjs
@@ -0,0 +1,102 @@
+/* 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/. */
+
+/**
+ * Provides methods for giving names and paths to files being downloaded.
+ */
+
+export var DownloadPaths = {
+  /**
+   * Sanitizes an arbitrary string via mimeSvc.validateFileNameForSaving.
+   *
+   * If the filename being validated is one that was returned from a
+   * file picker, pass false for compressWhitespaces and true for
+   * allowInvalidFilenames. Otherwise, the default values of the arguments
+   * should generally be used.
+   *
+   * @param {string} leafName The full leaf name to sanitize
+   * @param {boolean} [compressWhitespaces] Whether consecutive whitespaces
+   *        should be compressed. The default value is true.
+   * @param {boolean} [allowInvalidFilenames] Allow invalid and dangerous
+   *        filenames and extensions as is.
+   */
+  sanitize(
+    leafName,
+    { compressWhitespaces = true, allowInvalidFilenames = false } = {}
+  ) {
+    const mimeSvc = Cc["@mozilla.org/mime;1"].getService(Ci.nsIMIMEService);
+
+    let flags = mimeSvc.VALIDATE_SANITIZE_ONLY | mimeSvc.VALIDATE_DONT_TRUNCATE;
+    if (!compressWhitespaces) {
+      flags |= mimeSvc.VALIDATE_DONT_COLLAPSE_WHITESPACE;
+    }
+    if (allowInvalidFilenames) {
+      flags |= mimeSvc.VALIDATE_ALLOW_INVALID_FILENAMES;
+    }
+    return mimeSvc.validateFileNameForSaving(leafName, "", flags);
+  },
+
+  /**
+   * Creates a uniquely-named file starting from the name of the provided file.
+   * If a file with the provided name already exists, the function attempts to
+   * create nice alternatives, like "base(1).ext" (instead of "base-1.ext").
+   *
+   * If a unique name cannot be found, the function throws the XPCOM exception
+   * NS_ERROR_FILE_TOO_BIG. Other exceptions, like NS_ERROR_FILE_ACCESS_DENIED,
+   * can also be expected.
+   *
+   * @param templateFile
+   *        nsIFile whose leaf name is going to be used as a template. The
+   *        provided object is not modified.
+   *
+   * @return A new instance of an nsIFile object pointing to the newly created
+   *         empty file. On platforms that support permission bits, the file is
+   *         created with permissions 644.
+   */
+  createNiceUniqueFile(templateFile) {
+    // Work on a clone of the provided template file object.
+    let curFile = templateFile.clone().QueryInterface(Ci.nsIFile);
+    let [base, ext] = DownloadPaths.splitBaseNameAndExtension(curFile.leafName);
+    // Try other file names, for example "base(1).txt" or "base(1).tar.gz",
+    // only if the file name initially set already exists.
+    for (let i = 1; i < 10000 && curFile.exists(); i++) {
+      curFile.leafName = base + "(" + i + ")" + ext;
+    }
+    // At this point we hand off control to createUnique, which will create the
+    // file with the name we chose, if it is valid. If not, createUnique will
+    // attempt to modify it again, for example it will shorten very long names
+    // that can't be created on some platforms, and for which a normal call to
+    // nsIFile.create would result in NS_ERROR_FILE_NOT_FOUND. This can result
+    // very rarely in strange names like "base(9999).tar-1.gz" or "ba-1.gz".
+    curFile.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, 0o644);
+    return curFile;
+  },
+
+  /**
+   * Separates the base name from the extension in a file name, recognizing some
+   * double extensions like ".tar.gz".
+   *
+   * @param leafName
+   *        The full leaf name to be parsed. Be careful when processing names
+   *        containing leading or trailing dots or spaces.
+   *
+   * @return [base, ext]
+   *         The base name of the file, which can be empty, and its extension,
+   *         which always includes the leading dot unless it's an empty string.
+   *         Concatenating the two items always results in the original name.
+   */
+  splitBaseNameAndExtension(leafName) {
+    // The following regular expression is built from these key parts:
+    //  .*?                      Matches the base name non-greedily.
+    //  \.[A-Z0-9]{1,3}          Up to three letters or numbers preceding a
+    //                           double extension.
+    //  \.(?:gz|bz2|Z)           The second part of common double extensions.
+    //  \.[^.]*                  Matches any extension or a single trailing dot.
+    let [, base, ext] = /(.*?)(\.[A-Z0-9]{1,3}\.(?:gz|bz2|Z)|\.[^.]*)?$/i.exec(
+      leafName
+    );
+    // Return an empty string instead of undefined if no extension is found.
+    return [base, ext || ""];
+  },
+};
diff --git a/toolkit/components/downloads/DownloadPlatform.cpp b/toolkit/components/downloads/DownloadPlatform.cpp
new file mode 100644
index 0000000000..ee2e0a3248
--- /dev/null
+++ b/toolkit/components/downloads/DownloadPlatform.cpp
@@ -0,0 +1,319 @@
+/* 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 "DownloadPlatform.h"
+#include "nsNetUtil.h"
+#include "nsString.h"
+#include "nsINestedURI.h"
+#include "nsIProtocolHandler.h"
+#include "nsIURI.h"
+#include "nsIFile.h"
+#include "xpcpublic.h"
+
+#include "mozilla/dom/Promise.h"
+#include "mozilla/Preferences.h"
+
+#define PREF_BDM_ADDTORECENTDOCS "browser.download.manager.addToRecentDocs"
+
+#ifdef XP_WIN
+#  include <shlobj.h>
+#  include <urlmon.h>
+#  include "nsILocalFileWin.h"
+#  include "WinTaskbar.h"
+#endif
+
+#ifdef XP_MACOSX
+#  include <CoreFoundation/CoreFoundation.h>
+#  include "../../../xpcom/io/CocoaFileUtils.h"
+#endif
+
+#ifdef MOZ_WIDGET_GTK
+#  include <gtk/gtk.h>
+#endif
+
+using namespace mozilla;
+using dom::Promise;
+
+DownloadPlatform* DownloadPlatform::gDownloadPlatformService = nullptr;
+
+NS_IMPL_ISUPPORTS(DownloadPlatform, mozIDownloadPlatform);
+
+DownloadPlatform* DownloadPlatform::GetDownloadPlatform() {
+  if (!gDownloadPlatformService) {
+    gDownloadPlatformService = new DownloadPlatform();
+  }
+
+  NS_ADDREF(gDownloadPlatformService);
+
+  return gDownloadPlatformService;
+}
+
+#ifdef MOZ_WIDGET_GTK
+static void gio_set_metadata_done(GObject* source_obj, GAsyncResult* res,
+                                  gpointer user_data) {
+  GError* err = nullptr;
+  g_file_set_attributes_finish(G_FILE(source_obj), res, nullptr, &err);
+  if (err) {
+#  ifdef DEBUG
+    NS_DebugBreak(NS_DEBUG_WARNING, "Set file metadata failed: ", err->message,
+                  __FILE__, __LINE__);
+#  endif
+    g_error_free(err);
+  }
+}
+#endif
+
+#ifdef XP_MACOSX
+// Caller is responsible for freeing any result (CF Create Rule)
+CFURLRef CreateCFURLFromNSIURI(nsIURI* aURI) {
+  nsAutoCString spec;
+  if (aURI) {
+    aURI->GetSpec(spec);
+  }
+
+  CFStringRef urlStr = ::CFStringCreateWithCString(
+      kCFAllocatorDefault, spec.get(), kCFStringEncodingUTF8);
+  if (!urlStr) {
+    return NULL;
+  }
+
+  CFURLRef url = ::CFURLCreateWithString(kCFAllocatorDefault, urlStr, NULL);
+
+  ::CFRelease(urlStr);
+
+  return url;
+}
+#endif
+
+#ifdef XP_WIN
+static void AddToRecentDocs(nsIFile* aTarget, nsAutoString& aPath) {
+  nsString modelId;
+  if (mozilla::widget::WinTaskbar::GetAppUserModelID(modelId)) {
+    nsCOMPtr<nsIURI> uri;
+    if (NS_SUCCEEDED(NS_NewFileURI(getter_AddRefs(uri), aTarget)) && uri) {
+      nsCString spec;
+      if (NS_SUCCEEDED(uri->GetSpec(spec))) {
+        IShellItem2* psi = nullptr;
+        if (SUCCEEDED(
+                SHCreateItemFromParsingName(NS_ConvertASCIItoUTF16(spec).get(),
+                                            nullptr, IID_PPV_ARGS(&psi)))) {
+          SHARDAPPIDINFO info = {psi, modelId.get()};
+          ::SHAddToRecentDocs(SHARD_APPIDINFO, &info);
+          psi->Release();
+          return;
+        }
+      }
+    }
+  }
+
+  ::SHAddToRecentDocs(SHARD_PATHW, aPath.get());
+}
+#endif
+
+nsresult DownloadPlatform::DownloadDone(nsIURI* aSource, nsIURI* aReferrer,
+                                        nsIFile* aTarget,
+                                        const nsACString& aContentType,
+                                        bool aIsPrivate, JSContext* aCx,
+                                        Promise** aPromise) {
+  nsIGlobalObject* globalObject =
+      xpc::NativeGlobal(JS::CurrentGlobalOrNull(aCx));
+
+  if (NS_WARN_IF(!globalObject)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  ErrorResult result;
+  RefPtr<Promise> promise = Promise::Create(globalObject, result);
+
+  if (NS_WARN_IF(result.Failed())) {
+    return result.StealNSResult();
+  }
+
+  nsresult rv = NS_OK;
+  bool pendingAsyncOperations = false;
+
+#if defined(XP_WIN) || defined(XP_MACOSX) || defined(MOZ_WIDGET_ANDROID) || \
+    defined(MOZ_WIDGET_GTK)
+
+  nsAutoString path;
+  if (aTarget && NS_SUCCEEDED(aTarget->GetPath(path))) {
+#  if defined(XP_WIN) || defined(MOZ_WIDGET_GTK) || defined(MOZ_WIDGET_ANDROID)
+    // On Windows and Gtk, add the download to the system's "recent documents"
+    // list, with a pref to disable.
+    {
+#    ifndef MOZ_WIDGET_ANDROID
+      bool addToRecentDocs = Preferences::GetBool(PREF_BDM_ADDTORECENTDOCS);
+      if (addToRecentDocs && !aIsPrivate) {
+#      ifdef XP_WIN
+        AddToRecentDocs(aTarget, path);
+#      elif defined(MOZ_WIDGET_GTK)
+        GtkRecentManager* manager = gtk_recent_manager_get_default();
+
+        gchar* uri = g_filename_to_uri(NS_ConvertUTF16toUTF8(path).get(),
+                                       nullptr, nullptr);
+        if (uri) {
+          gtk_recent_manager_add_item(manager, uri);
+          g_free(uri);
+        }
+#      endif
+      }
+#    endif
+#    ifdef MOZ_WIDGET_GTK
+      // Private window should not leak URI to the system (Bug 1535950)
+      if (!aIsPrivate) {
+        // Use GIO to store the source URI for later display in the file
+        // manager.
+        GFile* gio_file =
+            g_file_new_for_path(NS_ConvertUTF16toUTF8(path).get());
+        nsCString source_uri;
+        nsresult rv = aSource->GetSpec(source_uri);
+        NS_ENSURE_SUCCESS(rv, rv);
+        GFileInfo* file_info = g_file_info_new();
+        g_file_info_set_attribute_string(file_info, "metadata::download-uri",
+                                         source_uri.get());
+        g_file_set_attributes_async(gio_file, file_info, G_FILE_QUERY_INFO_NONE,
+                                    G_PRIORITY_DEFAULT, nullptr,
+                                    gio_set_metadata_done, nullptr);
+        g_object_unref(file_info);
+        g_object_unref(gio_file);
+      }
+#    endif
+    }
+#  endif
+
+#  ifdef XP_MACOSX
+    // On OS X, make the downloads stack bounce.
+    CFStringRef observedObject = ::CFStringCreateWithCString(
+        kCFAllocatorDefault, NS_ConvertUTF16toUTF8(path).get(),
+        kCFStringEncodingUTF8);
+    CFNotificationCenterRef center =
+        ::CFNotificationCenterGetDistributedCenter();
+    ::CFNotificationCenterPostNotification(
+        center, CFSTR("com.apple.DownloadFileFinished"), observedObject,
+        nullptr, TRUE);
+    ::CFRelease(observedObject);
+
+    // Add OS X origin and referrer file metadata
+    CFStringRef pathCFStr = NULL;
+    if (!path.IsEmpty()) {
+      pathCFStr = ::CFStringCreateWithCharacters(
+          kCFAllocatorDefault, (const UniChar*)path.get(), path.Length());
+    }
+    if (pathCFStr && !aIsPrivate) {
+      bool isFromWeb = IsURLPossiblyFromWeb(aSource);
+      nsCOMPtr<nsIURI> source(aSource);
+      nsCOMPtr<nsIURI> referrer(aReferrer);
+
+      rv = NS_DispatchBackgroundTask(
+          NS_NewRunnableFunction(
+              "DownloadPlatform::DownloadDone",
+              [pathCFStr, isFromWeb, source, referrer, promise]() mutable {
+                CFURLRef sourceCFURL = CreateCFURLFromNSIURI(source);
+                CFURLRef referrerCFURL = CreateCFURLFromNSIURI(referrer);
+
+                CocoaFileUtils::AddOriginMetadataToFile(pathCFStr, sourceCFURL,
+                                                        referrerCFURL);
+                CocoaFileUtils::AddQuarantineMetadataToFile(
+                    pathCFStr, sourceCFURL, referrerCFURL, isFromWeb);
+                ::CFRelease(pathCFStr);
+                if (sourceCFURL) {
+                  ::CFRelease(sourceCFURL);
+                }
+                if (referrerCFURL) {
+                  ::CFRelease(referrerCFURL);
+                }
+
+                DebugOnly<nsresult> rv =
+                    NS_DispatchToMainThread(NS_NewRunnableFunction(
+                        "DownloadPlatform::DownloadDoneResolve",
+                        [promise = std::move(promise)]() {
+                          promise->MaybeResolveWithUndefined();
+                        }));
+                MOZ_ASSERT(NS_SUCCEEDED(rv));
+                // In non-debug builds, if we've for some reason failed to
+                // dispatch a runnable to the main thread to resolve the
+                // Promise, then it's unlikely we can reject it either. At that
+                // point, the Promise is going to remain in pending limbo until
+                // its global goes away.
+              }),
+          NS_DISPATCH_EVENT_MAY_BLOCK);
+
+      if (NS_SUCCEEDED(rv)) {
+        pendingAsyncOperations = true;
+      }
+    }
+#  endif
+  }
+
+#endif
+
+  if (!pendingAsyncOperations) {
+    promise->MaybeResolveWithUndefined();
+  }
+  promise.forget(aPromise);
+  return rv;
+}
+
+nsresult DownloadPlatform::MapUrlToZone(const nsAString& aURL,
+                                        uint32_t* aZone) {
+#ifdef XP_WIN
+  RefPtr<IInternetSecurityManager> inetSecMgr;
+  if (FAILED(CoCreateInstance(CLSID_InternetSecurityManager, NULL, CLSCTX_ALL,
+                              IID_IInternetSecurityManager,
+                              getter_AddRefs(inetSecMgr)))) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  DWORD zone;
+  if (inetSecMgr->MapUrlToZone(PromiseFlatString(aURL).get(), &zone, 0) !=
+      S_OK) {
+    return NS_ERROR_UNEXPECTED;
+  } else {
+    *aZone = zone;
+  }
+
+  return NS_OK;
+#else
+  return NS_ERROR_NOT_IMPLEMENTED;
+#endif
+}
+
+// Check if a URI is likely to be web-based, by checking its URI flags.
+// If in doubt (e.g. if anything fails during the check) claims things
+// are from the web.
+bool DownloadPlatform::IsURLPossiblyFromWeb(nsIURI* aURI) {
+  nsCOMPtr<nsIIOService> ios = do_GetIOService();
+  nsCOMPtr<nsIURI> uri = aURI;
+  if (!ios) {
+    return true;
+  }
+
+  while (uri) {
+    // We're not using NS_URIChainHasFlags because we're checking for *any* of 3
+    // flags to be present on *all* of the nested URIs, which it can't do.
+    uint32_t flags;
+    nsresult rv = ios->GetDynamicProtocolFlags(uri, &flags);
+    if (NS_FAILED(rv)) {
+      return true;
+    }
+    // If not dangerous to load, not a UI resource and not a local file,
+    // assume this is from the web:
+    if (!(flags & nsIProtocolHandler::URI_DANGEROUS_TO_LOAD) &&
+        !(flags & nsIProtocolHandler::URI_IS_UI_RESOURCE) &&
+        !(flags & nsIProtocolHandler::URI_IS_LOCAL_FILE)) {
+      return true;
+    }
+    // Otherwise, check if the URI is nested, and if so go through
+    // the loop again:
+    nsCOMPtr<nsINestedURI> nestedURI = do_QueryInterface(uri);
+    uri = nullptr;
+    if (nestedURI) {
+      rv = nestedURI->GetInnerURI(getter_AddRefs(uri));
+      if (NS_FAILED(rv)) {
+        return true;
+      }
+    }
+  }
+  return false;
+}
diff --git a/toolkit/components/downloads/DownloadPlatform.h b/toolkit/components/downloads/DownloadPlatform.h
new file mode 100644
index 0000000000..82e74f8c49
--- /dev/null
+++ b/toolkit/components/downloads/DownloadPlatform.h
@@ -0,0 +1,28 @@
+/* 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/. */
+
+#ifndef __DownloadPlatform_h__
+#define __DownloadPlatform_h__
+
+#include "mozIDownloadPlatform.h"
+
+#include "nsISupports.h"
+
+class DownloadPlatform : public mozIDownloadPlatform {
+ protected:
+  virtual ~DownloadPlatform() = default;
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_MOZIDOWNLOADPLATFORM
+
+  static DownloadPlatform* gDownloadPlatformService;
+
+  static DownloadPlatform* GetDownloadPlatform();
+
+ private:
+  static bool IsURLPossiblyFromWeb(nsIURI* aURI);
+};
+
+#endif
diff --git a/toolkit/components/downloads/DownloadStore.sys.mjs b/toolkit/components/downloads/DownloadStore.sys.mjs
new file mode 100644
index 0000000000..542a8c765a
--- /dev/null
+++ b/toolkit/components/downloads/DownloadStore.sys.mjs
@@ -0,0 +1,211 @@
+/* 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/. */
+
+/**
+ * Handles serialization of Download objects and persistence into a file, so
+ * that the state of downloads can be restored across sessions.
+ *
+ * The file is stored in JSON format, without indentation.  With indentation
+ * applied, the file would look like this:
+ *
+ * {
+ *   "list": [
+ *     {
+ *       "source": "http://www.example.com/download.txt",
+ *       "target": "/home/user/Downloads/download.txt"
+ *     },
+ *     {
+ *       "source": {
+ *         "url": "http://www.example.com/download.txt",
+ *         "referrerInfo": serialized string represents referrerInfo object
+ *       },
+ *       "target": "/home/user/Downloads/download-2.txt"
+ *     }
+ *   ]
+ * }
+ */
+
+// Time after which insecure downloads that have not been dealt with on shutdown
+// get removed (5 minutes).
+const MAX_INSECURE_DOWNLOAD_AGE_MS = 5 * 60 * 1000;
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  Downloads: "resource://gre/modules/Downloads.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(lazy, "gTextDecoder", function () {
+  return new TextDecoder();
+});
+
+ChromeUtils.defineLazyGetter(lazy, "gTextEncoder", function () {
+  return new TextEncoder();
+});
+
+/**
+ * Handles serialization of Download objects and persistence into a file, so
+ * that the state of downloads can be restored across sessions.
+ *
+ * @param aList
+ *        DownloadList object to be populated or serialized.
+ * @param aPath
+ *        String containing the file path where data should be saved.
+ */
+export var DownloadStore = function (aList, aPath) {
+  this.list = aList;
+  this.path = aPath;
+};
+
+DownloadStore.prototype = {
+  /**
+   * DownloadList object to be populated or serialized.
+   */
+  list: null,
+
+  /**
+   * String containing the file path where data should be saved.
+   */
+  path: "",
+
+  /**
+   * This function is called with a Download object as its first argument, and
+   * should return true if the item should be saved.
+   */
+  onsaveitem: () => true,
+
+  /**
+   * Loads persistent downloads from the file to the list.
+   *
+   * @return {Promise}
+   * @resolves When the operation finished successfully.
+   * @rejects JavaScript exception.
+   */
+  load: function DS_load() {
+    return (async () => {
+      let bytes;
+      try {
+        bytes = await IOUtils.read(this.path);
+      } catch (ex) {
+        if (!(ex.name == "NotFoundError")) {
+          throw ex;
+        }
+        // If the file does not exist, there are no downloads to load.
+        return;
+      }
+
+      // Set this to true when we make changes to the download list that should
+      // be reflected in the file again.
+      let storeChanges = false;
+      let removePromises = [];
+      let storeData = JSON.parse(lazy.gTextDecoder.decode(bytes));
+
+      // Create live downloads based on the static snapshot.
+      for (let downloadData of storeData.list) {
+        try {
+          let download = await lazy.Downloads.createDownload(downloadData);
+
+          // Insecure downloads that have not been dealt with on shutdown should
+          // get cleaned up and removed from the download list on restart unless
+          // they are very new
+          if (
+            download.error?.becauseBlockedByReputationCheck &&
+            download.error.reputationCheckVerdict == "Insecure" &&
+            Date.now() - download.startTime > MAX_INSECURE_DOWNLOAD_AGE_MS
+          ) {
+            removePromises.push(download.removePartialData());
+            storeChanges = true;
+            continue;
+          }
+
+          try {
+            if (!download.succeeded && !download.canceled && !download.error) {
+              // Try to restart the download if it was in progress during the
+              // previous session.  Ignore errors.
+              download.start().catch(() => {});
+            } else {
+              // If the download was not in progress, try to update the current
+              // progress from disk.  This is relevant in case we retained
+              // partially downloaded data.
+              await download.refresh();
+            }
+          } finally {
+            // Add the download to the list if we succeeded in creating it,
+            // after we have updated its initial state.
+            await this.list.add(download);
+          }
+        } catch (ex) {
+          // If an item is unrecognized, don't prevent others from being loaded.
+          console.error(ex);
+        }
+      }
+
+      if (storeChanges) {
+        try {
+          await Promise.all(removePromises);
+          await this.save();
+        } catch (ex) {
+          console.error(ex);
+        }
+      }
+    })();
+  },
+
+  /**
+   * Saves persistent downloads from the list to the file.
+   *
+   * If an error occurs, the previous file is not deleted.
+   *
+   * @return {Promise}
+   * @resolves When the operation finished successfully.
+   * @rejects JavaScript exception.
+   */
+  save: function DS_save() {
+    return (async () => {
+      let downloads = await this.list.getAll();
+
+      // Take a static snapshot of the current state of all the downloads.
+      let storeData = { list: [] };
+      let atLeastOneDownload = false;
+      for (let download of downloads) {
+        try {
+          if (!this.onsaveitem(download)) {
+            continue;
+          }
+
+          let serializable = download.toSerializable();
+          if (!serializable) {
+            // This item cannot be persisted across sessions.
+            continue;
+          }
+          storeData.list.push(serializable);
+          atLeastOneDownload = true;
+        } catch (ex) {
+          // If an item cannot be converted to a serializable form, don't
+          // prevent others from being saved.
+          console.error(ex);
+        }
+      }
+
+      if (atLeastOneDownload) {
+        // Create or overwrite the file if there are downloads to save.
+        let bytes = lazy.gTextEncoder.encode(JSON.stringify(storeData));
+        await IOUtils.write(this.path, bytes, {
+          tmpPath: this.path + ".tmp",
+        });
+      } else {
+        // Remove the file if there are no downloads to save at all.
+        try {
+          await IOUtils.remove(this.path);
+        } catch (ex) {
+          if (!(ex.name == "NotFoundError" || ex.name == "NotAllowedError")) {
+            throw ex;
+          }
+          // On Windows, we may get an access denied error instead of a no such
+          // file error if the file existed before, and was recently deleted.
+        }
+      }
+    })();
+  },
+};
diff --git a/toolkit/components/downloads/DownloadUIHelper.sys.mjs b/toolkit/components/downloads/DownloadUIHelper.sys.mjs
new file mode 100644
index 0000000000..0d1d0e0a31
--- /dev/null
+++ b/toolkit/components/downloads/DownloadUIHelper.sys.mjs
@@ -0,0 +1,240 @@
+/* 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/. */
+
+/**
+ * Provides functions to handle status and messages in the user interface.
+ */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+
+const lazy = {};
+
+// BrowserWindowTracker and PrivateBrowsingUtils are only used when opening downloaded files into a browser window
+ChromeUtils.defineESModuleGetters(lazy, {
+  BrowserWindowTracker: "resource:///modules/BrowserWindowTracker.sys.mjs",
+  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+});
+
+ChromeUtils.defineLazyGetter(
+  lazy,
+  "l10n",
+  () => new Localization(["toolkit/downloads/downloadUI.ftl"], true)
+);
+
+/**
+ * Provides functions to handle status and messages in the user interface.
+ */
+export var DownloadUIHelper = {
+  /**
+   * Returns an object that can be used to display prompts related to downloads.
+   *
+   * The prompts may be either anchored to a specified window, or anchored to
+   * the most recently active window, for example if the prompt is displayed in
+   * response to global notifications that are not associated with any window.
+   *
+   * @param aParent
+   *        If specified, should reference the nsIDOMWindow to which the prompts
+   *        should be attached.  If omitted, the prompts will be attached to the
+   *        most recently active window.
+   *
+   * @return A DownloadPrompter object.
+   */
+  getPrompter(aParent) {
+    return new DownloadPrompter(aParent || null);
+  },
+
+  /**
+   * Open the given file as a file: URI in the active window
+   *
+   * @param nsIFile file         The downloaded file
+   * @param options.chromeWindow Optional chrome window where we could open the file URI
+   * @param options.openWhere    String indicating how to open the URI.
+   *                             One of "window", "tab", "tabshifted"
+   * @param options.isPrivate    Open in private window or not
+   * @param options.browsingContextId BrowsingContext ID of the initiating document
+   * @param options.userContextId UserContextID of the initiating document
+   */
+  loadFileIn(
+    file,
+    {
+      chromeWindow: browserWin,
+      openWhere = "tab",
+      isPrivate,
+      userContextId = 0,
+      browsingContextId = 0,
+    } = {}
+  ) {
+    let fileURI = Services.io.newFileURI(file);
+    let allowPrivate =
+      isPrivate || lazy.PrivateBrowsingUtils.permanentPrivateBrowsing;
+
+    if (
+      !browserWin ||
+      browserWin.document.documentElement.getAttribute("windowtype") !==
+        "navigator:browser"
+    ) {
+      // we'll need a private window for a private download, or if we're in private-only mode
+      // but otherwise we want to open files in a non-private window
+      browserWin = lazy.BrowserWindowTracker.getTopWindow({
+        private: allowPrivate,
+      });
+    }
+    // if there is no suitable browser window, we'll need to open one and ignore any other `openWhere` value
+    // this can happen if the library dialog is the only open window
+    if (!browserWin) {
+      // There is no browser window open, so open a new one.
+      let args = Cc["@mozilla.org/array;1"].createInstance(Ci.nsIMutableArray);
+      let strURI = Cc["@mozilla.org/supports-string;1"].createInstance(
+        Ci.nsISupportsString
+      );
+      strURI.data = fileURI.spec;
+      args.appendElement(strURI);
+      let features = "chrome,dialog=no,all";
+      if (isPrivate) {
+        features += ",private";
+      }
+      browserWin = Services.ww.openWindow(
+        null,
+        AppConstants.BROWSER_CHROME_URL,
+        null,
+        features,
+        args
+      );
+      return;
+    }
+
+    // a browser window will have the helpers from utilityOverlay.js
+    let browsingContext = browserWin?.BrowsingContext.get(browsingContextId);
+    browserWin.openTrustedLinkIn(fileURI.spec, openWhere, {
+      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
+      private: isPrivate,
+      userContextId,
+      openerBrowser: browsingContext?.top?.embedderElement,
+    });
+  },
+};
+
+/**
+ * Allows displaying prompts related to downloads.
+ *
+ * @param aParent
+ *        The nsIDOMWindow to which prompts should be attached, or null to
+ *        attach prompts to the most recently active window.
+ */
+var DownloadPrompter = function (aParent) {
+  this._prompter = Services.ww.getNewPrompter(aParent);
+};
+
+DownloadPrompter.prototype = {
+  /**
+   * Constants with the different type of prompts.
+   */
+  ON_QUIT: "prompt-on-quit",
+  ON_OFFLINE: "prompt-on-offline",
+  ON_LEAVE_PRIVATE_BROWSING: "prompt-on-leave-private-browsing",
+
+  /**
+   * nsIPrompt instance for displaying messages.
+   */
+  _prompter: null,
+
+  /**
+   * Displays a warning message box that informs that the specified file is
+   * executable, and asks whether the user wants to launch it.
+   *
+   * @param path
+   *        String containing the full path to the file to be opened.
+   *
+   * @resolves Boolean indicating whether the launch operation can continue.
+   */
+  async confirmLaunchExecutable(path) {
+    const kPrefSkipConfirm = "browser.download.skipConfirmLaunchExecutable";
+
+    // Always launch in case we have no prompter implementation.
+    if (!this._prompter) {
+      return true;
+    }
+
+    try {
+      if (Services.prefs.getBoolPref(kPrefSkipConfirm)) {
+        return true;
+      }
+    } catch (ex) {
+      // If the preference does not exist, continue with the prompt.
+    }
+
+    const title = lazy.l10n.formatValueSync(
+      "download-ui-file-executable-security-warning-title"
+    );
+    const message = lazy.l10n.formatValueSync(
+      "download-ui-file-executable-security-warning",
+      { executable: PathUtils.filename(path) }
+    );
+    return this._prompter.confirm(title, message);
+  },
+
+  /**
+   * Displays a warning message box that informs that there are active
+   * downloads, and asks whether the user wants to cancel them or not.
+   *
+   * @param aDownloadsCount
+   *        The current downloads count.
+   * @param aPromptType
+   *        The type of prompt notification depending on the observer.
+   *
+   * @return False to cancel the downloads and continue, true to abort the
+   *         operation.
+   */
+  confirmCancelDownloads: function DP_confirmCancelDownload(
+    aDownloadsCount,
+    aPromptType
+  ) {
+    // Always continue in case we have no prompter implementation, or if there
+    // are no active downloads.
+    if (!this._prompter || aDownloadsCount <= 0) {
+      return false;
+    }
+
+    let message, cancelButton;
+
+    switch (aPromptType) {
+      case this.ON_QUIT:
+        message =
+          AppConstants.platform == "macosx"
+            ? "download-ui-confirm-quit-cancel-downloads-mac"
+            : "download-ui-confirm-quit-cancel-downloads";
+        cancelButton = "download-ui-dont-quit-button";
+        break;
+
+      case this.ON_OFFLINE:
+        message = "download-ui-confirm-offline-cancel-downloads";
+        cancelButton = "download-ui-dont-go-offline-button";
+        break;
+
+      case this.ON_LEAVE_PRIVATE_BROWSING:
+        message =
+          "download-ui-confirm-leave-private-browsing-windows-cancel-downloads";
+        cancelButton = "download-ui-dont-leave-private-browsing-button";
+        break;
+    }
+
+    const buttonFlags =
+      Ci.nsIPrompt.BUTTON_TITLE_IS_STRING * Ci.nsIPrompt.BUTTON_POS_0 +
+      Ci.nsIPrompt.BUTTON_TITLE_IS_STRING * Ci.nsIPrompt.BUTTON_POS_1;
+
+    let rv = this._prompter.confirmEx(
+      lazy.l10n.formatValueSync("download-ui-confirm-title"),
+      lazy.l10n.formatValueSync(message, { downloadsCount: aDownloadsCount }),
+      buttonFlags,
+      lazy.l10n.formatValueSync("download-ui-cancel-downloads-ok", {
+        downloadsCount: aDownloadsCount,
+      }),
+      lazy.l10n.formatValueSync(cancelButton),
+      null,
+      null,
+      {}
+    );
+    return rv == 1;
+  },
+};
diff --git a/toolkit/components/downloads/Downloads.sys.mjs b/toolkit/components/downloads/Downloads.sys.mjs
new file mode 100644
index 0000000000..1443a69e55
--- /dev/null
+++ b/toolkit/components/downloads/Downloads.sys.mjs
@@ -0,0 +1,294 @@
+/* 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/. */
+
+/**
+ * Main entry point to get references to all the back-end objects.
+ */
+
+import { Integration } from "resource://gre/modules/Integration.sys.mjs";
+
+import {
+  Download,
+  DownloadError,
+} from "resource://gre/modules/DownloadCore.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  DownloadCombinedList: "resource://gre/modules/DownloadList.sys.mjs",
+  DownloadList: "resource://gre/modules/DownloadList.sys.mjs",
+  DownloadSummary: "resource://gre/modules/DownloadList.sys.mjs",
+});
+
+Integration.downloads.defineESModuleGetter(
+  lazy,
+  "DownloadIntegration",
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+/**
+ * This object is exposed directly to the consumers of this JavaScript module,
+ * and provides the only entry point to get references to back-end objects.
+ */
+export const Downloads = {
+  /**
+   * Work on downloads that were not started from a private browsing window.
+   */
+  get PUBLIC() {
+    return "{Downloads.PUBLIC}";
+  },
+  /**
+   * Work on downloads that were started from a private browsing window.
+   */
+  get PRIVATE() {
+    return "{Downloads.PRIVATE}";
+  },
+  /**
+   * Work on both Downloads.PRIVATE and Downloads.PUBLIC downloads.
+   */
+  get ALL() {
+    return "{Downloads.ALL}";
+  },
+
+  /**
+   * Creates a new Download object.
+   *
+   * @param properties
+   *        Provides the initial properties for the newly created download.
+   *        This matches the serializable representation of a Download object.
+   *        Some of the most common properties in this object include:
+   *        {
+   *          source: String containing the URI for the download source.
+   *                  Alternatively, may be an nsIURI, a DownloadSource object,
+   *                  or an object with the following properties:
+   *          {
+   *            url: String containing the URI for the download source.
+   *            isPrivate: Indicates whether the download originated from a
+   *                       private window.  If omitted, the download is public.
+   *            referrerInfo: String or nsIReferrerInfo object represents the
+   *                          referrerInfo of the download source.  Can be
+   *                          omitted or null for example when the download
+   *                          source is not HTTP.
+   *            cookieJarSettings: The nsICookieJarSettings object represents
+   *                               the cookieJarSetting of the download source.
+   *                               Can be omitted or null if the download source
+   *                               is not from a document.
+   *          },
+   *          target: String containing the path of the target file.
+   *                  Alternatively, may be an nsIFile, a DownloadTarget object,
+   *                  or an object with the following properties:
+   *          {
+   *            path: String containing the path of the target file.
+   *          },
+   *          saver: String representing the class of the download operation.
+   *                 If omitted, defaults to "copy".  Alternatively, may be the
+   *                 serializable representation of a DownloadSaver object.
+   *        }
+   *
+   * @return {Promise}
+   * @resolves The newly created Download object.
+   * @rejects JavaScript exception.
+   */
+  async createDownload(properties) {
+    return Download.fromSerializable(properties);
+  },
+
+  /**
+   * Downloads data from a remote network location to a local file.
+   *
+   * This download method does not provide user interface, or the ability to
+   * cancel or restart the download programmatically.  For that, you should
+   * obtain a reference to a Download object using the createDownload function.
+   *
+   * Since the download cannot be restarted, any partially downloaded data will
+   * not be kept in case the download fails.
+   *
+   * @param source
+   *        String containing the URI for the download source.  Alternatively,
+   *        may be an nsIURI or a DownloadSource object.
+   * @param target
+   *        String containing the path of the target file.  Alternatively, may
+   *        be an nsIFile or a DownloadTarget object.
+   * @param options
+   *        An optional object used to control the behavior of this function.
+   *        You may pass an object with a subset of the following fields:
+   *        {
+   *          isPrivate: Indicates whether the download originated from a
+   *                     private window.
+   *        }
+   *
+   * @return {Promise}
+   * @resolves When the download has finished successfully.
+   * @rejects JavaScript exception if the download failed.
+   */
+  async fetch(source, target, options) {
+    const download = await this.createDownload({ source, target });
+
+    if (options?.isPrivate) {
+      download.source.isPrivate = options.isPrivate;
+    }
+    return download.start();
+  },
+
+  /**
+   * Retrieves the specified type of DownloadList object.  There is one download
+   * list for each type, and this method always retrieves a reference to the
+   * same download list when called with the same argument.
+   *
+   * Calling this function may cause the list of public downloads to be reloaded
+   * from the previous session, if it wasn't loaded already.
+   *
+   * @param type
+   *        This can be Downloads.PUBLIC, Downloads.PRIVATE, or Downloads.ALL.
+   *        Downloads added to the Downloads.PUBLIC and Downloads.PRIVATE lists
+   *        are reflected in the Downloads.ALL list, and downloads added to the
+   *        Downloads.ALL list are also added to either the Downloads.PUBLIC or
+   *        the Downloads.PRIVATE list based on their properties.
+   *
+   * @return {Promise}
+   * @resolves The requested DownloadList or DownloadCombinedList object.
+   * @rejects JavaScript exception.
+   */
+  async getList(type) {
+    if (!this._promiseListsInitialized) {
+      this._promiseListsInitialized = (async () => {
+        let publicList = new lazy.DownloadList();
+        let privateList = new lazy.DownloadList();
+        let combinedList = new lazy.DownloadCombinedList(
+          publicList,
+          privateList
+        );
+
+        try {
+          await lazy.DownloadIntegration.addListObservers(publicList, false);
+          await lazy.DownloadIntegration.addListObservers(privateList, true);
+          await lazy.DownloadIntegration.initializePublicDownloadList(
+            publicList
+          );
+        } catch (err) {
+          console.error(err);
+        }
+
+        let publicSummary = await this.getSummary(Downloads.PUBLIC);
+        let privateSummary = await this.getSummary(Downloads.PRIVATE);
+        let combinedSummary = await this.getSummary(Downloads.ALL);
+
+        await publicSummary.bindToList(publicList);
+        await privateSummary.bindToList(privateList);
+        await combinedSummary.bindToList(combinedList);
+
+        this._lists[Downloads.PUBLIC] = publicList;
+        this._lists[Downloads.PRIVATE] = privateList;
+        this._lists[Downloads.ALL] = combinedList;
+      })();
+    }
+
+    await this._promiseListsInitialized;
+
+    return this._lists[type];
+  },
+
+  /**
+   * Promise resolved when the initialization of the download lists has
+   * completed, or null if initialization has never been requested.
+   */
+  _promiseListsInitialized: null,
+
+  /**
+   * After initialization, this object is populated with one key for each type
+   * of download list that can be returned (Downloads.PUBLIC, Downloads.PRIVATE,
+   * or Downloads.ALL).  The values are the DownloadList objects.
+   */
+  _lists: {},
+
+  /**
+   * Retrieves the specified type of DownloadSummary object.  There is one
+   * download summary for each type, and this method always retrieves a
+   * reference to the same download summary when called with the same argument.
+   *
+   * Calling this function does not cause the list of public downloads to be
+   * reloaded from the previous session.  The summary will behave as if no
+   * downloads are present until the getList method is called.
+   *
+   * @param type
+   *        This can be Downloads.PUBLIC, Downloads.PRIVATE, or Downloads.ALL.
+   *
+   * @return {Promise}
+   * @resolves The requested DownloadList or DownloadCombinedList object.
+   * @rejects JavaScript exception.
+   */
+  async getSummary(type) {
+    if (
+      type != Downloads.PUBLIC &&
+      type != Downloads.PRIVATE &&
+      type != Downloads.ALL
+    ) {
+      throw new Error("Invalid type argument.");
+    }
+
+    if (!(type in this._summaries)) {
+      this._summaries[type] = new lazy.DownloadSummary();
+    }
+
+    return this._summaries[type];
+  },
+
+  /**
+   * This object is populated by the getSummary method with one key for each
+   * type of object that can be returned (Downloads.PUBLIC, Downloads.PRIVATE,
+   * or Downloads.ALL).  The values are the DownloadSummary objects.
+   */
+  _summaries: {},
+
+  /**
+   * Returns the system downloads directory asynchronously.
+   *   Mac OSX:
+   *     User downloads directory
+   *   XP/2K:
+   *     My Documents/Downloads
+   *   Vista and others:
+   *     User downloads directory
+   *   Linux:
+   *     XDG user dir spec, with a fallback to Home/Downloads
+   *   Android:
+   *     standard downloads directory i.e. /sdcard
+   *
+   * @return {Promise}
+   * @resolves The downloads directory string path.
+   */
+  getSystemDownloadsDirectory() {
+    return lazy.DownloadIntegration.getSystemDownloadsDirectory();
+  },
+
+  /**
+   * Returns the preferred downloads directory based on the user preferences
+   * in the current profile asynchronously.
+   *
+   * @return {Promise}
+   * @resolves The downloads directory string path.
+   */
+  getPreferredDownloadsDirectory() {
+    return lazy.DownloadIntegration.getPreferredDownloadsDirectory();
+  },
+
+  /**
+   * Returns the temporary directory where downloads are placed before the
+   * final location is chosen, or while the document is opened temporarily
+   * with an external application. This may or may not be the system temporary
+   * directory, based on the platform asynchronously.
+   *
+   * @return {Promise}
+   * @resolves The downloads directory string path.
+   */
+  getTemporaryDownloadsDirectory() {
+    return lazy.DownloadIntegration.getTemporaryDownloadsDirectory();
+  },
+
+  /**
+   * Constructor for a DownloadError object.  When you catch an exception during
+   * a download, you can use this to verify if "ex instanceof Downloads.Error",
+   * before reading the exception properties with the error details.
+   */
+  Error: DownloadError,
+};
diff --git a/toolkit/components/downloads/components.conf b/toolkit/components/downloads/components.conf
new file mode 100644
index 0000000000..63223bfe75
--- /dev/null
+++ b/toolkit/components/downloads/components.conf
@@ -0,0 +1,14 @@
+# -*- 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/.
+
+Classes = [
+    {
+        'cid': '{1b4c85df-cbdd-4bb6-b04e-613caece083c}',
+        'contract_ids': ['@mozilla.org/transfer;1'],
+        'esModule': 'resource://gre/modules/DownloadLegacy.sys.mjs',
+        'constructor': 'DownloadLegacyTransfer',
+    },
+]
diff --git a/toolkit/components/downloads/moz.build b/toolkit/components/downloads/moz.build
new file mode 100644
index 0000000000..a6eea0a2d4
--- /dev/null
+++ b/toolkit/components/downloads/moz.build
@@ -0,0 +1,55 @@
+# -*- 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/.
+
+XPCSHELL_TESTS_MANIFESTS += ["test/unit/xpcshell.toml"]
+
+TEST_HARNESS_FILES.xpcshell.toolkit.components.downloads.test.data += [
+    "test/data/empty.txt",
+    "test/data/source.txt",
+]
+
+XPIDL_SOURCES += [
+    "mozIDownloadPlatform.idl",
+]
+
+XPIDL_MODULE = "downloads"
+
+if CONFIG["MOZ_WIDGET_TOOLKIT"] == "gtk":
+    CXXFLAGS += CONFIG["MOZ_GTK3_CFLAGS"]
+
+SOURCES += [
+    "DownloadPlatform.cpp",
+]
+
+if CONFIG["OS_ARCH"] == "WINNT":
+    LOCAL_INCLUDES += [
+        "/widget/windows",
+    ]
+
+EXTRA_JS_MODULES += [
+    "DownloadCore.sys.mjs",
+    "DownloadIntegration.sys.mjs",
+    "DownloadLegacy.sys.mjs",
+    "DownloadList.sys.mjs",
+    "DownloadPaths.sys.mjs",
+    "Downloads.sys.mjs",
+    "DownloadStore.sys.mjs",
+    "DownloadUIHelper.sys.mjs",
+]
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+if CONFIG["MOZ_PLACES"]:
+    EXTRA_JS_MODULES += [
+        "DownloadHistory.sys.mjs",
+    ]
+
+FINAL_LIBRARY = "xul"
+
+with Files("**"):
+    BUG_COMPONENT = ("Toolkit", "Downloads API")
diff --git a/toolkit/components/downloads/mozIDownloadPlatform.idl b/toolkit/components/downloads/mozIDownloadPlatform.idl
new file mode 100644
index 0000000000..30f1cc1f24
--- /dev/null
+++ b/toolkit/components/downloads/mozIDownloadPlatform.idl
@@ -0,0 +1,62 @@
+/* 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 "nsISupports.idl"
+
+interface nsIURI;
+interface nsIFile;
+
+[scriptable, uuid(9f556e4a-d9b3-46c3-9f8f-d0db1ac6c8c1)]
+interface mozIDownloadPlatform : nsISupports
+{
+  /**
+   * Perform platform specific operations when a download is done.
+   *
+   *   Windows:
+   *     Add the download to the recent documents list
+   *     Set the file to be indexed for searching
+   *   Mac:
+   *     Bounce the downloads dock icon
+   *   GTK:
+   *     Add the download to the recent documents list
+   *     Save the source uri in the downloaded file's metadata
+   *   Android:
+   *     Scan media
+   *
+   * @param aSource
+   *        Source URI of the download
+   * @param aReferrer
+   *        Referrer URI of the download
+   * @param aTarget
+   *        Downloaded file
+   * @param aContentType
+   *        The source's content type
+   * @param aIsPrivate
+   *        True for private downloads
+   * @return Promise that resolves once operations have completed.
+   */
+  [implicit_jscontext]
+  Promise downloadDone(in nsIURI aSource, in nsIURI aReferrer, in nsIFile aTarget,
+                       in ACString aContentType, in boolean aIsPrivate);
+
+  /**
+   * Security Zone constants. Used by mapUrlToZone().
+   */
+  const unsigned long ZONE_MY_COMPUTER = 0;
+  const unsigned long ZONE_INTRANET = 1;
+  const unsigned long ZONE_TRUSTED = 2;
+  const unsigned long ZONE_INTERNET = 3;
+  const unsigned long ZONE_RESTRICTED = 4;
+
+  /**
+   * Proxy for IInternetSecurityManager::MapUrlToZone().
+   *
+   *   Windows only.
+   *
+   * @param aURL
+   *        URI of the download
+   * @return Security Zone corresponding to aURL.
+   */
+  unsigned long mapUrlToZone(in AString aURL);
+};
diff --git a/toolkit/components/downloads/test/data/empty.txt b/toolkit/components/downloads/test/data/empty.txt
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/toolkit/components/downloads/test/data/empty.txt
diff --git a/toolkit/components/downloads/test/data/source.txt b/toolkit/components/downloads/test/data/source.txt
new file mode 100644
index 0000000000..2156cb8c03
--- /dev/null
+++ b/toolkit/components/downloads/test/data/source.txt
@@ -0,0 +1 @@
+This test string is downloaded.
\ No newline at end of file
diff --git a/toolkit/components/downloads/test/unit/common_test_Download.js b/toolkit/components/downloads/test/unit/common_test_Download.js
new file mode 100644
index 0000000000..1360271686
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/common_test_Download.js
@@ -0,0 +1,2753 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * This script is loaded by "test_DownloadCore.js" and "test_DownloadLegacy.js"
+ * with different values of the gUseLegacySaver variable, to apply tests to both
+ * the "copy" and "legacy" saver implementations.
+ */
+
+/* import-globals-from head.js */
+/* global gUseLegacySaver */
+
+"use strict";
+
+// Globals
+
+const kDeleteTempFileOnExit = "browser.helperApps.deleteTempFileOnExit";
+
+ChromeUtils.defineESModuleGetters(this, {
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+});
+
+/**
+ * Creates and starts a new download, using either DownloadCopySaver or
+ * DownloadLegacySaver based on the current test run.
+ *
+ * @return {Promise}
+ * @resolves The newly created Download object.  The download may be in progress
+ *           or already finished.  The promiseDownloadStopped function can be
+ *           used to wait for completion.
+ * @rejects JavaScript exception.
+ */
+function promiseStartDownload(aSourceUrl) {
+  if (gUseLegacySaver) {
+    return promiseStartLegacyDownload(aSourceUrl);
+  }
+
+  return promiseNewDownload(aSourceUrl).then(download => {
+    download.start().catch(() => {});
+    return download;
+  });
+}
+
+/**
+ * Checks that the actual data written to disk matches the expected data as well
+ * as the properties of the given DownloadTarget object.
+ *
+ * @param downloadTarget
+ *        The DownloadTarget object whose details have to be verified.
+ * @param expectedContents
+ *        String containing the octets that are expected in the file.
+ *
+ * @return {Promise}
+ * @resolves When the properties have been verified.
+ * @rejects JavaScript exception.
+ */
+var promiseVerifyTarget = async function (downloadTarget, expectedContents) {
+  Assert.ok(downloadTarget.exists);
+  Assert.equal(
+    await expectNonZeroDownloadTargetSize(downloadTarget),
+    expectedContents.length
+  );
+  await promiseVerifyContents(downloadTarget.path, expectedContents);
+};
+
+/**
+ * This is a temporary workaround for frequent intermittent Bug 1760112.
+ * For some reason the download target size is not updated, even if the code
+ * is "apparently" already executing and awaiting for refresh().
+ * TODO(Bug 1814364): Figure out a proper fix for this.
+ */
+async function expectNonZeroDownloadTargetSize(downloadTarget) {
+  todo_check_true(downloadTarget.size, "Size should not be zero.");
+  if (!downloadTarget.size) {
+    await downloadTarget.refresh();
+  }
+  return downloadTarget.size;
+}
+
+/**
+ * Waits for an attempt to launch a file, and returns the nsIMIMEInfo used for
+ * the launch, or null if the file was launched with the default handler.
+ */
+function waitForFileLaunched() {
+  return new Promise(resolve => {
+    let waitFn = base => ({
+      launchFile(file, mimeInfo) {
+        Integration.downloads.unregister(waitFn);
+        if (
+          !mimeInfo ||
+          mimeInfo.preferredAction == Ci.nsIMIMEInfo.useSystemDefault
+        ) {
+          resolve(null);
+        } else {
+          resolve(mimeInfo);
+        }
+        return Promise.resolve();
+      },
+    });
+    Integration.downloads.register(waitFn);
+  });
+}
+
+/**
+ * Waits for an attempt to show the directory where a file is located, and
+ * returns the path of the file.
+ */
+function waitForDirectoryShown() {
+  return new Promise(resolve => {
+    let waitFn = base => ({
+      showContainingDirectory(path) {
+        Integration.downloads.unregister(waitFn);
+        resolve(path);
+        return Promise.resolve();
+      },
+    });
+    Integration.downloads.register(waitFn);
+  });
+}
+
+// Tests
+
+/**
+ * Executes a download and checks its basic properties after construction.
+ * The download is started by constructing the simplest Download object with
+ * the "copy" saver, or using the legacy nsITransfer interface.
+ */
+add_task(async function test_basic() {
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+
+  let download;
+  if (!gUseLegacySaver) {
+    // When testing DownloadCopySaver, we have control over the download, thus
+    // we can check its basic properties before it starts.
+    download = await Downloads.createDownload({
+      source: { url: httpUrl("source.txt") },
+      target: { path: targetFile.path },
+      saver: { type: "copy" },
+    });
+
+    Assert.equal(download.source.url, httpUrl("source.txt"));
+    Assert.equal(download.target.path, targetFile.path);
+
+    await download.start();
+  } else {
+    // When testing DownloadLegacySaver, the download is already started when it
+    // is created, thus we must check its basic properties while in progress.
+    download = await promiseStartLegacyDownload(null, { targetFile });
+
+    Assert.equal(download.source.url, httpUrl("source.txt"));
+    Assert.equal(download.target.path, targetFile.path);
+
+    await promiseDownloadStopped(download);
+  }
+
+  // Check additional properties on the finished download.
+  Assert.equal(download.source.referrerInfo, null);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT);
+});
+
+/**
+ * Executes a download with the tryToKeepPartialData property set, and ensures
+ * that the file is saved correctly.  When testing DownloadLegacySaver, the
+ * download is executed using the nsIExternalHelperAppService component.
+ */
+add_task(async function test_basic_tryToKeepPartialData() {
+  let download = await promiseStartDownload_tryToKeepPartialData({
+    useLegacySaver: gUseLegacySaver,
+  });
+  continueResponses();
+  await promiseDownloadStopped(download);
+
+  // The target file should now have been created, and the ".part" file deleted.
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  Assert.equal(32, download.saver.getSha256Hash().length);
+});
+
+/**
+ * Tests that the channelIsForDownload property is set for the network request,
+ * and that the request is marked as throttleable.
+ */
+add_task(async function test_channelIsForDownload_classFlags() {
+  let downloadChannel = null;
+
+  // We use a different method based on whether we are testing legacy downloads.
+  if (!gUseLegacySaver) {
+    let download = await Downloads.createDownload({
+      source: {
+        url: httpUrl("interruptible_resumable.txt"),
+        async adjustChannel(channel) {
+          downloadChannel = channel;
+        },
+      },
+      target: getTempFile(TEST_TARGET_FILE_NAME).path,
+    });
+    await download.start();
+  } else {
+    // Start a download using nsIExternalHelperAppService, but ensure it cannot
+    // finish before we retrieve the "request" property.
+    mustInterruptResponses();
+    let download = await promiseStartExternalHelperAppServiceDownload();
+    downloadChannel = download.saver.request;
+    continueResponses();
+    await promiseDownloadStopped(download);
+  }
+
+  Assert.ok(
+    downloadChannel.QueryInterface(Ci.nsIHttpChannelInternal)
+      .channelIsForDownload
+  );
+
+  // Throttleable is the only class flag assigned to downloads.
+  Assert.equal(
+    downloadChannel.QueryInterface(Ci.nsIClassOfService).classFlags,
+    Ci.nsIClassOfService.Throttleable
+  );
+});
+
+/**
+ * Tests the permissions of the final target file once the download finished.
+ */
+add_task(async function test_unix_permissions() {
+  // This test is only executed on some Desktop systems.
+  if (
+    Services.appinfo.OS != "Darwin" &&
+    Services.appinfo.OS != "Linux" &&
+    Services.appinfo.OS != "WINNT"
+  ) {
+    info("Skipping test.");
+    return;
+  }
+
+  let launcherPath = getTempFile("app-launcher").path;
+
+  for (let autoDelete of [false, true]) {
+    for (let isPrivate of [false, true]) {
+      for (let launchWhenSucceeded of [false, true]) {
+        info(
+          "Checking " +
+            JSON.stringify({ autoDelete, isPrivate, launchWhenSucceeded })
+        );
+
+        Services.prefs.setBoolPref(kDeleteTempFileOnExit, autoDelete);
+
+        let download;
+        if (!gUseLegacySaver) {
+          download = await Downloads.createDownload({
+            source: { url: httpUrl("source.txt"), isPrivate },
+            target: getTempFile(TEST_TARGET_FILE_NAME).path,
+            launchWhenSucceeded,
+            launcherPath,
+          });
+          await download.start();
+        } else {
+          download = await promiseStartLegacyDownload(httpUrl("source.txt"), {
+            isPrivate,
+            launchWhenSucceeded,
+            launcherPath: launchWhenSucceeded && launcherPath,
+          });
+          await promiseDownloadStopped(download);
+        }
+
+        let isTemporary = launchWhenSucceeded && isPrivate;
+        let stat = await IOUtils.stat(download.target.path);
+        if (Services.appinfo.OS == "WINNT") {
+          // On Windows
+          // Temporary downloads should be read-only
+          Assert.equal(stat.permissions, isTemporary ? 0o444 : 0o666);
+        } else {
+          // On Linux, Mac
+          // Temporary downloads should be read-only and not accessible to other
+          // users, while permanently downloaded files should be readable and
+          // writable as specified by the system umask.
+          Assert.equal(
+            stat.permissions,
+            isTemporary ? 0o400 : 0o666 & ~Services.sysinfo.getProperty("umask")
+          );
+        }
+      }
+    }
+  }
+
+  // Clean up the changes to the preference.
+  Services.prefs.clearUserPref(kDeleteTempFileOnExit);
+});
+
+/**
+ * Tests the zone information of the final target once the download finished.
+ */
+add_task(async function test_windows_zoneInformation() {
+  // This test is only executed on Windows, and in order to work correctly it
+  // requires the local user applicaton data directory to be on an NTFS file
+  // system. We use this directory because it is more likely to be on the local
+  // system installation drive, while the temporary directory used by the test
+  // environment is on the same drive as the test sources.
+  if (Services.appinfo.OS != "WINNT") {
+    info("Skipping test.");
+    return;
+  }
+
+  let normalTargetFile = await IOUtils.getFile(
+    Services.dirsvc.get("LocalAppData", Ci.nsIFile).path,
+    "xpcshell-download-test.txt"
+  );
+
+  // The template file name lenght is more than MAX_PATH characters. The final
+  // full path will be shortened to MAX_PATH length by the createUnique call.
+  let longTargetFile = await IOUtils.getFile(
+    Services.dirsvc.get("LocalAppData", Ci.nsIFile).path,
+    "T".repeat(256) + ".txt"
+  );
+  longTargetFile.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, 0o600);
+
+  const httpSourceUrl = httpUrl("source.txt");
+  const dataSourceUrl = "data:text/html," + TEST_DATA_SHORT;
+
+  function createReferrerInfo(
+    aReferrer,
+    aRefererPolicy = Ci.nsIReferrerInfo.EMPTY
+  ) {
+    return new ReferrerInfo(aRefererPolicy, true, NetUtil.newURI(aReferrer));
+  }
+
+  const tests = [
+    {
+      expectedZoneId:
+        "[ZoneTransfer]\r\nZoneId=3\r\nHostUrl=" + httpSourceUrl + "\r\n",
+    },
+    {
+      targetFile: longTargetFile,
+      expectedZoneId:
+        "[ZoneTransfer]\r\nZoneId=3\r\nHostUrl=" + httpSourceUrl + "\r\n",
+    },
+    {
+      sourceUrl: dataSourceUrl,
+      expectedZoneId:
+        "[ZoneTransfer]\r\nZoneId=3\r\nHostUrl=about:internet\r\n",
+    },
+    {
+      options: {
+        referrerInfo: createReferrerInfo(
+          TEST_REFERRER_URL,
+          Ci.nsIReferrerInfo.UNSAFE_URL
+        ),
+      },
+      expectedZoneId:
+        "[ZoneTransfer]\r\nZoneId=3\r\n" +
+        "ReferrerUrl=" +
+        TEST_REFERRER_URL +
+        "\r\n" +
+        "HostUrl=" +
+        httpSourceUrl +
+        "\r\n",
+    },
+    {
+      options: { referrerInfo: createReferrerInfo(dataSourceUrl) },
+      expectedZoneId:
+        "[ZoneTransfer]\r\nZoneId=3\r\nHostUrl=" + httpSourceUrl + "\r\n",
+    },
+    {
+      options: {
+        referrerInfo: createReferrerInfo("http://example.com/a\rb\nc"),
+      },
+      expectedZoneId:
+        "[ZoneTransfer]\r\nZoneId=3\r\n" +
+        "ReferrerUrl=http://example.com/\r\n" +
+        "HostUrl=" +
+        httpSourceUrl +
+        "\r\n",
+    },
+    {
+      options: { isPrivate: true },
+      expectedZoneId: "[ZoneTransfer]\r\nZoneId=3\r\n",
+    },
+    {
+      options: {
+        referrerInfo: createReferrerInfo(TEST_REFERRER_URL),
+        isPrivate: true,
+      },
+      expectedZoneId: "[ZoneTransfer]\r\nZoneId=3\r\n",
+    },
+  ];
+  for (const test of tests) {
+    const sourceUrl = test.sourceUrl || httpSourceUrl;
+    const targetFile = test.targetFile || normalTargetFile;
+    info(targetFile.path);
+    try {
+      if (!gUseLegacySaver) {
+        let download = await Downloads.createDownload({
+          source: test.options
+            ? Object.assign({ url: sourceUrl }, test.options)
+            : sourceUrl,
+          target: targetFile.path,
+        });
+        await download.start();
+      } else {
+        let download = await promiseStartLegacyDownload(
+          sourceUrl,
+          Object.assign({ targetFile }, test.options || {})
+        );
+        await promiseDownloadStopped(download);
+      }
+      await promiseVerifyContents(targetFile.path, TEST_DATA_SHORT);
+
+      let path = targetFile.path + ":Zone.Identifier";
+      if (Services.appinfo.OS === "WINNT") {
+        path = PathUtils.toExtendedWindowsPath(path);
+      }
+
+      Assert.equal(await IOUtils.readUTF8(path), test.expectedZoneId);
+    } finally {
+      await IOUtils.remove(targetFile.path);
+    }
+  }
+});
+
+/**
+ * Checks the referrer for downloads.
+ */
+add_task(async function test_referrer() {
+  let sourcePath = "/test_referrer.txt";
+  let sourceUrl = httpUrl("test_referrer.txt");
+  let dataSourceUrl = "data:text/html,<html><body></body></html>";
+
+  function cleanup() {
+    gHttpServer.registerPathHandler(sourcePath, null);
+  }
+  registerCleanupFunction(cleanup);
+
+  gHttpServer.registerPathHandler(sourcePath, function (aRequest, aResponse) {
+    aResponse.setHeader("Content-Type", "text/plain", false);
+
+    Assert.ok(aRequest.hasHeader("Referer"));
+    Assert.equal(aRequest.getHeader("Referer"), TEST_REFERRER_URL);
+  });
+  let download;
+  let referrerInfo = new ReferrerInfo(
+    Ci.nsIReferrerInfo.UNSAFE_URL,
+    true,
+    NetUtil.newURI(TEST_REFERRER_URL)
+  );
+
+  if (!gUseLegacySaver) {
+    let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+    let targetPath = targetFile.path;
+
+    download = await Downloads.createDownload({
+      source: { url: sourceUrl, referrerInfo },
+      target: targetPath,
+    });
+
+    Assert.ok(download.source.referrerInfo.equals(referrerInfo));
+    await download.start();
+
+    download = await Downloads.createDownload({
+      source: { url: sourceUrl, referrerInfo, isPrivate: true },
+      target: targetPath,
+    });
+    Assert.ok(download.source.referrerInfo.equals(referrerInfo));
+    await download.start();
+
+    // Test the download still works for non-HTTP channel with referrer.
+    download = await Downloads.createDownload({
+      source: { url: dataSourceUrl, referrerInfo },
+      target: targetPath,
+    });
+    Assert.ok(download.source.referrerInfo.equals(referrerInfo));
+    await download.start();
+  } else {
+    download = await promiseStartLegacyDownload(sourceUrl, {
+      referrerInfo,
+    });
+    await promiseDownloadStopped(download);
+    checkEqualReferrerInfos(download.source.referrerInfo, referrerInfo);
+
+    download = await promiseStartLegacyDownload(sourceUrl, {
+      referrerInfo,
+      isPrivate: true,
+    });
+    await promiseDownloadStopped(download);
+    checkEqualReferrerInfos(download.source.referrerInfo, referrerInfo);
+
+    download = await promiseStartLegacyDownload(dataSourceUrl, {
+      referrerInfo,
+    });
+    await promiseDownloadStopped(download);
+    Assert.equal(download.source.referrerInfo, null);
+  }
+
+  cleanup();
+});
+
+/**
+ * Checks the adjustChannel callback for downloads.
+ */
+add_task(async function test_adjustChannel() {
+  const sourcePath = "/test_post.txt";
+  const sourceUrl = httpUrl("test_post.txt");
+  const targetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  const customHeader = { name: "X-Answer", value: "42" };
+  const postData = "Don't Panic";
+
+  function cleanup() {
+    gHttpServer.registerPathHandler(sourcePath, null);
+  }
+  registerCleanupFunction(cleanup);
+
+  gHttpServer.registerPathHandler(sourcePath, aRequest => {
+    Assert.equal(aRequest.method, "POST");
+
+    Assert.ok(aRequest.hasHeader(customHeader.name));
+    Assert.equal(aRequest.getHeader(customHeader.name), customHeader.value);
+
+    const stream = aRequest.bodyInputStream;
+    const body = NetUtil.readInputStreamToString(stream, stream.available());
+    Assert.equal(body, postData);
+  });
+
+  function adjustChannel(channel) {
+    channel.QueryInterface(Ci.nsIHttpChannel);
+    channel.setRequestHeader(customHeader.name, customHeader.value, false);
+
+    const stream = Cc["@mozilla.org/io/string-input-stream;1"].createInstance(
+      Ci.nsIStringInputStream
+    );
+    stream.setData(postData, postData.length);
+
+    channel.QueryInterface(Ci.nsIUploadChannel2);
+    channel.explicitSetUploadStream(stream, null, -1, "POST", false);
+
+    return Promise.resolve();
+  }
+
+  const download = await Downloads.createDownload({
+    source: { url: sourceUrl, adjustChannel },
+    target: targetPath,
+  });
+  Assert.equal(download.source.adjustChannel, adjustChannel);
+  Assert.equal(download.toSerializable(), null);
+  await download.start();
+
+  cleanup();
+});
+
+/**
+ * Checks initial and final state and progress for a successful download.
+ */
+add_task(async function test_initial_final_state() {
+  let download;
+  if (!gUseLegacySaver) {
+    // When testing DownloadCopySaver, we have control over the download, thus
+    // we can check its state before it starts.
+    download = await promiseNewDownload();
+
+    Assert.ok(download.stopped);
+    Assert.ok(!download.succeeded);
+    Assert.ok(!download.canceled);
+    Assert.ok(download.error === null);
+    Assert.equal(download.progress, 0);
+    Assert.ok(download.startTime === null);
+    Assert.ok(!download.target.exists);
+    Assert.equal(download.target.size, 0);
+
+    await download.start();
+  } else {
+    // When testing DownloadLegacySaver, the download is already started when it
+    // is created, thus we cannot check its initial state.
+    download = await promiseStartLegacyDownload();
+    await promiseDownloadStopped(download);
+  }
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+  Assert.equal(download.progress, 100);
+  Assert.ok(isValidDate(download.startTime));
+  Assert.ok(download.target.exists);
+  Assert.equal(
+    await expectNonZeroDownloadTargetSize(download.target),
+    TEST_DATA_SHORT.length
+  );
+});
+
+/**
+ * Checks the notification of the final download state.
+ */
+add_task(async function test_final_state_notified() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+
+  let onchangeNotified = false;
+  let lastNotifiedStopped;
+  let lastNotifiedProgress;
+  download.onchange = function () {
+    onchangeNotified = true;
+    lastNotifiedStopped = download.stopped;
+    lastNotifiedProgress = download.progress;
+  };
+
+  // Allow the download to complete.
+  let promiseAttempt = download.start();
+  continueResponses();
+  await promiseAttempt;
+
+  // The view should have been notified before the download completes.
+  Assert.ok(onchangeNotified);
+  Assert.ok(lastNotifiedStopped);
+  Assert.equal(lastNotifiedProgress, 100);
+});
+
+/**
+ * Checks intermediate progress for a successful download.
+ */
+add_task(async function test_intermediate_progress() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+
+  await promiseDownloadMidway(download);
+
+  Assert.ok(download.hasProgress);
+  Assert.equal(download.currentBytes, TEST_DATA_SHORT.length);
+  Assert.equal(download.totalBytes, TEST_DATA_SHORT.length * 2);
+
+  // The final file size should not be computed for in-progress downloads.
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+
+  // Continue after the first chunk of data is fully received.
+  continueResponses();
+  await promiseDownloadStopped(download);
+
+  Assert.ok(download.stopped);
+  Assert.equal(download.progress, 100);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+});
+
+/**
+ * Downloads a file with a "Content-Length" of 0 and checks the progress.
+ */
+add_task(async function test_empty_progress() {
+  let download = await promiseStartDownload(httpUrl("empty.txt"));
+  await promiseDownloadStopped(download);
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.hasProgress);
+  Assert.equal(download.progress, 100);
+  Assert.equal(download.currentBytes, 0);
+  Assert.equal(download.totalBytes, 0);
+
+  // We should have received the content type even for an empty file.
+  Assert.equal(download.contentType, "text/plain");
+
+  Assert.equal((await IOUtils.stat(download.target.path)).size, 0);
+  Assert.ok(download.target.exists);
+  Assert.equal(download.target.size, 0);
+});
+
+/**
+ * Downloads a file with a "Content-Length" of 0 with the tryToKeepPartialData
+ * property set, and ensures that the file is saved correctly.
+ */
+add_task(async function test_empty_progress_tryToKeepPartialData() {
+  // Start a new download and configure it to keep partially downloaded data.
+  let download;
+  if (!gUseLegacySaver) {
+    let targetFilePath = getTempFile(TEST_TARGET_FILE_NAME).path;
+    download = await Downloads.createDownload({
+      source: httpUrl("empty.txt"),
+      target: { path: targetFilePath, partFilePath: targetFilePath + ".part" },
+    });
+    download.tryToKeepPartialData = true;
+    download.start().catch(() => {});
+  } else {
+    // Start a download using nsIExternalHelperAppService, that is configured
+    // to keep partially downloaded data by default.
+    download = await promiseStartExternalHelperAppServiceDownload(
+      httpUrl("empty.txt")
+    );
+  }
+  await promiseDownloadStopped(download);
+
+  // The target file should now have been created, and the ".part" file deleted.
+  Assert.equal((await IOUtils.stat(download.target.path)).size, 0);
+  Assert.ok(download.target.exists);
+  Assert.equal(download.target.size, 0);
+
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  Assert.equal(32, download.saver.getSha256Hash().length);
+});
+
+/**
+ * Downloads an empty file with no "Content-Length" and checks the progress.
+ */
+add_task(async function test_empty_noprogress() {
+  let sourcePath = "/test_empty_noprogress.txt";
+  let sourceUrl = httpUrl("test_empty_noprogress.txt");
+  let deferRequestReceived = Promise.withResolvers();
+
+  // Register an interruptible handler that notifies us when the request occurs.
+  function cleanup() {
+    gHttpServer.registerPathHandler(sourcePath, null);
+  }
+  registerCleanupFunction(cleanup);
+
+  registerInterruptibleHandler(
+    sourcePath,
+    function firstPart(aRequest, aResponse) {
+      aResponse.setHeader("Content-Type", "text/plain", false);
+      deferRequestReceived.resolve();
+    },
+    function secondPart(aRequest, aResponse) {}
+  );
+
+  // Start the download, without allowing the request to finish.
+  mustInterruptResponses();
+  let download;
+  if (!gUseLegacySaver) {
+    // When testing DownloadCopySaver, we have control over the download, thus
+    // we can hook its onchange callback that will be notified when the
+    // download starts.
+    download = await promiseNewDownload(sourceUrl);
+
+    download.onchange = function () {
+      if (!download.stopped) {
+        Assert.ok(!download.hasProgress);
+        Assert.equal(download.currentBytes, 0);
+        Assert.equal(download.totalBytes, 0);
+      }
+    };
+
+    download.start().catch(() => {});
+  } else {
+    // When testing DownloadLegacySaver, the download is already started when it
+    // is created, and it may have already made all needed property change
+    // notifications, thus there is no point in checking the onchange callback.
+    download = await promiseStartLegacyDownload(sourceUrl);
+  }
+
+  // Wait for the request to be received by the HTTP server, but don't allow the
+  // request to finish yet.  Before checking the download state, wait for the
+  // events to be processed by the client.
+  await deferRequestReceived.promise;
+  await promiseExecuteSoon();
+
+  // Check that this download has no progress report.
+  Assert.ok(!download.stopped);
+  Assert.ok(!download.hasProgress);
+  Assert.equal(download.currentBytes, 0);
+  Assert.equal(download.totalBytes, 0);
+
+  // Now allow the response to finish.
+  continueResponses();
+  await promiseDownloadStopped(download);
+
+  // We should have received the content type even if no progress is reported.
+  Assert.equal(download.contentType, "text/plain");
+
+  // Verify the state of the completed download.
+  Assert.ok(download.stopped);
+  Assert.ok(!download.hasProgress);
+  Assert.equal(download.progress, 100);
+  Assert.equal(download.currentBytes, 0);
+  Assert.equal(download.totalBytes, 0);
+  Assert.ok(download.target.exists);
+  Assert.equal(download.target.size, 0);
+
+  Assert.equal((await IOUtils.stat(download.target.path)).size, 0);
+});
+
+/**
+ * Calls the "start" method two times before the download is finished.
+ */
+add_task(async function test_start_twice() {
+  mustInterruptResponses();
+
+  let download;
+  if (!gUseLegacySaver) {
+    // When testing DownloadCopySaver, we have control over the download, thus
+    // we can start the download later during the test.
+    download = await promiseNewDownload(httpUrl("interruptible.txt"));
+  } else {
+    // When testing DownloadLegacySaver, the download is already started when it
+    // is created.  Effectively, we are starting the download three times.
+    download = await promiseStartLegacyDownload(httpUrl("interruptible.txt"));
+  }
+
+  // Call the start method two times.
+  let promiseAttempt1 = download.start();
+  let promiseAttempt2 = download.start();
+
+  // Allow the download to finish.
+  continueResponses();
+
+  // Both promises should now be resolved.
+  await promiseAttempt1;
+  await promiseAttempt2;
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+});
+
+/**
+ * Cancels a download and verifies that its state is reported correctly.
+ */
+add_task(async function test_cancel_midway() {
+  mustInterruptResponses();
+
+  // In this test case, we execute different checks that are only possible with
+  // DownloadCopySaver or DownloadLegacySaver respectively.
+  let download;
+  let options = {};
+  if (!gUseLegacySaver) {
+    download = await promiseNewDownload(httpUrl("interruptible.txt"));
+  } else {
+    download = await promiseStartLegacyDownload(
+      httpUrl("interruptible.txt"),
+      options
+    );
+  }
+
+  // Cancel the download after receiving the first part of the response.
+  let deferCancel = Promise.withResolvers();
+  let onchange = function () {
+    if (!download.stopped && !download.canceled && download.progress == 50) {
+      // Cancel the download immediately during the notification.
+      deferCancel.resolve(download.cancel());
+
+      // The state change happens immediately after calling "cancel", but
+      // temporary files or part files may still exist at this point.
+      Assert.ok(download.canceled);
+    }
+  };
+
+  // Register for the notification, but also call the function directly in
+  // case the download already reached the expected progress.  This may happen
+  // when using DownloadLegacySaver.
+  download.onchange = onchange;
+  onchange();
+
+  let promiseAttempt;
+  if (!gUseLegacySaver) {
+    promiseAttempt = download.start();
+  }
+
+  // Wait on the promise returned by the "cancel" method to ensure that the
+  // cancellation process finished and temporary files were removed.
+  await deferCancel.promise;
+
+  if (gUseLegacySaver) {
+    // The nsIWebBrowserPersist instance should have been canceled now.
+    Assert.equal(options.outPersist.result, Cr.NS_ERROR_ABORT);
+  }
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.canceled);
+  Assert.ok(download.error === null);
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+
+  // Progress properties are not reset by canceling.
+  Assert.equal(download.progress, 50);
+  Assert.equal(download.totalBytes, TEST_DATA_SHORT.length * 2);
+  Assert.equal(download.currentBytes, TEST_DATA_SHORT.length);
+
+  if (!gUseLegacySaver) {
+    // The promise returned by "start" should have been rejected meanwhile.
+    try {
+      await promiseAttempt;
+      do_throw("The download should have been canceled.");
+    } catch (ex) {
+      if (!(ex instanceof Downloads.Error)) {
+        throw ex;
+      }
+      Assert.ok(!ex.becauseSourceFailed);
+      Assert.ok(!ex.becauseTargetFailed);
+    }
+  }
+});
+
+/**
+ * Cancels a download while keeping partially downloaded data, and verifies that
+ * both the target file and the ".part" file are deleted.
+ */
+add_task(async function test_cancel_midway_tryToKeepPartialData() {
+  let download = await promiseStartDownload_tryToKeepPartialData({
+    useLegacySaver: gUseLegacySaver,
+  });
+
+  Assert.ok(await IOUtils.exists(download.target.path));
+  Assert.ok(await IOUtils.exists(download.target.partFilePath));
+
+  await download.cancel();
+  await download.removePartialData();
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.canceled);
+  Assert.ok(download.error === null);
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+});
+
+/**
+ * Cancels a download right after starting it.
+ */
+add_task(async function test_cancel_immediately() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+
+  let promiseAttempt = download.start();
+  Assert.ok(!download.stopped);
+
+  let promiseCancel = download.cancel();
+  Assert.ok(download.canceled);
+
+  // At this point, we don't know whether the download has already stopped or
+  // is still waiting for cancellation.  We can wait on the promise returned
+  // by the "start" method to know for sure.
+  try {
+    await promiseAttempt;
+    do_throw("The download should have been canceled.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error)) {
+      throw ex;
+    }
+    Assert.ok(!ex.becauseSourceFailed);
+    Assert.ok(!ex.becauseTargetFailed);
+  }
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.canceled);
+  Assert.ok(download.error === null);
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+
+  // Check that the promise returned by the "cancel" method has been resolved.
+  await promiseCancel;
+});
+
+/**
+ * Cancels and restarts a download sequentially.
+ */
+add_task(async function test_cancel_midway_restart() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+
+  // The first time, cancel the download midway.
+  await promiseDownloadMidway(download);
+  await download.cancel();
+
+  Assert.ok(download.stopped);
+
+  // The second time, we'll provide the entire interruptible response.
+  continueResponses();
+  download.onchange = null;
+  let promiseAttempt = download.start();
+
+  // Download state should have already been reset.
+  Assert.ok(!download.stopped);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  // For the following test, we rely on the network layer reporting its progress
+  // asynchronously.  Otherwise, there is nothing stopping the restarted
+  // download from reaching the same progress as the first request already.
+  Assert.equal(download.progress, 0);
+  Assert.equal(download.totalBytes, 0);
+  Assert.equal(download.currentBytes, 0);
+
+  await promiseAttempt;
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+});
+
+/**
+ * Cancels a download and restarts it from where it stopped.
+ */
+add_task(async function test_cancel_midway_restart_tryToKeepPartialData() {
+  let download = await promiseStartDownload_tryToKeepPartialData({
+    useLegacySaver: gUseLegacySaver,
+  });
+  await download.cancel();
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.hasPartialData);
+
+  // We should have kept the partial data and an empty target file placeholder.
+  Assert.ok(await IOUtils.exists(download.target.path));
+  await promiseVerifyContents(download.target.partFilePath, TEST_DATA_SHORT);
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+
+  // Verify that the server sent the response from the start.
+  Assert.equal(gMostRecentFirstBytePos, 0);
+
+  // The second time, we'll request and obtain the second part of the response,
+  // but we still stop when half of the remaining progress is reached.
+  let deferMidway = Promise.withResolvers();
+  download.onchange = function () {
+    if (
+      !download.stopped &&
+      !download.canceled &&
+      download.currentBytes == Math.floor((TEST_DATA_SHORT.length * 3) / 2)
+    ) {
+      download.onchange = null;
+      deferMidway.resolve();
+    }
+  };
+
+  mustInterruptResponses();
+  let promiseAttempt = download.start();
+
+  // Continue when the number of bytes we received is correct, then check that
+  // progress is at about 75 percent.  The exact figure may vary because of
+  // rounding issues, since the total number of bytes in the response might not
+  // be a multiple of four.
+  await deferMidway.promise;
+  Assert.ok(download.progress > 72 && download.progress < 78);
+
+  // Now we allow the download to finish.
+  continueResponses();
+  await promiseAttempt;
+
+  // Check that the server now sent the second part only.
+  Assert.equal(gMostRecentFirstBytePos, TEST_DATA_SHORT.length);
+
+  // The target file should now have been created, and the ".part" file deleted.
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+});
+
+/**
+ * Cancels a download while keeping partially downloaded data, then removes the
+ * data and restarts the download from the beginning.
+ */
+add_task(async function test_cancel_midway_restart_removePartialData() {
+  let download = await promiseStartDownload_tryToKeepPartialData({
+    useLegacySaver: gUseLegacySaver,
+  });
+  await download.cancel();
+  await download.removePartialData();
+
+  Assert.ok(!download.hasPartialData);
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+
+  // The second time, we'll request and obtain the entire response again.
+  continueResponses();
+  await download.start();
+
+  // Verify that the server sent the response from the start.
+  Assert.equal(gMostRecentFirstBytePos, 0);
+
+  // The target file should now have been created, and the ".part" file deleted.
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+});
+
+/**
+ * Cancels a download while keeping partially downloaded data, then removes the
+ * data and restarts the download from the beginning without keeping the partial
+ * data anymore.
+ */
+add_task(
+  async function test_cancel_midway_restart_tryToKeepPartialData_false() {
+    let download = await promiseStartDownload_tryToKeepPartialData({
+      useLegacySaver: gUseLegacySaver,
+    });
+    await download.cancel();
+
+    download.tryToKeepPartialData = false;
+
+    // The above property change does not affect existing partial data.
+    Assert.ok(download.hasPartialData);
+    await promiseVerifyContents(download.target.partFilePath, TEST_DATA_SHORT);
+
+    await download.removePartialData();
+    Assert.equal(false, await IOUtils.exists(download.target.path));
+    Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+
+    // Restart the download from the beginning.
+    mustInterruptResponses();
+    download.start().catch(() => {});
+
+    await promiseDownloadMidway(download);
+    await promisePartFileReady(download);
+
+    // While the download is in progress, we should still have a ".part" file.
+    Assert.ok(!download.hasPartialData);
+    Assert.ok(await IOUtils.exists(download.target.path));
+    Assert.ok(await IOUtils.exists(download.target.partFilePath));
+
+    // On Unix, verify that the file with the partially downloaded data is not
+    // accessible by other users on the system.
+    if (Services.appinfo.OS == "Darwin" || Services.appinfo.OS == "Linux") {
+      Assert.equal(
+        (await IOUtils.stat(download.target.partFilePath)).permissions,
+        0o600
+      );
+    }
+
+    await download.cancel();
+
+    // The ".part" file should be deleted now that the download is canceled.
+    Assert.ok(!download.hasPartialData);
+    Assert.equal(false, await IOUtils.exists(download.target.path));
+    Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+
+    // The third time, we'll request and obtain the entire response again.
+    continueResponses();
+    await download.start();
+
+    // Verify that the server sent the response from the start.
+    Assert.equal(gMostRecentFirstBytePos, 0);
+
+    // The target file should now have been created, and the ".part" file deleted.
+    await promiseVerifyTarget(
+      download.target,
+      TEST_DATA_SHORT + TEST_DATA_SHORT
+    );
+    Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  }
+);
+
+/**
+ * Cancels a download right after starting it, then restarts it immediately.
+ */
+add_task(async function test_cancel_immediately_restart_immediately() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+  let promiseAttempt = download.start();
+
+  Assert.ok(!download.stopped);
+
+  download.cancel();
+  Assert.ok(download.canceled);
+
+  let promiseRestarted = download.start();
+  Assert.ok(!download.stopped);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  // For the following test, we rely on the network layer reporting its progress
+  // asynchronously.  Otherwise, there is nothing stopping the restarted
+  // download from reaching the same progress as the first request already.
+  Assert.equal(download.hasProgress, false);
+  Assert.equal(download.progress, 0);
+  Assert.equal(download.totalBytes, 0);
+  Assert.equal(download.currentBytes, 0);
+
+  // Ensure the next request is now allowed to complete, regardless of whether
+  // the canceled request was received by the server or not.
+  continueResponses();
+  try {
+    await promiseAttempt;
+    // If we get here, it means that the first attempt actually succeeded.  In
+    // fact, this could be a valid outcome, because the cancellation request may
+    // not have been processed in time before the download finished.
+    info("The download should have been canceled.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error)) {
+      throw ex;
+    }
+    Assert.ok(!ex.becauseSourceFailed);
+    Assert.ok(!ex.becauseTargetFailed);
+  }
+
+  await promiseRestarted;
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+});
+
+/**
+ * Cancels a download midway, then restarts it immediately.
+ */
+add_task(async function test_cancel_midway_restart_immediately() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+  let promiseAttempt = download.start();
+
+  // The first time, cancel the download midway.
+  await promiseDownloadMidway(download);
+  download.cancel();
+  Assert.ok(download.canceled);
+
+  let promiseRestarted = download.start();
+  Assert.ok(!download.stopped);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  // For the following test, we rely on the network layer reporting its progress
+  // asynchronously.  Otherwise, there is nothing stopping the restarted
+  // download from reaching the same progress as the first request already.
+  Assert.equal(download.hasProgress, false);
+  Assert.equal(download.progress, 0);
+  Assert.equal(download.totalBytes, 0);
+  Assert.equal(download.currentBytes, 0);
+
+  // The second request is allowed to complete.
+  continueResponses();
+  try {
+    await promiseAttempt;
+    do_throw("The download should have been canceled.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error)) {
+      throw ex;
+    }
+    Assert.ok(!ex.becauseSourceFailed);
+    Assert.ok(!ex.becauseTargetFailed);
+  }
+
+  await promiseRestarted;
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+});
+
+/**
+ * Calls the "cancel" method on a successful download.
+ */
+add_task(async function test_cancel_successful() {
+  let download = await promiseStartDownload();
+  await promiseDownloadStopped(download);
+
+  // The cancel method should succeed with no effect.
+  await download.cancel();
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT);
+});
+
+/**
+ * Calls the "cancel" method two times in a row.
+ */
+add_task(async function test_cancel_twice() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+
+  let promiseAttempt = download.start();
+  Assert.ok(!download.stopped);
+
+  let promiseCancel1 = download.cancel();
+  Assert.ok(download.canceled);
+  let promiseCancel2 = download.cancel();
+
+  try {
+    await promiseAttempt;
+    do_throw("The download should have been canceled.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error)) {
+      throw ex;
+    }
+    Assert.ok(!ex.becauseSourceFailed);
+    Assert.ok(!ex.becauseTargetFailed);
+  }
+
+  // Both promises should now be resolved.
+  await promiseCancel1;
+  await promiseCancel2;
+
+  Assert.ok(download.stopped);
+  Assert.ok(!download.succeeded);
+  Assert.ok(download.canceled);
+  Assert.ok(download.error === null);
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+});
+
+/**
+ * Checks the "refresh" method for succeeded downloads.
+ */
+add_task(async function test_refresh_succeeded() {
+  let download = await promiseStartDownload();
+  await promiseDownloadStopped(download);
+
+  // The DownloadTarget properties should be the same after calling "refresh".
+  await download.refresh();
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT);
+
+  // If the file is removed, only the "exists" property should change, and the
+  // "size" property should keep its previous value.
+  await IOUtils.move(download.target.path, `${download.target.path}.old`);
+  await download.refresh();
+  Assert.ok(!download.target.exists);
+  Assert.equal(
+    await expectNonZeroDownloadTargetSize(download.target),
+    TEST_DATA_SHORT.length
+  );
+
+  // The DownloadTarget properties should be restored when the file is put back.
+  await IOUtils.move(`${download.target.path}.old`, download.target.path);
+  await download.refresh();
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT);
+});
+
+/**
+ * Checks that a download cannot be restarted after the "finalize" method.
+ */
+add_task(async function test_finalize() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible.txt"));
+
+  let promiseFinalized = download.finalize();
+
+  try {
+    await download.start();
+    do_throw("It should not be possible to restart after finalization.");
+  } catch (ex) {}
+
+  await promiseFinalized;
+
+  Assert.ok(download.stopped);
+  Assert.ok(!download.succeeded);
+  Assert.ok(download.canceled);
+  Assert.ok(download.error === null);
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+});
+
+/**
+ * Checks that the "finalize" method can remove partially downloaded data.
+ */
+add_task(async function test_finalize_tryToKeepPartialData() {
+  // Check finalization without removing partial data.
+  let download = await promiseStartDownload_tryToKeepPartialData({
+    useLegacySaver: gUseLegacySaver,
+  });
+  await download.finalize();
+
+  Assert.ok(download.hasPartialData);
+  Assert.ok(await IOUtils.exists(download.target.path));
+  Assert.ok(await IOUtils.exists(download.target.partFilePath));
+
+  // Clean up.
+  await download.removePartialData();
+
+  // Check finalization while removing partial data.
+  download = await promiseStartDownload_tryToKeepPartialData({
+    useLegacySaver: gUseLegacySaver,
+  });
+  await download.finalize(true);
+
+  Assert.ok(!download.hasPartialData);
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+});
+
+/**
+ * Checks that whenSucceeded returns a promise that is resolved after a restart.
+ */
+add_task(async function test_whenSucceeded_after_restart() {
+  mustInterruptResponses();
+
+  let promiseSucceeded;
+
+  let download;
+  if (!gUseLegacySaver) {
+    // When testing DownloadCopySaver, we have control over the download, thus
+    // we can verify getting a reference before the first download attempt.
+    download = await promiseNewDownload(httpUrl("interruptible.txt"));
+    promiseSucceeded = download.whenSucceeded();
+    download.start().catch(() => {});
+  } else {
+    // When testing DownloadLegacySaver, the download is already started when it
+    // is created, thus we cannot get the reference before the first attempt.
+    download = await promiseStartLegacyDownload(httpUrl("interruptible.txt"));
+    promiseSucceeded = download.whenSucceeded();
+  }
+
+  // Cancel the first download attempt.
+  await download.cancel();
+
+  // The second request is allowed to complete.
+  continueResponses();
+  download.start().catch(() => {});
+
+  // Wait for the download to finish by waiting on the whenSucceeded promise.
+  await promiseSucceeded;
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+});
+
+/**
+ * Ensures download error details are reported on network failures.
+ */
+add_task(async function test_error_source() {
+  let serverSocket = startFakeServer();
+  try {
+    let sourceUrl = "http://localhost:" + serverSocket.port + "/source.txt";
+
+    let download;
+    try {
+      if (!gUseLegacySaver) {
+        // When testing DownloadCopySaver, we want to check that the promise
+        // returned by the "start" method is rejected.
+        download = await promiseNewDownload(sourceUrl);
+
+        Assert.ok(download.error === null);
+
+        await download.start();
+      } else {
+        // When testing DownloadLegacySaver, we cannot be sure whether we are
+        // testing the promise returned by the "start" method or we are testing
+        // the "error" property checked by promiseDownloadStopped.  This happens
+        // because we don't have control over when the download is started.
+        download = await promiseStartLegacyDownload(sourceUrl);
+        await promiseDownloadStopped(download);
+      }
+      do_throw("The download should have failed.");
+    } catch (ex) {
+      if (!(ex instanceof Downloads.Error) || !ex.becauseSourceFailed) {
+        throw ex;
+      }
+      // A specific error object is thrown when reading from the source fails.
+    }
+
+    // Check the properties now that the download stopped.
+    Assert.ok(download.stopped);
+    Assert.ok(!download.canceled);
+    Assert.ok(download.error !== null);
+    Assert.ok(download.error.becauseSourceFailed);
+    Assert.ok(!download.error.becauseTargetFailed);
+
+    Assert.equal(false, await IOUtils.exists(download.target.path));
+    Assert.ok(!download.target.exists);
+    Assert.equal(download.target.size, 0);
+  } finally {
+    serverSocket.close();
+  }
+});
+
+/**
+ * Ensures a download error is reported when receiving less bytes than what was
+ * specified in the Content-Length header.
+ */
+add_task(async function test_error_source_partial() {
+  let sourceUrl = httpUrl("shorter-than-content-length-http-1-1.txt");
+
+  let enforcePref = Services.prefs.getBoolPref(
+    "network.http.enforce-framing.http1"
+  );
+  Services.prefs.setBoolPref("network.http.enforce-framing.http1", true);
+
+  function cleanup() {
+    Services.prefs.setBoolPref(
+      "network.http.enforce-framing.http1",
+      enforcePref
+    );
+  }
+  registerCleanupFunction(cleanup);
+
+  let download;
+  try {
+    if (!gUseLegacySaver) {
+      // When testing DownloadCopySaver, we want to check that the promise
+      // returned by the "start" method is rejected.
+      download = await promiseNewDownload(sourceUrl);
+
+      Assert.ok(download.error === null);
+
+      await download.start();
+    } else {
+      // When testing DownloadLegacySaver, we cannot be sure whether we are
+      // testing the promise returned by the "start" method or we are testing
+      // the "error" property checked by promiseDownloadStopped.  This happens
+      // because we don't have control over when the download is started.
+      download = await promiseStartLegacyDownload(sourceUrl);
+      await promiseDownloadStopped(download);
+    }
+    do_throw("The download should have failed.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error) || !ex.becauseSourceFailed) {
+      throw ex;
+    }
+    // A specific error object is thrown when reading from the source fails.
+  }
+
+  // Check the properties now that the download stopped.
+  Assert.ok(download.stopped);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error !== null);
+  Assert.ok(download.error.becauseSourceFailed);
+  Assert.ok(!download.error.becauseTargetFailed);
+  Assert.equal(download.error.result, Cr.NS_ERROR_NET_PARTIAL_TRANSFER);
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+});
+
+/**
+ * Ensures a download error is reported when an RST packet is received.
+ */
+add_task(async function test_error_source_netreset() {
+  if (AppConstants.platform == "win") {
+    return;
+  }
+
+  let download;
+  try {
+    if (!gUseLegacySaver) {
+      // When testing DownloadCopySaver, we want to check that the promise
+      // returned by the "start" method is rejected.
+      download = await promiseNewDownload(httpUrl("netreset.txt"));
+
+      Assert.ok(download.error === null);
+
+      await download.start();
+    } else {
+      // When testing DownloadLegacySaver, we cannot be sure whether we are
+      // testing the promise returned by the "start" method or we are testing
+      // the "error" property checked by promiseDownloadStopped.  This happens
+      // because we don't have control over when the download is started.
+      download = await promiseStartLegacyDownload(httpUrl("netreset.txt"));
+      await promiseDownloadStopped(download);
+    }
+    do_throw("The download should have failed.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error) || !ex.becauseSourceFailed) {
+      throw ex;
+    }
+    // A specific error object is thrown when reading from the source fails.
+  }
+
+  // Check the properties now that the download stopped.
+  Assert.ok(download.stopped);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error !== null);
+  Assert.ok(download.error.becauseSourceFailed);
+  Assert.ok(!download.error.becauseTargetFailed);
+  Assert.equal(download.error.result, Cr.NS_ERROR_NET_RESET);
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+});
+
+/**
+ * Ensures download error details are reported on local writing failures.
+ */
+add_task(async function test_error_target() {
+  // Create a file without write access permissions before downloading.
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  targetFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, 0);
+  try {
+    let download;
+    try {
+      if (!gUseLegacySaver) {
+        // When testing DownloadCopySaver, we want to check that the promise
+        // returned by the "start" method is rejected.
+        download = await Downloads.createDownload({
+          source: httpUrl("source.txt"),
+          target: targetFile,
+        });
+        await download.start();
+      } else {
+        // When testing DownloadLegacySaver, we cannot be sure whether we are
+        // testing the promise returned by the "start" method or we are testing
+        // the "error" property checked by promiseDownloadStopped.  This happens
+        // because we don't have control over when the download is started.
+        download = await promiseStartLegacyDownload(null, { targetFile });
+        await promiseDownloadStopped(download);
+      }
+      do_throw("The download should have failed.");
+    } catch (ex) {
+      if (!(ex instanceof Downloads.Error) || !ex.becauseTargetFailed) {
+        throw ex;
+      }
+      // A specific error object is thrown when writing to the target fails.
+    }
+
+    // Check the properties now that the download stopped.
+    Assert.ok(download.stopped);
+    Assert.ok(!download.canceled);
+    Assert.ok(download.error !== null);
+    Assert.ok(download.error.becauseTargetFailed);
+    Assert.ok(!download.error.becauseSourceFailed);
+
+    // Check unserializing a download with an errorObj and restarting it will
+    // clear the errorObj initially.
+    let serializable = download.toSerializable();
+    Assert.ok(serializable.errorObj, "Ensure we have an errorObj initially");
+    let reserialized = JSON.parse(JSON.stringify(serializable));
+    download = await Downloads.createDownload(reserialized);
+    let promise = download.start().catch(() => {});
+    serializable = download.toSerializable();
+    Assert.ok(!serializable.errorObj, "Ensure we didn't persist the errorObj");
+    await promise;
+  } finally {
+    // Restore the default permissions to allow deleting the file on Windows.
+    if (targetFile.exists()) {
+      targetFile.permissions = FileUtils.PERMS_FILE;
+      targetFile.remove(false);
+    }
+  }
+});
+
+/**
+ * Restarts a failed download.
+ */
+add_task(async function test_error_restart() {
+  let download;
+
+  // Create a file without write access permissions before downloading.
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  targetFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, 0);
+  try {
+    // Use DownloadCopySaver or DownloadLegacySaver based on the test run,
+    // specifying the target file we created.
+    if (!gUseLegacySaver) {
+      download = await Downloads.createDownload({
+        source: httpUrl("source.txt"),
+        target: targetFile,
+      });
+      download.start().catch(() => {});
+    } else {
+      download = await promiseStartLegacyDownload(null, { targetFile });
+    }
+    await promiseDownloadStopped(download);
+    do_throw("The download should have failed.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error) || !ex.becauseTargetFailed) {
+      throw ex;
+    }
+    // A specific error object is thrown when writing to the target fails.
+  } finally {
+    // Restore the default permissions to allow deleting the file on Windows.
+    if (targetFile.exists()) {
+      targetFile.permissions = FileUtils.PERMS_FILE;
+
+      // Also for Windows, rename the file before deleting.  This makes the
+      // current file name available immediately for a new file, while deleting
+      // in place prevents creation of a file with the same name for some time.
+      targetFile.moveTo(null, targetFile.leafName + ".delete.tmp");
+      targetFile.remove(false);
+    }
+  }
+
+  // Restart the download and wait for completion.
+  await download.start();
+
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.canceled);
+  Assert.ok(download.error === null);
+  Assert.equal(download.progress, 100);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT);
+});
+
+/**
+ * Executes download in both public and private modes.
+ */
+add_task(async function test_public_and_private() {
+  let sourcePath = "/test_public_and_private.txt";
+  let sourceUrl = httpUrl("test_public_and_private.txt");
+  let testCount = 0;
+
+  // Apply pref to allow all cookies.
+  Services.prefs.setIntPref("network.cookie.cookieBehavior", 0);
+
+  function cleanup() {
+    Services.prefs.clearUserPref("network.cookie.cookieBehavior");
+    Services.cookies.removeAll();
+    gHttpServer.registerPathHandler(sourcePath, null);
+  }
+  registerCleanupFunction(cleanup);
+
+  gHttpServer.registerPathHandler(sourcePath, function (aRequest, aResponse) {
+    aResponse.setHeader("Content-Type", "text/plain", false);
+
+    if (testCount == 0) {
+      // No cookies should exist for first public download.
+      Assert.ok(!aRequest.hasHeader("Cookie"));
+      aResponse.setHeader("Set-Cookie", "foobar=1", false);
+      testCount++;
+    } else if (testCount == 1) {
+      // The cookie should exists for second public download.
+      Assert.ok(aRequest.hasHeader("Cookie"));
+      Assert.equal(aRequest.getHeader("Cookie"), "foobar=1");
+      testCount++;
+    } else if (testCount == 2) {
+      // No cookies should exist for first private download.
+      Assert.ok(!aRequest.hasHeader("Cookie"));
+    }
+  });
+
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  await Downloads.fetch(sourceUrl, targetFile);
+  await Downloads.fetch(sourceUrl, targetFile);
+
+  if (!gUseLegacySaver) {
+    let download = await Downloads.createDownload({
+      source: { url: sourceUrl, isPrivate: true },
+      target: targetFile,
+    });
+    await download.start();
+  } else {
+    let download = await promiseStartLegacyDownload(sourceUrl, {
+      isPrivate: true,
+    });
+    await promiseDownloadStopped(download);
+  }
+
+  cleanup();
+});
+
+/**
+ * Checks the startTime gets updated even after a restart.
+ */
+add_task(async function test_cancel_immediately_restart_and_check_startTime() {
+  let download = await promiseStartDownload();
+
+  let startTime = download.startTime;
+  Assert.ok(isValidDate(download.startTime));
+
+  await download.cancel();
+  Assert.equal(download.startTime.getTime(), startTime.getTime());
+
+  // Wait for a timeout.
+  await promiseTimeout(10);
+
+  await download.start();
+  Assert.ok(download.startTime.getTime() > startTime.getTime());
+});
+
+/**
+ * Executes download with content-encoding.
+ */
+add_task(async function test_with_content_encoding() {
+  let sourcePath = "/test_with_content_encoding.txt";
+  let sourceUrl = httpUrl("test_with_content_encoding.txt");
+
+  function cleanup() {
+    gHttpServer.registerPathHandler(sourcePath, null);
+  }
+  registerCleanupFunction(cleanup);
+
+  gHttpServer.registerPathHandler(sourcePath, function (aRequest, aResponse) {
+    aResponse.setHeader("Content-Type", "text/plain", false);
+    aResponse.setHeader("Content-Encoding", "gzip", false);
+    aResponse.setHeader(
+      "Content-Length",
+      "" + TEST_DATA_SHORT_GZIP_ENCODED.length,
+      false
+    );
+
+    let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
+    bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED);
+  });
+
+  let download = await promiseStartDownload(sourceUrl);
+  await promiseDownloadStopped(download);
+
+  Assert.equal(download.progress, 100);
+  Assert.equal(download.totalBytes, TEST_DATA_SHORT_GZIP_ENCODED.length);
+
+  // Ensure the content matches the decoded test data.
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT);
+
+  cleanup();
+});
+
+/**
+ * Checks that the file is not decoded if the extension matches the encoding.
+ */
+add_task(async function test_with_content_encoding_ignore_extension() {
+  let sourcePath = "/test_with_content_encoding_ignore_extension.gz";
+  let sourceUrl = httpUrl("test_with_content_encoding_ignore_extension.gz");
+
+  function cleanup() {
+    gHttpServer.registerPathHandler(sourcePath, null);
+  }
+  registerCleanupFunction(cleanup);
+
+  gHttpServer.registerPathHandler(sourcePath, function (aRequest, aResponse) {
+    aResponse.setHeader("Content-Type", "text/plain", false);
+    aResponse.setHeader("Content-Encoding", "gzip", false);
+    aResponse.setHeader(
+      "Content-Length",
+      "" + TEST_DATA_SHORT_GZIP_ENCODED.length,
+      false
+    );
+
+    let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
+    bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED);
+  });
+
+  let download = await promiseStartDownload(sourceUrl);
+  await promiseDownloadStopped(download);
+
+  Assert.equal(download.progress, 100);
+  Assert.equal(download.totalBytes, TEST_DATA_SHORT_GZIP_ENCODED.length);
+  Assert.equal(
+    await expectNonZeroDownloadTargetSize(download.target),
+    TEST_DATA_SHORT_GZIP_ENCODED.length
+  );
+
+  // Ensure the content matches the encoded test data.  We convert the data to a
+  // string before executing the content check.
+  await promiseVerifyTarget(
+    download.target,
+    String.fromCharCode.apply(String, TEST_DATA_SHORT_GZIP_ENCODED)
+  );
+
+  cleanup();
+});
+
+/**
+ * Cancels and restarts a download sequentially with content-encoding.
+ */
+add_task(async function test_cancel_midway_restart_with_content_encoding() {
+  mustInterruptResponses();
+
+  let download = await promiseStartDownload(httpUrl("interruptible_gzip.txt"));
+
+  // The first time, cancel the download midway.
+  await new Promise(resolve => {
+    let onchange = function () {
+      if (
+        !download.stopped &&
+        !download.canceled &&
+        download.currentBytes == TEST_DATA_SHORT_GZIP_ENCODED_FIRST.length
+      ) {
+        resolve(download.cancel());
+      }
+    };
+
+    // Register for the notification, but also call the function directly in
+    // case the download already reached the expected progress.
+    download.onchange = onchange;
+    onchange();
+  });
+
+  Assert.ok(download.stopped);
+
+  // The second time, we'll provide the entire interruptible response.
+  continueResponses();
+  download.onchange = null;
+  await download.start();
+
+  Assert.equal(download.progress, 100);
+  Assert.equal(download.totalBytes, TEST_DATA_SHORT_GZIP_ENCODED.length);
+
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT);
+});
+
+/**
+ * Download with parental controls enabled.
+ */
+add_task(async function test_blocked_parental_controls() {
+  let blockFn = base => ({
+    shouldBlockForParentalControls: () => Promise.resolve(true),
+  });
+
+  Integration.downloads.register(blockFn);
+  function cleanup() {
+    Integration.downloads.unregister(blockFn);
+  }
+  registerCleanupFunction(cleanup);
+
+  let download;
+  try {
+    if (!gUseLegacySaver) {
+      // When testing DownloadCopySaver, we want to check that the promise
+      // returned by the "start" method is rejected.
+      download = await promiseNewDownload();
+      await download.start();
+    } else {
+      // When testing DownloadLegacySaver, we cannot be sure whether we are
+      // testing the promise returned by the "start" method or we are testing
+      // the "error" property checked by promiseDownloadStopped.  This happens
+      // because we don't have control over when the download is started.
+      download = await promiseStartLegacyDownload();
+      await promiseDownloadStopped(download);
+    }
+    do_throw("The download should have blocked.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error) || !ex.becauseBlocked) {
+      throw ex;
+    }
+    Assert.ok(ex.becauseBlockedByParentalControls);
+    Assert.ok(download.error.becauseBlockedByParentalControls);
+  }
+
+  // Now that the download stopped, the target file should not exist.
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+
+  cleanup();
+});
+
+/**
+ * Test a download that will be blocked by Windows parental controls by
+ * resulting in an HTTP status code of 450.
+ */
+add_task(async function test_blocked_parental_controls_httpstatus450() {
+  let download;
+  try {
+    if (!gUseLegacySaver) {
+      download = await promiseNewDownload(httpUrl("parentalblocked.zip"));
+      await download.start();
+    } else {
+      download = await promiseStartLegacyDownload(
+        httpUrl("parentalblocked.zip")
+      );
+      await promiseDownloadStopped(download);
+    }
+    do_throw("The download should have blocked.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error) || !ex.becauseBlocked) {
+      throw ex;
+    }
+    Assert.ok(ex.becauseBlockedByParentalControls);
+    Assert.ok(download.error.becauseBlockedByParentalControls);
+    Assert.ok(download.stopped);
+  }
+
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+});
+
+/**
+ * Check that DownloadCopySaver can always retrieve the hash.
+ * DownloadLegacySaver can only retrieve the hash when
+ * nsIExternalHelperAppService is invoked.
+ */
+add_task(async function test_getSha256Hash() {
+  if (!gUseLegacySaver) {
+    let download = await promiseStartDownload(httpUrl("source.txt"));
+    await promiseDownloadStopped(download);
+    Assert.ok(download.stopped);
+    Assert.equal(32, download.saver.getSha256Hash().length);
+  }
+});
+
+/**
+ * Checks that application reputation blocks the download and the target file
+ * does not exist.
+ */
+add_task(async function test_blocked_applicationReputation() {
+  let download = await promiseBlockedDownload({
+    keepPartialData: false,
+    keepBlockedData: false,
+  });
+
+  // Now that the download is blocked, the target file should not exist.
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+
+  // There should also be no blocked data in this case
+  Assert.ok(!download.hasBlockedData);
+});
+
+/**
+ * Checks that if a download restarts while processing an application reputation
+ * request, the status is handled correctly.
+ */
+add_task(async function test_blocked_applicationReputation_race() {
+  let isFirstShouldBlockCall = true;
+
+  let blockFn = base => ({
+    shouldBlockForReputationCheck(download) {
+      if (isFirstShouldBlockCall) {
+        isFirstShouldBlockCall = false;
+
+        // 2. Cancel and restart the download before the first attempt has a
+        //    chance to finish.
+        download.cancel();
+        download.removePartialData();
+        download.start();
+
+        // 3. Allow the first attempt to finish with a blocked response.
+        return Promise.resolve({
+          shouldBlock: true,
+          verdict: Downloads.Error.BLOCK_VERDICT_UNCOMMON,
+        });
+      }
+
+      // 4/5. Don't block the download the second time. The race condition would
+      //      occur with the first attempt regardless of whether the second one
+      //      is blocked, but not blocking here makes the test simpler.
+      return Promise.resolve({
+        shouldBlock: false,
+        verdict: "",
+      });
+    },
+    shouldKeepBlockedData: () => Promise.resolve(true),
+  });
+
+  Integration.downloads.register(blockFn);
+  function cleanup() {
+    Integration.downloads.unregister(blockFn);
+  }
+  registerCleanupFunction(cleanup);
+
+  let download;
+
+  try {
+    // 1. Start the download and get a reference to the promise asociated with
+    //    the first attempt, before allowing the response to continue.
+    download = await promiseStartDownload_tryToKeepPartialData({
+      useLegacySaver: gUseLegacySaver,
+    });
+    let firstAttempt = promiseDownloadStopped(download);
+    continueResponses();
+
+    // 4/5. Wait for the first attempt to be completed. The result of this
+    //      should appear as a cancellation.
+    await firstAttempt;
+
+    do_throw("The first attempt should have been canceled.");
+  } catch (ex) {
+    // The "becauseBlocked" property should be false.
+    if (!(ex instanceof Downloads.Error) || ex.becauseBlocked) {
+      throw ex;
+    }
+  }
+
+  // 6. Wait for the second attempt to be completed.
+  await promiseDownloadStopped(download);
+
+  // 7. At this point, "hasBlockedData" should be false.
+  Assert.ok(!download.hasBlockedData);
+
+  cleanup();
+});
+
+/**
+ * Checks that application reputation blocks the download but maintains the
+ * blocked data, which will be deleted when the block is confirmed.
+ */
+add_task(async function test_blocked_applicationReputation_confirmBlock() {
+  let download = await promiseBlockedDownload({
+    keepPartialData: true,
+    keepBlockedData: true,
+  });
+
+  Assert.ok(download.hasBlockedData);
+  Assert.equal((await IOUtils.stat(download.target.path)).size, 0);
+  Assert.ok(await IOUtils.exists(download.target.partFilePath));
+
+  await download.confirmBlock();
+
+  // After confirming the block the download should be in a failed state and
+  // have no downloaded data left on disk.
+  Assert.ok(download.stopped);
+  Assert.ok(!download.succeeded);
+  Assert.ok(!download.hasBlockedData);
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+});
+
+/**
+ * Checks that application reputation blocks the download but maintains the
+ * blocked data, which will be used to complete the download when unblocking.
+ */
+add_task(async function test_blocked_applicationReputation_unblock() {
+  let download = await promiseBlockedDownload({
+    keepPartialData: true,
+    keepBlockedData: true,
+    useLegacySaver: gUseLegacySaver,
+  });
+
+  Assert.ok(download.hasBlockedData);
+  Assert.equal((await IOUtils.stat(download.target.path)).size, 0);
+  Assert.ok(await IOUtils.exists(download.target.partFilePath));
+
+  await download.unblock();
+
+  // After unblocking the download should have succeeded and be
+  // present at the final path.
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.hasBlockedData);
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  await promiseVerifyTarget(download.target, TEST_DATA_SHORT + TEST_DATA_SHORT);
+
+  // The only indication the download was previously blocked is the
+  // existence of the error, so we make sure it's still set.
+  Assert.ok(download.error instanceof Downloads.Error);
+  Assert.ok(download.error.becauseBlocked);
+  Assert.ok(download.error.becauseBlockedByReputationCheck);
+});
+
+/**
+ * Check that calling cancel on a blocked download will not cause errors
+ */
+add_task(async function test_blocked_applicationReputation_cancel() {
+  let download = await promiseBlockedDownload({
+    keepPartialData: true,
+    keepBlockedData: true,
+  });
+
+  // This call should succeed on a blocked download.
+  await download.cancel();
+
+  // Calling cancel should not have changed the current state, the download
+  // should still be blocked.
+  Assert.ok(download.error.becauseBlockedByReputationCheck);
+  Assert.ok(download.stopped);
+  Assert.ok(!download.succeeded);
+  Assert.ok(download.hasBlockedData);
+});
+
+/**
+ * Checks that unblock and confirmBlock cannot race on a blocked download
+ */
+add_task(async function test_blocked_applicationReputation_decisionRace() {
+  let download = await promiseBlockedDownload({
+    keepPartialData: true,
+    keepBlockedData: true,
+  });
+
+  let unblockPromise = download.unblock();
+  let confirmBlockPromise = download.confirmBlock();
+
+  await confirmBlockPromise.then(
+    () => {
+      do_throw("confirmBlock should have failed.");
+    },
+    () => {}
+  );
+
+  await unblockPromise;
+
+  // After unblocking the download should have succeeded and be
+  // present at the final path.
+  Assert.ok(download.stopped);
+  Assert.ok(download.succeeded);
+  Assert.ok(!download.hasBlockedData);
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  Assert.ok(await IOUtils.exists(download.target.path));
+
+  download = await promiseBlockedDownload({
+    keepPartialData: true,
+    keepBlockedData: true,
+  });
+
+  confirmBlockPromise = download.confirmBlock();
+  unblockPromise = download.unblock();
+
+  await unblockPromise.then(
+    () => {
+      do_throw("unblock should have failed.");
+    },
+    () => {}
+  );
+
+  await confirmBlockPromise;
+
+  // After confirming the block the download should be in a failed state and
+  // have no downloaded data left on disk.
+  Assert.ok(download.stopped);
+  Assert.ok(!download.succeeded);
+  Assert.ok(!download.hasBlockedData);
+  Assert.equal(false, await IOUtils.exists(download.target.partFilePath));
+  Assert.equal(false, await IOUtils.exists(download.target.path));
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+});
+
+/**
+ * Checks that unblocking a blocked download fails if the blocked data has been
+ * removed.
+ */
+add_task(async function test_blocked_applicationReputation_unblock() {
+  let download = await promiseBlockedDownload({
+    keepPartialData: true,
+    keepBlockedData: true,
+  });
+
+  Assert.ok(download.hasBlockedData);
+  Assert.ok(await IOUtils.exists(download.target.partFilePath));
+
+  // Remove the blocked data without telling the download.
+  await IOUtils.remove(download.target.partFilePath);
+
+  let unblockPromise = download.unblock();
+  await unblockPromise.then(
+    () => {
+      do_throw("unblock should have failed.");
+    },
+    () => {}
+  );
+
+  // Even though unblocking failed the download state should have been updated
+  // to reflect the lack of blocked data.
+  Assert.ok(!download.hasBlockedData);
+  Assert.ok(download.stopped);
+  Assert.ok(!download.succeeded);
+  Assert.ok(!download.target.exists);
+  Assert.equal(download.target.size, 0);
+});
+
+/**
+ * download.showContainingDirectory() action
+ */
+add_task(async function test_showContainingDirectory() {
+  let targetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+
+  let download = await Downloads.createDownload({
+    source: { url: httpUrl("source.txt") },
+    target: "",
+  });
+
+  let promiseDirectoryShown = waitForDirectoryShown();
+  await download.showContainingDirectory();
+  let path = await promiseDirectoryShown;
+  try {
+    new FileUtils.File(path);
+    do_throw("Should have failed because of an invalid path.");
+  } catch (ex) {
+    if (!(ex instanceof Components.Exception)) {
+      throw ex;
+    }
+    // Invalid paths on Windows are reported with NS_ERROR_FAILURE,
+    // but with NS_ERROR_FILE_UNRECOGNIZED_PATH on Mac/Linux
+    let validResult =
+      ex.result == Cr.NS_ERROR_FILE_UNRECOGNIZED_PATH ||
+      ex.result == Cr.NS_ERROR_FAILURE;
+    Assert.ok(validResult);
+  }
+
+  download = await Downloads.createDownload({
+    source: { url: httpUrl("source.txt") },
+    target: targetPath,
+  });
+
+  promiseDirectoryShown = waitForDirectoryShown();
+  download.showContainingDirectory();
+  await promiseDirectoryShown;
+});
+
+/**
+ * download.launch() action
+ */
+add_task(async function test_launch() {
+  let customLauncher = getTempFile("app-launcher");
+
+  // Test both with and without setting a custom application.
+  for (let launcherPath of [null, customLauncher.path]) {
+    let download;
+    if (!gUseLegacySaver) {
+      // When testing DownloadCopySaver, we have control over the download, thus
+      // we can test that file is not launched if download.succeeded is not set.
+      download = await Downloads.createDownload({
+        source: httpUrl("source.txt"),
+        target: getTempFile(TEST_TARGET_FILE_NAME).path,
+        launcherPath,
+        launchWhenSucceeded: true,
+      });
+
+      try {
+        await download.launch();
+        do_throw("Can't launch download file as it has not completed yet");
+      } catch (ex) {
+        Assert.equal(
+          ex.message,
+          "launch can only be called if the download succeeded"
+        );
+      }
+
+      Assert.ok(download.launchWhenSucceeded);
+      await download.start();
+    } else {
+      // When testing DownloadLegacySaver, the download is already started when
+      // it is created, thus we don't test calling "launch" before starting.
+      download = await promiseStartLegacyDownload(httpUrl("source.txt"), {
+        launcherPath,
+        launchWhenSucceeded: true,
+      });
+      Assert.ok(download.launchWhenSucceeded);
+      await promiseDownloadStopped(download);
+    }
+
+    let promiseFileLaunched = waitForFileLaunched();
+    download.launch();
+    let result = await promiseFileLaunched;
+
+    // Verify that the results match the test case.
+    if (!launcherPath) {
+      // This indicates that the default handler has been chosen.
+      Assert.ok(result === null);
+    } else {
+      // Check the nsIMIMEInfo instance that would have been used for launching.
+      Assert.equal(result.preferredAction, Ci.nsIMIMEInfo.useHelperApp);
+      Assert.ok(
+        result.preferredApplicationHandler
+          .QueryInterface(Ci.nsILocalHandlerApp)
+          .executable.equals(customLauncher)
+      );
+    }
+  }
+});
+
+/**
+ * Test passing an invalid path as the launcherPath property.
+ */
+add_task(async function test_launcherPath_invalid() {
+  let download = await Downloads.createDownload({
+    source: { url: httpUrl("source.txt") },
+    target: { path: getTempFile(TEST_TARGET_FILE_NAME).path },
+    launcherPath: " ",
+  });
+
+  let promiseDownloadLaunched = new Promise(resolve => {
+    let waitFn = base => {
+      let launchOverride = {
+        launchDownload() {
+          Integration.downloads.unregister(waitFn);
+          let superPromise = super.launchDownload(...arguments);
+          resolve(superPromise);
+          return superPromise;
+        },
+      };
+      Object.setPrototypeOf(launchOverride, base);
+      return launchOverride;
+    };
+    Integration.downloads.register(waitFn);
+  });
+
+  await download.start();
+  try {
+    download.launch();
+    await promiseDownloadLaunched;
+    do_throw("Can't launch file with invalid custom launcher");
+  } catch (ex) {
+    if (!(ex instanceof Components.Exception)) {
+      throw ex;
+    }
+    // Invalid paths on Windows are reported with NS_ERROR_FAILURE,
+    // but with NS_ERROR_FILE_UNRECOGNIZED_PATH on Mac/Linux
+    let validResult =
+      ex.result == Cr.NS_ERROR_FILE_UNRECOGNIZED_PATH ||
+      ex.result == Cr.NS_ERROR_FAILURE;
+    Assert.ok(validResult);
+  }
+});
+
+/**
+ * Tests that download.launch() is automatically called after
+ * the download finishes if download.launchWhenSucceeded = true
+ */
+add_task(async function test_launchWhenSucceeded() {
+  let customLauncher = getTempFile("app-launcher");
+
+  // Test both with and without setting a custom application.
+  for (let launcherPath of [null, customLauncher.path]) {
+    let promiseFileLaunched = waitForFileLaunched();
+
+    if (!gUseLegacySaver) {
+      let download = await Downloads.createDownload({
+        source: httpUrl("source.txt"),
+        target: getTempFile(TEST_TARGET_FILE_NAME).path,
+        launchWhenSucceeded: true,
+        launcherPath,
+      });
+      await download.start();
+    } else {
+      let download = await promiseStartLegacyDownload(httpUrl("source.txt"), {
+        launcherPath,
+        launchWhenSucceeded: true,
+      });
+      await promiseDownloadStopped(download);
+    }
+
+    let result = await promiseFileLaunched;
+
+    // Verify that the results match the test case.
+    if (!launcherPath) {
+      // This indicates that the default handler has been chosen.
+      Assert.ok(result === null);
+    } else {
+      // Check the nsIMIMEInfo instance that would have been used for launching.
+      Assert.equal(result.preferredAction, Ci.nsIMIMEInfo.useHelperApp);
+      Assert.ok(
+        result.preferredApplicationHandler
+          .QueryInterface(Ci.nsILocalHandlerApp)
+          .executable.equals(customLauncher)
+      );
+    }
+  }
+});
+
+/**
+ * Tests that the proper content type is set for a normal download.
+ */
+add_task(async function test_contentType() {
+  let download = await promiseStartDownload(httpUrl("source.txt"));
+  await promiseDownloadStopped(download);
+
+  Assert.equal("text/plain", download.contentType);
+});
+
+/**
+ * Tests that the serialization/deserialization of the startTime Date
+ * object works correctly.
+ */
+add_task(async function test_toSerializable_startTime() {
+  let download1 = await promiseStartDownload(httpUrl("source.txt"));
+  await promiseDownloadStopped(download1);
+
+  let serializable = download1.toSerializable();
+  let reserialized = JSON.parse(JSON.stringify(serializable));
+
+  let download2 = await Downloads.createDownload(reserialized);
+
+  Assert.equal(download1.startTime.constructor.name, "Date");
+  Assert.equal(download2.startTime.constructor.name, "Date");
+  Assert.equal(download1.startTime.toJSON(), download2.startTime.toJSON());
+});
+
+/**
+ * Checks that downloads are added to browsing history when they start.
+ */
+add_task(async function test_history() {
+  mustInterruptResponses();
+
+  let sourceUrl = httpUrl("interruptible.txt");
+
+  // We will wait for the visit to be notified during the download.
+  await PlacesUtils.history.clear();
+  let promiseVisit = promiseWaitForVisit(sourceUrl);
+
+  // Start a download that is not allowed to finish yet.
+  let download = await promiseStartDownload(sourceUrl);
+  let expectedFile = new FileUtils.File(download.target.path);
+  let expectedFileURI = Services.io.newFileURI(expectedFile);
+  let promiseAnnotation = waitForAnnotation(
+    sourceUrl,
+    "downloads/destinationFileURI",
+    expectedFileURI.spec
+  );
+
+  // The history and annotation notifications should be received before the download completes.
+  let [time, transitionType, lastKnownTitle] = await promiseVisit;
+  await promiseAnnotation;
+
+  Assert.equal(time, download.startTime.getTime());
+  Assert.equal(transitionType, Ci.nsINavHistoryService.TRANSITION_DOWNLOAD);
+  Assert.equal(lastKnownTitle, expectedFile.leafName);
+
+  let pageInfo = await PlacesUtils.history.fetch(sourceUrl, {
+    includeAnnotations: true,
+  });
+  Assert.equal(
+    pageInfo.annotations.get("downloads/destinationFileURI"),
+    expectedFileURI.spec,
+    "Should have saved the correct download target annotation."
+  );
+
+  // Restart and complete the download after clearing history.
+  await PlacesUtils.history.clear();
+  download.cancel();
+  continueResponses();
+  await download.start();
+
+  // The restart should not have added a new history visit.
+  Assert.equal(false, await PlacesUtils.history.hasVisits(sourceUrl));
+});
+
+/**
+ * Checks that downloads started by nsIHelperAppService are added to the
+ * browsing history when they start.
+ */
+add_task(async function test_history_tryToKeepPartialData() {
+  // We will wait for the visit to be notified during the download.
+  await PlacesUtils.history.clear();
+  let promiseVisit = promiseWaitForVisit(
+    httpUrl("interruptible_resumable.txt")
+  );
+
+  // Start a download that is not allowed to finish yet.
+  let beforeStartTimeMs = Date.now();
+  let download = await promiseStartDownload_tryToKeepPartialData({
+    useLegacySaver: gUseLegacySaver,
+  });
+
+  // The history notifications should be received before the download completes.
+  let [time, transitionType] = await promiseVisit;
+  Assert.equal(transitionType, Ci.nsINavHistoryService.TRANSITION_DOWNLOAD);
+
+  // The time set by nsIHelperAppService may be different than the start time in
+  // the download object, thus we only check that it is a meaningful time.  Note
+  // that we subtract one second from the earliest time to account for rounding.
+  Assert.ok(time >= beforeStartTimeMs - 1000);
+
+  // Complete the download before finishing the test.
+  continueResponses();
+  await promiseDownloadStopped(download);
+});
+
+/**
+ * Checks that finished downloads are not removed.
+ */
+add_task(async function test_download_cancel_retry_finalize() {
+  // Start a download that is not allowed to finish yet.
+  let sourceUrl = httpUrl("interruptible.txt");
+  let targetFilePath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  mustInterruptResponses();
+  let download1 = await Downloads.createDownload({
+    source: sourceUrl,
+    target: { path: targetFilePath, partFilePath: targetFilePath + ".part" },
+  });
+  download1.start().catch(() => {});
+  await promiseDownloadMidway(download1);
+  await promisePartFileReady(download1);
+
+  // Cancel the download and make sure that the partial data do not exist.
+  await download1.cancel();
+  Assert.equal(targetFilePath, download1.target.path);
+  Assert.equal(false, await IOUtils.exists(download1.target.path));
+  Assert.equal(false, await IOUtils.exists(download1.target.partFilePath));
+  continueResponses();
+
+  // Download the same file again with a different download session.
+  let download2 = await Downloads.createDownload({
+    source: sourceUrl,
+    target: { path: targetFilePath, partFilePath: targetFilePath + ".part" },
+  });
+  download2.start().catch(() => {});
+
+  // Wait for download to be completed.
+  await promiseDownloadStopped(download2);
+  Assert.equal(targetFilePath, download2.target.path);
+  Assert.ok(await IOUtils.exists(download2.target.path));
+  Assert.equal(false, await IOUtils.exists(download2.target.partFilePath));
+
+  // Finalize the first download session.
+  await download1.finalize(true);
+
+  // The complete download should not have been removed.
+  Assert.ok(await IOUtils.exists(download2.target.path));
+  Assert.equal(false, await IOUtils.exists(download2.target.partFilePath));
+});
+
+/**
+ * Checks that confirmBlock does not clobber unrelated safe files.
+ */
+add_task(async function test_blocked_removeByHand_confirmBlock() {
+  let download1 = await promiseBlockedDownload({
+    keepPartialData: true,
+    keepBlockedData: true,
+  });
+
+  Assert.ok(download1.hasBlockedData);
+  Assert.equal((await IOUtils.stat(download1.target.path)).size, 0);
+  Assert.ok(await IOUtils.exists(download1.target.partFilePath));
+
+  // Remove the placeholder without telling the download.
+  await IOUtils.remove(download1.target.path);
+  Assert.equal(false, await IOUtils.exists(download1.target.path));
+
+  // Download a file with the same name as the blocked download.
+  let download2 = await Downloads.createDownload({
+    source: httpUrl("interruptible_resumable.txt"),
+    target: {
+      path: download1.target.path,
+      partFilePath: download1.target.path + ".part",
+    },
+  });
+  download2.start().catch(() => {});
+
+  // Wait for download to be completed.
+  await promiseDownloadStopped(download2);
+  Assert.equal(download1.target.path, download2.target.path);
+  Assert.ok(await IOUtils.exists(download2.target.path));
+
+  // Remove the blocked download.
+  await download1.confirmBlock();
+
+  // After confirming the complete download should not have been removed.
+  Assert.ok(await IOUtils.exists(download2.target.path));
+});
+
+/**
+ * Tests that the temp download files are removed on exit and exiting private
+ * mode after they have been launched.
+ */
+add_task(async function test_launchWhenSucceeded_deleteTempFileOnExit() {
+  let customLauncherPath = getTempFile("app-launcher").path;
+  let autoDeleteTargetPathOne = getTempFile(TEST_TARGET_FILE_NAME).path;
+  let autoDeleteTargetPathTwo = getTempFile(TEST_TARGET_FILE_NAME).path;
+  let noAutoDeleteTargetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+
+  let autoDeleteDownloadOne = await Downloads.createDownload({
+    source: { url: httpUrl("source.txt"), isPrivate: true },
+    target: autoDeleteTargetPathOne,
+    launchWhenSucceeded: true,
+    launcherPath: customLauncherPath,
+  });
+  await autoDeleteDownloadOne.start();
+
+  Services.prefs.setBoolPref(kDeleteTempFileOnExit, true);
+  let autoDeleteDownloadTwo = await Downloads.createDownload({
+    source: httpUrl("source.txt"),
+    target: autoDeleteTargetPathTwo,
+    launchWhenSucceeded: true,
+    launcherPath: customLauncherPath,
+  });
+  await autoDeleteDownloadTwo.start();
+
+  Services.prefs.setBoolPref(kDeleteTempFileOnExit, false);
+  let noAutoDeleteDownload = await Downloads.createDownload({
+    source: httpUrl("source.txt"),
+    target: noAutoDeleteTargetPath,
+    launchWhenSucceeded: true,
+    launcherPath: customLauncherPath,
+  });
+  await noAutoDeleteDownload.start();
+
+  Services.prefs.clearUserPref(kDeleteTempFileOnExit);
+
+  Assert.ok(await IOUtils.exists(autoDeleteTargetPathOne));
+  Assert.ok(await IOUtils.exists(autoDeleteTargetPathTwo));
+  Assert.ok(await IOUtils.exists(noAutoDeleteTargetPath));
+
+  // Simulate leaving private browsing mode
+  Services.obs.notifyObservers(null, "last-pb-context-exited");
+  Assert.equal(false, await IOUtils.exists(autoDeleteTargetPathOne));
+
+  // Simulate browser shutdown
+  let expire = Cc[
+    "@mozilla.org/uriloader/external-helper-app-service;1"
+  ].getService(Ci.nsIObserver);
+  expire.observe(null, "profile-before-change", null);
+
+  // The file should still exist following the simulated shutdown.
+  Assert.ok(await IOUtils.exists(autoDeleteTargetPathTwo));
+  Assert.ok(await IOUtils.exists(noAutoDeleteTargetPath));
+});
+
+add_task(async function test_partitionKey() {
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  Services.prefs.setBoolPref("privacy.partition.network_state", true);
+
+  function promiseVerifyDownloadChannel(url, partitionKey) {
+    return TestUtils.topicObserved(
+      "http-on-modify-request",
+      (subject, data) => {
+        let httpChannel = subject.QueryInterface(Ci.nsIHttpChannel);
+        if (httpChannel.URI.spec != url) {
+          return false;
+        }
+
+        let reqLoadInfo = httpChannel.loadInfo;
+        let cookieJarSettings = reqLoadInfo.cookieJarSettings;
+
+        // Check the partitionKey of the cookieJarSettings.
+        Assert.equal(cookieJarSettings.partitionKey, partitionKey);
+
+        return true;
+      }
+    );
+  }
+
+  let test_url = httpUrl("source.txt");
+  let uri = Services.io.newURI(test_url);
+  let cookieJarSettings = Cc["@mozilla.org/cookieJarSettings;1"].createInstance(
+    Ci.nsICookieJarSettings
+  );
+  cookieJarSettings.initWithURI(uri, false);
+  let expectedPartitionKey = cookieJarSettings.partitionKey;
+
+  let verifyPromise;
+
+  let download;
+  if (!gUseLegacySaver) {
+    // When testing DownloadCopySaver, we have control over the download, thus
+    // we can check its basic properties before it starts.
+    download = await Downloads.createDownload({
+      source: { url: test_url, cookieJarSettings },
+      target: { path: targetFile.path },
+      saver: { type: "copy" },
+    });
+
+    Assert.equal(download.source.url, test_url);
+    Assert.equal(download.target.path, targetFile.path);
+
+    verifyPromise = promiseVerifyDownloadChannel(
+      test_url,
+      expectedPartitionKey
+    );
+
+    await download.start();
+  } else {
+    verifyPromise = promiseVerifyDownloadChannel(
+      test_url,
+      expectedPartitionKey
+    );
+
+    // When testing DownloadLegacySaver, the download is already started when it
+    // is created, thus we must check its basic properties while in progress.
+    download = await promiseStartLegacyDownload(null, {
+      targetFile,
+      cookieJarSettings,
+    });
+
+    Assert.equal(download.source.url, test_url);
+    Assert.equal(download.target.path, targetFile.path);
+
+    await promiseDownloadStopped(download);
+  }
+
+  await verifyPromise;
+
+  Services.prefs.clearUserPref("privacy.partition.network_state");
+});
diff --git a/toolkit/components/downloads/test/unit/head.js b/toolkit/components/downloads/test/unit/head.js
new file mode 100644
index 0000000000..19d85b6f7c
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/head.js
@@ -0,0 +1,1183 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Provides infrastructure for automated download components tests.
+ */
+
+"use strict";
+
+var { Integration } = ChromeUtils.importESModule(
+  "resource://gre/modules/Integration.sys.mjs"
+);
+var { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+ChromeUtils.defineESModuleGetters(this, {
+  DownloadPaths: "resource://gre/modules/DownloadPaths.sys.mjs",
+  Downloads: "resource://gre/modules/Downloads.sys.mjs",
+  E10SUtils: "resource://gre/modules/E10SUtils.sys.mjs",
+  FileTestUtils: "resource://testing-common/FileTestUtils.sys.mjs",
+  FileUtils: "resource://gre/modules/FileUtils.sys.mjs",
+  HttpServer: "resource://testing-common/httpd.sys.mjs",
+  MockRegistrar: "resource://testing-common/MockRegistrar.sys.mjs",
+  NetUtil: "resource://gre/modules/NetUtil.sys.mjs",
+  PlacesUtils: "resource://gre/modules/PlacesUtils.sys.mjs",
+  TelemetryTestUtils: "resource://testing-common/TelemetryTestUtils.sys.mjs",
+  TestUtils: "resource://testing-common/TestUtils.sys.mjs",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "gExternalHelperAppService",
+  "@mozilla.org/uriloader/external-helper-app-service;1",
+  Ci.nsIExternalHelperAppService
+);
+
+/* global DownloadIntegration */
+Integration.downloads.defineESModuleGetter(
+  this,
+  "DownloadIntegration",
+  "resource://gre/modules/DownloadIntegration.sys.mjs"
+);
+
+const ServerSocket = Components.Constructor(
+  "@mozilla.org/network/server-socket;1",
+  "nsIServerSocket",
+  "init"
+);
+const BinaryOutputStream = Components.Constructor(
+  "@mozilla.org/binaryoutputstream;1",
+  "nsIBinaryOutputStream",
+  "setOutputStream"
+);
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "gMIMEService",
+  "@mozilla.org/mime;1",
+  "nsIMIMEService"
+);
+
+const ReferrerInfo = Components.Constructor(
+  "@mozilla.org/referrer-info;1",
+  "nsIReferrerInfo",
+  "init"
+);
+
+const TEST_TARGET_FILE_NAME = "test-download.txt";
+const TEST_STORE_FILE_NAME = "test-downloads.json";
+
+// We are testing an HTTPS referrer with HTTP downloads in order to verify that
+// the default policy will not prevent the referrer from being passed around.
+const TEST_REFERRER_URL = "https://www.example.com/referrer.html";
+
+const TEST_DATA_SHORT = "This test string is downloaded.";
+// Generate using gzipCompressString in TelemetryController.sys.mjs.
+const TEST_DATA_SHORT_GZIP_ENCODED_FIRST = [
+  31, 139, 8, 0, 0, 0, 0, 0, 0, 3, 11, 201, 200, 44, 86, 40, 73, 45, 46, 81, 40,
+  46, 41, 202, 204,
+];
+const TEST_DATA_SHORT_GZIP_ENCODED_SECOND = [
+  75, 87, 0, 114, 83, 242, 203, 243, 114, 242, 19, 83, 82, 83, 244, 0, 151, 222,
+  109, 43, 31, 0, 0, 0,
+];
+const TEST_DATA_SHORT_GZIP_ENCODED = TEST_DATA_SHORT_GZIP_ENCODED_FIRST.concat(
+  TEST_DATA_SHORT_GZIP_ENCODED_SECOND
+);
+
+/**
+ * All the tests are implemented with add_task, this starts them automatically.
+ */
+function run_test() {
+  do_get_profile();
+  run_next_test();
+}
+
+// Support functions
+
+/**
+ * HttpServer object initialized before tests start.
+ */
+var gHttpServer;
+
+/**
+ * Given a file name, returns a string containing an URI that points to the file
+ * on the currently running instance of the test HTTP server.
+ */
+function httpUrl(aFileName) {
+  return (
+    "http://www.example.com:" +
+    gHttpServer.identity.primaryPort +
+    "/" +
+    aFileName
+  );
+}
+
+/**
+ * Returns a reference to a temporary file that is guaranteed not to exist and
+ * is cleaned up later. See FileTestUtils.getTempFile for details.
+ */
+function getTempFile(leafName) {
+  return FileTestUtils.getTempFile(leafName);
+}
+
+/**
+ * Check for file existence.
+ * @param {string} path The file path.
+ */
+async function fileExists(path) {
+  try {
+    return (await IOUtils.stat(path)).type == "regular";
+  } catch (ex) {
+    return false;
+  }
+}
+
+/**
+ * Waits for pending events to be processed.
+ *
+ * @return {Promise}
+ * @resolves When pending events have been processed.
+ * @rejects Never.
+ */
+function promiseExecuteSoon() {
+  return new Promise(resolve => {
+    executeSoon(resolve);
+  });
+}
+
+/**
+ * Waits for a pending events to be processed after a timeout.
+ *
+ * @return {Promise}
+ * @resolves When pending events have been processed.
+ * @rejects Never.
+ */
+function promiseTimeout(aTime) {
+  return new Promise(resolve => {
+    do_timeout(aTime, resolve);
+  });
+}
+
+/**
+ * Waits for a new history visit to be notified for the specified URI.
+ *
+ * @param aUrl
+ *        String containing the URI that will be visited.
+ *
+ * @return {Promise}
+ * @resolves Array [aTime, aTransitionType] from page-visited places event.
+ * @rejects Never.
+ */
+function promiseWaitForVisit(aUrl) {
+  return new Promise(resolve => {
+    function listener(aEvents) {
+      Assert.equal(aEvents.length, 1);
+      let event = aEvents[0];
+      Assert.equal(event.type, "page-visited");
+      if (event.url == aUrl) {
+        PlacesObservers.removeListener(["page-visited"], listener);
+        resolve([event.visitTime, event.transitionType, event.lastKnownTitle]);
+      }
+    }
+    PlacesObservers.addListener(["page-visited"], listener);
+  });
+}
+
+/**
+ * Creates a new Download object, setting a temporary file as the target.
+ *
+ * @param aSourceUrl
+ *        String containing the URI for the download source, or null to use
+ *        httpUrl("source.txt").
+ *
+ * @return {Promise}
+ * @resolves The newly created Download object.
+ * @rejects JavaScript exception.
+ */
+function promiseNewDownload(aSourceUrl) {
+  return Downloads.createDownload({
+    source: aSourceUrl || httpUrl("source.txt"),
+    target: getTempFile(TEST_TARGET_FILE_NAME),
+  });
+}
+
+/**
+ * Starts a new download using the nsIWebBrowserPersist interface, and controls
+ * it using the legacy nsITransfer interface.
+ *
+ * @param aSourceUrl
+ *        String containing the URI for the download source, or null to use
+ *        httpUrl("source.txt").
+ * @param aOptions
+ *        An optional object used to control the behavior of this function.
+ *        You may pass an object with a subset of the following fields:
+ *        {
+ *          isPrivate: Boolean indicating whether the download originated from a
+ *                     private window.
+ *          referrerInfo: referrerInfo for the download source.
+ *          cookieJarSettings: cookieJarSettings for the download source.
+ *          targetFile: nsIFile for the target, or null to use a temporary file.
+ *          outPersist: Receives a reference to the created nsIWebBrowserPersist
+ *                      instance.
+ *          launchWhenSucceeded: Boolean indicating whether the target should
+ *                               be launched when it has completed successfully.
+ *          launcherPath: String containing the path of the custom executable to
+ *                        use to launch the target of the download.
+ *        }
+ *
+ * @return {Promise}
+ * @resolves The Download object created as a consequence of controlling the
+ *           download through the legacy nsITransfer interface.
+ * @rejects Never.  The current test fails in case of exceptions.
+ */
+function promiseStartLegacyDownload(aSourceUrl, aOptions) {
+  let sourceURI = NetUtil.newURI(aSourceUrl || httpUrl("source.txt"));
+  let targetFile =
+    (aOptions && aOptions.targetFile) || getTempFile(TEST_TARGET_FILE_NAME);
+
+  let persist = Cc[
+    "@mozilla.org/embedding/browser/nsWebBrowserPersist;1"
+  ].createInstance(Ci.nsIWebBrowserPersist);
+  if (aOptions) {
+    aOptions.outPersist = persist;
+  }
+
+  let fileExtension = null,
+    mimeInfo = null;
+  let match = sourceURI.pathQueryRef.match(/\.([^.\/]+)$/);
+  if (match) {
+    fileExtension = match[1];
+  }
+
+  if (fileExtension) {
+    try {
+      mimeInfo = gMIMEService.getFromTypeAndExtension(null, fileExtension);
+      mimeInfo.preferredAction = Ci.nsIMIMEInfo.saveToDisk;
+    } catch (ex) {}
+  }
+
+  if (aOptions && aOptions.launcherPath) {
+    Assert.ok(mimeInfo != null);
+
+    let localHandlerApp = Cc[
+      "@mozilla.org/uriloader/local-handler-app;1"
+    ].createInstance(Ci.nsILocalHandlerApp);
+    localHandlerApp.executable = new FileUtils.File(aOptions.launcherPath);
+
+    mimeInfo.preferredApplicationHandler = localHandlerApp;
+    mimeInfo.preferredAction = Ci.nsIMIMEInfo.useHelperApp;
+  }
+
+  if (aOptions && aOptions.launchWhenSucceeded) {
+    Assert.ok(mimeInfo != null);
+
+    mimeInfo.preferredAction = Ci.nsIMIMEInfo.useHelperApp;
+  }
+
+  // Apply decoding if required by the "Content-Encoding" header.
+  persist.persistFlags &= ~Ci.nsIWebBrowserPersist.PERSIST_FLAGS_NO_CONVERSION;
+  persist.persistFlags |=
+    Ci.nsIWebBrowserPersist.PERSIST_FLAGS_AUTODETECT_APPLY_CONVERSION;
+
+  let transfer = Cc["@mozilla.org/transfer;1"].createInstance(Ci.nsITransfer);
+
+  return new Promise(resolve => {
+    Downloads.getList(Downloads.ALL)
+      .then(function (aList) {
+        // Temporarily register a view that will get notified when the download we
+        // are controlling becomes visible in the list of downloads.
+        aList
+          .addView({
+            onDownloadAdded(aDownload) {
+              aList.removeView(this).catch(do_report_unexpected_exception);
+
+              // Remove the download to keep the list empty for the next test.  This
+              // also allows the caller to register the "onchange" event directly.
+              let promise = aList.remove(aDownload);
+
+              // When the download object is ready, make it available to the caller.
+              promise.then(
+                () => resolve(aDownload),
+                do_report_unexpected_exception
+              );
+            },
+          })
+          .catch(do_report_unexpected_exception);
+
+        let isPrivate = aOptions && aOptions.isPrivate;
+        let referrerInfo = aOptions ? aOptions.referrerInfo : null;
+        let cookieJarSettings = aOptions ? aOptions.cookieJarSettings : null;
+        let classification =
+          aOptions?.downloadClassification ??
+          Ci.nsITransfer.DOWNLOAD_ACCEPTABLE;
+        // Initialize the components so they reference each other.  This will cause
+        // the Download object to be created and added to the public downloads.
+        transfer.init(
+          sourceURI,
+          null,
+          NetUtil.newURI(targetFile),
+          null,
+          mimeInfo,
+          null,
+          null,
+          persist,
+          isPrivate,
+          classification,
+          null
+        );
+        persist.progressListener = transfer;
+
+        // Start the actual download process.
+        persist.saveURI(
+          sourceURI,
+          Services.scriptSecurityManager.getSystemPrincipal(),
+          0,
+          referrerInfo,
+          cookieJarSettings,
+          null,
+          null,
+          targetFile,
+          Ci.nsIContentPolicy.TYPE_SAVEAS_DOWNLOAD,
+          isPrivate
+        );
+      })
+      .catch(do_report_unexpected_exception);
+  });
+}
+
+/**
+ * Starts a new download using the nsIHelperAppService interface, and controls
+ * it using the legacy nsITransfer interface.  The source of the download will
+ * be "interruptible_resumable.txt" and partially downloaded data will be kept.
+ *
+ * @param aSourceUrl
+ *        String containing the URI for the download source, or null to use
+ *        httpUrl("interruptible_resumable.txt").
+ *
+ * @return {Promise}
+ * @resolves The Download object created as a consequence of controlling the
+ *           download through the legacy nsITransfer interface.
+ * @rejects Never.  The current test fails in case of exceptions.
+ */
+function promiseStartExternalHelperAppServiceDownload(aSourceUrl) {
+  let sourceURI = NetUtil.newURI(
+    aSourceUrl || httpUrl("interruptible_resumable.txt")
+  );
+
+  return new Promise(resolve => {
+    Downloads.getList(Downloads.PUBLIC)
+      .then(function (aList) {
+        // Temporarily register a view that will get notified when the download we
+        // are controlling becomes visible in the list of downloads.
+        aList
+          .addView({
+            onDownloadAdded(aDownload) {
+              aList.removeView(this).catch(do_report_unexpected_exception);
+
+              // Remove the download to keep the list empty for the next test.  This
+              // also allows the caller to register the "onchange" event directly.
+              let promise = aList.remove(aDownload);
+
+              // When the download object is ready, make it available to the caller.
+              promise.then(
+                () => resolve(aDownload),
+                do_report_unexpected_exception
+              );
+            },
+          })
+          .catch(do_report_unexpected_exception);
+
+        let channel = NetUtil.newChannel({
+          uri: sourceURI,
+          loadUsingSystemPrincipal: true,
+        });
+
+        // Start the actual download process.
+        channel.asyncOpen({
+          contentListener: null,
+
+          onStartRequest(aRequest) {
+            let requestChannel = aRequest.QueryInterface(Ci.nsIChannel);
+            this.contentListener = gExternalHelperAppService.doContent(
+              requestChannel.contentType,
+              aRequest,
+              null,
+              true
+            );
+            this.contentListener.onStartRequest(aRequest);
+          },
+
+          onStopRequest(aRequest, aStatusCode) {
+            this.contentListener.onStopRequest(aRequest, aStatusCode);
+          },
+
+          onDataAvailable(aRequest, aInputStream, aOffset, aCount) {
+            this.contentListener.onDataAvailable(
+              aRequest,
+              aInputStream,
+              aOffset,
+              aCount
+            );
+          },
+        });
+      })
+      .catch(do_report_unexpected_exception);
+  });
+}
+
+/**
+ * Waits for a download to reach half of its progress, in case it has not
+ * reached the expected progress already.
+ *
+ * @param aDownload
+ *        The Download object to wait upon.
+ *
+ * @return {Promise}
+ * @resolves When the download has reached half of its progress.
+ * @rejects Never.
+ */
+function promiseDownloadMidway(aDownload) {
+  return new Promise(resolve => {
+    // Wait for the download to reach half of its progress.
+    let onchange = function () {
+      if (
+        !aDownload.stopped &&
+        !aDownload.canceled &&
+        aDownload.progress == 50
+      ) {
+        aDownload.onchange = null;
+        resolve();
+      }
+    };
+
+    // Register for the notification, but also call the function directly in
+    // case the download already reached the expected progress.
+    aDownload.onchange = onchange;
+    onchange();
+  });
+}
+
+/**
+ * Waits for a download to make any amount of progress.
+ *
+ * @param aDownload
+ *        The Download object to wait upon.
+ *
+ * @return {Promise}
+ * @resolves When the download has transfered any number of bytes.
+ * @rejects Never.
+ */
+function promiseDownloadStarted(aDownload) {
+  return new Promise(resolve => {
+    // Wait for the download to transfer some amount of bytes.
+    let onchange = function () {
+      if (
+        !aDownload.stopped &&
+        !aDownload.canceled &&
+        aDownload.currentBytes > 0
+      ) {
+        aDownload.onchange = null;
+        resolve();
+      }
+    };
+
+    // Register for the notification, but also call the function directly in
+    // case the download already reached the expected progress.
+    aDownload.onchange = onchange;
+    onchange();
+  });
+}
+
+/**
+ * Waits for a download to finish.
+ *
+ * @param aDownload
+ *        The Download object to wait upon.
+ *
+ * @return {Promise}
+ * @resolves When the download succeeded or errored.
+ * @rejects Never.
+ */
+function promiseDownloadFinished(aDownload) {
+  return new Promise(resolve => {
+    // Wait for the download to finish.
+    let onchange = function () {
+      if (aDownload.succeeded || aDownload.error) {
+        aDownload.onchange = null;
+        resolve();
+      }
+    };
+
+    // Register for the notification, but also call the function directly in
+    // case the download already reached the expected progress.
+    aDownload.onchange = onchange;
+    onchange();
+  });
+}
+
+/**
+ * Waits for a download to finish, in case it has not finished already.
+ *
+ * @param aDownload
+ *        The Download object to wait upon.
+ *
+ * @return {Promise}
+ * @resolves When the download has finished successfully.
+ * @rejects JavaScript exception if the download failed.
+ */
+function promiseDownloadStopped(aDownload) {
+  if (!aDownload.stopped) {
+    // The download is in progress, wait for the current attempt to finish and
+    // report any errors that may occur.
+    return aDownload.start();
+  }
+
+  if (aDownload.succeeded) {
+    return Promise.resolve();
+  }
+
+  // The download failed or was canceled.
+  return Promise.reject(aDownload.error || new Error("Download canceled."));
+}
+
+/**
+ * Returns a new public or private DownloadList object.
+ *
+ * @param aIsPrivate
+ *        True for the private list, false or undefined for the public list.
+ *
+ * @return {Promise}
+ * @resolves The newly created DownloadList object.
+ * @rejects JavaScript exception.
+ */
+function promiseNewList(aIsPrivate) {
+  // We need to clear all the internal state for the list and summary objects,
+  // since all the objects are interdependent internally.
+  Downloads._promiseListsInitialized = null;
+  Downloads._lists = {};
+  Downloads._summaries = {};
+
+  return Downloads.getList(aIsPrivate ? Downloads.PRIVATE : Downloads.PUBLIC);
+}
+
+/**
+ * Ensures that the given file contents are equal to the given string.
+ *
+ * @param aPath
+ *        String containing the path of the file whose contents should be
+ *        verified.
+ * @param aExpectedContents
+ *        String containing the octets that are expected in the file.
+ *
+ * @return {Promise}
+ * @resolves When the operation completes.
+ * @rejects Never.
+ */
+async function promiseVerifyContents(aPath, aExpectedContents) {
+  let file = new FileUtils.File(aPath);
+
+  if (!(await IOUtils.exists(aPath))) {
+    do_throw("File does not exist: " + aPath);
+  }
+
+  if ((await IOUtils.stat(aPath)).size == 0) {
+    do_throw("File is empty: " + aPath);
+  }
+
+  await new Promise(resolve => {
+    NetUtil.asyncFetch(
+      { uri: NetUtil.newURI(file), loadUsingSystemPrincipal: true },
+      function (aInputStream, aStatus) {
+        Assert.ok(Components.isSuccessCode(aStatus));
+        let contents = NetUtil.readInputStreamToString(
+          aInputStream,
+          aInputStream.available()
+        );
+        if (
+          contents.length > TEST_DATA_SHORT.length * 2 ||
+          /[^\x20-\x7E]/.test(contents)
+        ) {
+          // Do not print the entire content string to the test log.
+          Assert.equal(contents.length, aExpectedContents.length);
+          Assert.ok(contents == aExpectedContents);
+        } else {
+          // Print the string if it is short and made of printable characters.
+          Assert.equal(contents, aExpectedContents);
+        }
+        resolve();
+      }
+    );
+  });
+}
+
+/**
+ * Creates and starts a new download, configured to keep partial data, and
+ * returns only when the first part of "interruptible_resumable.txt" has been
+ * saved to disk.  You must call "continueResponses" to allow the interruptible
+ * request to continue.
+ *
+ * This function uses either DownloadCopySaver or DownloadLegacySaver based on
+ * the current test run.
+ *
+ * @param aOptions
+ *        An optional object used to control the behavior of this function.
+ *        You may pass an object with a subset of the following fields:
+ *        {
+ *          useLegacySaver: Boolean indicating whether to launch a legacy download.
+ *        }
+ *
+ * @return {Promise}
+ * @resolves The newly created Download object, still in progress.
+ * @rejects JavaScript exception.
+ */
+async function promiseStartDownload_tryToKeepPartialData({
+  useLegacySaver = false,
+} = {}) {
+  mustInterruptResponses();
+
+  // Start a new download and configure it to keep partially downloaded data.
+  let download;
+  if (!useLegacySaver) {
+    let targetFilePath = getTempFile(TEST_TARGET_FILE_NAME).path;
+    download = await Downloads.createDownload({
+      source: httpUrl("interruptible_resumable.txt"),
+      target: {
+        path: targetFilePath,
+        partFilePath: targetFilePath + ".part",
+      },
+    });
+    download.tryToKeepPartialData = true;
+    download.start().catch(() => {});
+  } else {
+    // Start a download using nsIExternalHelperAppService, that is configured
+    // to keep partially downloaded data by default.
+    download = await promiseStartExternalHelperAppServiceDownload();
+  }
+
+  await promiseDownloadMidway(download);
+  await promisePartFileReady(download);
+
+  return download;
+}
+
+/**
+ * This function should be called after the progress notification for a download
+ * is received, and waits for the worker thread of BackgroundFileSaver to
+ * receive the data to be written to the ".part" file on disk.
+ *
+ * @return {Promise}
+ * @resolves When the ".part" file has been written to disk.
+ * @rejects JavaScript exception.
+ */
+async function promisePartFileReady(aDownload) {
+  // We don't have control over the file output code in BackgroundFileSaver.
+  // After we receive the download progress notification, we may only check
+  // that the ".part" file has been created, while its size cannot be
+  // determined because the file is currently open.
+  try {
+    do {
+      await promiseTimeout(50);
+    } while (!(await IOUtils.exists(aDownload.target.partFilePath)));
+  } catch (ex) {
+    if (!(ex instanceof IOUtils.Error)) {
+      throw ex;
+    }
+    // This indicates that the file has been created and cannot be accessed.
+    // The specific error might vary with the platform.
+    info("Expected exception while checking existence: " + ex.toString());
+    // Wait some more time to allow the write to complete.
+    await promiseTimeout(100);
+  }
+}
+
+/**
+ * Create a download which will be reputation blocked.
+ *
+ * @param options
+ *        {
+ *           keepPartialData: bool,
+ *           keepBlockedData: bool,
+ *           useLegacySaver: bool,
+ *           verdict: string indicating the detailed reason for the block,
+ *        }
+ * @return {Promise}
+ * @resolves The reputation blocked download.
+ * @rejects JavaScript exception.
+ */
+async function promiseBlockedDownload({
+  keepPartialData,
+  keepBlockedData,
+  useLegacySaver,
+  verdict = Downloads.Error.BLOCK_VERDICT_UNCOMMON,
+} = {}) {
+  let blockFn = base => ({
+    shouldBlockForReputationCheck: () =>
+      Promise.resolve({
+        shouldBlock: true,
+        verdict,
+      }),
+    shouldKeepBlockedData: () => Promise.resolve(keepBlockedData),
+  });
+
+  Integration.downloads.register(blockFn);
+  function cleanup() {
+    Integration.downloads.unregister(blockFn);
+  }
+  registerCleanupFunction(cleanup);
+
+  let download;
+
+  try {
+    if (keepPartialData) {
+      download = await promiseStartDownload_tryToKeepPartialData({
+        useLegacySaver,
+      });
+      continueResponses();
+    } else if (useLegacySaver) {
+      download = await promiseStartLegacyDownload();
+    } else {
+      download = await promiseNewDownload();
+      await download.start();
+      do_throw("The download should have blocked.");
+    }
+
+    await promiseDownloadStopped(download);
+    do_throw("The download should have blocked.");
+  } catch (ex) {
+    if (!(ex instanceof Downloads.Error) || !ex.becauseBlocked) {
+      throw ex;
+    }
+    Assert.ok(ex.becauseBlockedByReputationCheck);
+    Assert.equal(ex.reputationCheckVerdict, verdict);
+    Assert.ok(download.error.becauseBlockedByReputationCheck);
+    Assert.equal(download.error.reputationCheckVerdict, verdict);
+  }
+
+  Assert.ok(download.stopped);
+  Assert.ok(!download.succeeded);
+
+  cleanup();
+  return download;
+}
+
+/**
+ * Starts a socket listener that closes each incoming connection.
+ *
+ * @returns nsIServerSocket that listens for connections.  Call its "close"
+ *          method to stop listening and free the server port.
+ */
+function startFakeServer() {
+  let serverSocket = new ServerSocket(-1, true, -1);
+  serverSocket.asyncListen({
+    onSocketAccepted(aServ, aTransport) {
+      aTransport.close(Cr.NS_BINDING_ABORTED);
+    },
+    onStopListening() {},
+  });
+  return serverSocket;
+}
+
+/**
+ * This is an internal reference that should not be used directly by tests.
+ */
+var _gDeferResponses = Promise.withResolvers();
+
+/**
+ * Ensures that all the interruptible requests started after this function is
+ * called won't complete until the continueResponses function is called.
+ *
+ * Normally, the internal HTTP server returns all the available data as soon as
+ * a request is received.  In order for some requests to be served one part at a
+ * time, special interruptible handlers are registered on the HTTP server.  This
+ * allows testing events or actions that need to happen in the middle of a
+ * download.
+ *
+ * For example, the handler accessible at the httpUri("interruptible.txt")
+ * address returns the TEST_DATA_SHORT text, then it may block until the
+ * continueResponses method is called.  At this point, the handler sends the
+ * TEST_DATA_SHORT text again to complete the response.
+ *
+ * If an interruptible request is started before the function is called, it may
+ * or may not be blocked depending on the actual sequence of events.
+ */
+function mustInterruptResponses() {
+  // If there are pending blocked requests, allow them to complete.  This is
+  // done to prevent requests from being blocked forever, but should not affect
+  // the test logic, since previously started requests should not be monitored
+  // on the client side anymore.
+  _gDeferResponses.resolve();
+
+  info("Interruptible responses will be blocked midway.");
+  _gDeferResponses = Promise.withResolvers();
+}
+
+/**
+ * Allows all the current and future interruptible requests to complete.
+ */
+function continueResponses() {
+  info("Interruptible responses are now allowed to continue.");
+  _gDeferResponses.resolve();
+}
+
+/**
+ * Registers an interruptible response handler.
+ *
+ * @param aPath
+ *        Path passed to nsIHttpServer.registerPathHandler.
+ * @param aFirstPartFn
+ *        This function is called when the response is received, with the
+ *        aRequest and aResponse arguments of the server.
+ * @param aSecondPartFn
+ *        This function is called with the aRequest and aResponse arguments of
+ *        the server, when the continueResponses function is called.
+ */
+function registerInterruptibleHandler(aPath, aFirstPartFn, aSecondPartFn) {
+  gHttpServer.registerPathHandler(aPath, function (aRequest, aResponse) {
+    info("Interruptible request started.");
+
+    // Process the first part of the response.
+    aResponse.processAsync();
+    aFirstPartFn(aRequest, aResponse);
+
+    // Wait on the current deferred object, then finish the request.
+    _gDeferResponses.promise
+      .then(function RIH_onSuccess() {
+        aSecondPartFn(aRequest, aResponse);
+        aResponse.finish();
+        info("Interruptible request finished.");
+      })
+      .catch(console.error);
+  });
+}
+
+/**
+ * Ensure the given date object is valid.
+ *
+ * @param aDate
+ *        The date object to be checked. This value can be null.
+ */
+function isValidDate(aDate) {
+  return aDate && aDate.getTime && !isNaN(aDate.getTime());
+}
+
+/**
+ * Check actual ReferrerInfo is the same as expected.
+ * Because the actual download's referrer info's computedReferrer is computed
+ * from referrerPolicy and originalReferrer and is non-null, and the expected
+ * referrer info was constructed in isolation and therefore the computedReferrer
+ * is null, it isn't possible to use equals here. */
+function checkEqualReferrerInfos(aActualInfo, aExpectedInfo) {
+  Assert.equal(
+    !!aExpectedInfo.originalReferrer,
+    !!aActualInfo.originalReferrer
+  );
+  if (aExpectedInfo.originalReferrer && aActualInfo.originalReferrer) {
+    Assert.equal(
+      aExpectedInfo.originalReferrer.spec,
+      aActualInfo.originalReferrer.spec
+    );
+  }
+
+  Assert.equal(aExpectedInfo.sendReferrer, aActualInfo.sendReferrer);
+  Assert.equal(aExpectedInfo.referrerPolicy, aActualInfo.referrerPolicy);
+}
+
+/**
+ * Waits for the download annotations to be set for the given page, required
+ * because the addDownload method will add these to the database asynchronously.
+ */
+function waitForAnnotation(sourceUriSpec, annotationName, optionalValue) {
+  return TestUtils.waitForCondition(async () => {
+    let pageInfo = await PlacesUtils.history.fetch(sourceUriSpec, {
+      includeAnnotations: true,
+    });
+    if (optionalValue) {
+      return pageInfo?.annotations.get(annotationName) == optionalValue;
+    }
+    return pageInfo?.annotations.has(annotationName);
+  }, `Should have found annotation ${annotationName} for ${sourceUriSpec}`);
+}
+
+/**
+ * Position of the first byte served by the "interruptible_resumable.txt"
+ * handler during the most recent response.
+ */
+var gMostRecentFirstBytePos;
+
+// Initialization functions common to all tests
+
+add_setup(function test_common_initialize() {
+  // Start the HTTP server.
+  gHttpServer = new HttpServer();
+  gHttpServer.registerDirectory("/", do_get_file("../data"));
+  gHttpServer.start(-1);
+  registerCleanupFunction(() => {
+    return new Promise(resolve => {
+      // Ensure all the pending HTTP requests have a chance to finish.
+      continueResponses();
+      // Stop the HTTP server, calling resolve when it's done.
+      gHttpServer.stop(resolve);
+    });
+  });
+
+  // Serve the downloads from a domain located in the Internet zone on Windows.
+  gHttpServer.identity.setPrimary(
+    "http",
+    "www.example.com",
+    gHttpServer.identity.primaryPort
+  );
+  Services.prefs.setCharPref("network.dns.localDomains", "www.example.com");
+  registerCleanupFunction(function () {
+    Services.prefs.clearUserPref("network.dns.localDomains");
+  });
+
+  // Cache locks might prevent concurrent requests to the same resource, and
+  // this may block tests that use the interruptible handlers.
+  Services.prefs.setBoolPref("browser.cache.disk.enable", false);
+  Services.prefs.setBoolPref("browser.cache.memory.enable", false);
+  registerCleanupFunction(function () {
+    Services.prefs.clearUserPref("browser.cache.disk.enable");
+    Services.prefs.clearUserPref("browser.cache.memory.enable");
+  });
+
+  // Allow relaxing default referrer.
+  Services.prefs.setBoolPref(
+    "network.http.referer.disallowCrossSiteRelaxingDefault",
+    false
+  );
+  registerCleanupFunction(function () {
+    Services.prefs.clearUserPref(
+      "network.http.referer.disallowCrossSiteRelaxingDefault"
+    );
+  });
+
+  registerInterruptibleHandler(
+    "/interruptible.txt",
+    function firstPart(aRequest, aResponse) {
+      aResponse.setHeader("Content-Type", "text/plain", false);
+      aResponse.setHeader(
+        "Content-Length",
+        "" + TEST_DATA_SHORT.length * 2,
+        false
+      );
+      aResponse.write(TEST_DATA_SHORT);
+    },
+    function secondPart(aRequest, aResponse) {
+      aResponse.write(TEST_DATA_SHORT);
+    }
+  );
+
+  registerInterruptibleHandler(
+    "/interruptible_nosize.txt",
+    function firstPart(aRequest, aResponse) {
+      aResponse.setHeader("Content-Type", "text/plain", false);
+      aResponse.write(TEST_DATA_SHORT);
+    },
+    function secondPart(aRequest, aResponse) {
+      aResponse.write(TEST_DATA_SHORT);
+    }
+  );
+
+  registerInterruptibleHandler(
+    "/interruptible_resumable.txt",
+    function firstPart(aRequest, aResponse) {
+      aResponse.setHeader("Content-Type", "text/plain", false);
+
+      // Determine if only part of the data should be sent.
+      let data = TEST_DATA_SHORT + TEST_DATA_SHORT;
+      if (aRequest.hasHeader("Range")) {
+        var matches = aRequest
+          .getHeader("Range")
+          .match(/^\s*bytes=(\d+)?-(\d+)?\s*$/);
+        var firstBytePos = matches[1] === undefined ? 0 : matches[1];
+        var lastBytePos =
+          matches[2] === undefined ? data.length - 1 : matches[2];
+        if (firstBytePos >= data.length) {
+          aResponse.setStatusLine(
+            aRequest.httpVersion,
+            416,
+            "Requested Range Not Satisfiable"
+          );
+          aResponse.setHeader("Content-Range", "*/" + data.length, false);
+          aResponse.finish();
+          return;
+        }
+
+        aResponse.setStatusLine(aRequest.httpVersion, 206, "Partial Content");
+        aResponse.setHeader(
+          "Content-Range",
+          firstBytePos + "-" + lastBytePos + "/" + data.length,
+          false
+        );
+
+        data = data.substring(firstBytePos, lastBytePos + 1);
+
+        gMostRecentFirstBytePos = firstBytePos;
+      } else {
+        gMostRecentFirstBytePos = 0;
+      }
+
+      aResponse.setHeader("Content-Length", "" + data.length, false);
+
+      aResponse.write(data.substring(0, data.length / 2));
+
+      // Store the second part of the data on the response object, so that it
+      // can be used by the secondPart function.
+      aResponse.secondPartData = data.substring(data.length / 2);
+    },
+    function secondPart(aRequest, aResponse) {
+      aResponse.write(aResponse.secondPartData);
+    }
+  );
+
+  registerInterruptibleHandler(
+    "/interruptible_gzip.txt",
+    function firstPart(aRequest, aResponse) {
+      aResponse.setHeader("Content-Type", "text/plain", false);
+      aResponse.setHeader("Content-Encoding", "gzip", false);
+      aResponse.setHeader(
+        "Content-Length",
+        "" + TEST_DATA_SHORT_GZIP_ENCODED.length
+      );
+
+      let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
+      bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED_FIRST);
+    },
+    function secondPart(aRequest, aResponse) {
+      let bos = new BinaryOutputStream(aResponse.bodyOutputStream);
+      bos.writeByteArray(TEST_DATA_SHORT_GZIP_ENCODED_SECOND);
+    }
+  );
+
+  gHttpServer.registerPathHandler(
+    "/shorter-than-content-length-http-1-1.txt",
+    function (aRequest, aResponse) {
+      aResponse.processAsync();
+      aResponse.setStatusLine("1.1", 200, "OK");
+      aResponse.setHeader("Content-Type", "text/plain", false);
+      aResponse.setHeader(
+        "Content-Length",
+        "" + TEST_DATA_SHORT.length * 2,
+        false
+      );
+      aResponse.write(TEST_DATA_SHORT);
+      aResponse.finish();
+    }
+  );
+
+  gHttpServer.registerPathHandler("/busy.txt", function (aRequest, aResponse) {
+    aResponse.setStatusLine("1.1", 504, "Gateway Timeout");
+    aResponse.setHeader("Content-Type", "text/plain", false);
+    aResponse.setHeader("Content-Length", "" + TEST_DATA_SHORT.length, false);
+    aResponse.write(TEST_DATA_SHORT);
+  });
+
+  gHttpServer.registerPathHandler("/redirect", function (aRequest, aResponse) {
+    aResponse.setStatusLine("1.1", 301, "Moved Permanently");
+    aResponse.setHeader("Location", httpUrl("busy.txt"), false);
+    aResponse.setHeader("Content-Type", "text/javascript", false);
+    aResponse.setHeader("Content-Length", "0", false);
+  });
+
+  // This URL will emulate being blocked by Windows Parental controls
+  gHttpServer.registerPathHandler(
+    "/parentalblocked.zip",
+    function (aRequest, aResponse) {
+      aResponse.setStatusLine(
+        aRequest.httpVersion,
+        450,
+        "Blocked by Windows Parental Controls"
+      );
+    }
+  );
+
+  // This URL sends some data followed by an RST packet
+  gHttpServer.registerPathHandler(
+    "/netreset.txt",
+    function (aRequest, aResponse) {
+      info("Starting response that will be aborted.");
+      aResponse.processAsync();
+      aResponse.setHeader("Content-Type", "text/plain", false);
+      aResponse.write(TEST_DATA_SHORT);
+      promiseExecuteSoon()
+        .then(() => {
+          aResponse.abort(null, true);
+          aResponse.finish();
+          info("Aborting response with network reset.");
+        })
+        .then(null, console.error);
+    }
+  );
+
+  // During unit tests, most of the functions that require profile access or
+  // operating system features will be disabled. Individual tests may override
+  // them again to check for specific behaviors.
+  Integration.downloads.register(base => {
+    let override = {
+      loadPublicDownloadListFromStore: () => Promise.resolve(),
+      shouldKeepBlockedData: () => Promise.resolve(false),
+      shouldBlockForParentalControls: () => Promise.resolve(false),
+      shouldBlockForReputationCheck: () =>
+        Promise.resolve({
+          shouldBlock: false,
+          verdict: "",
+        }),
+      confirmLaunchExecutable: () => Promise.resolve(),
+      launchFile: () => Promise.resolve(),
+      showContainingDirectory: () => Promise.resolve(),
+      // This flag allows re-enabling the default observers during their tests.
+      allowObservers: false,
+      addListObservers() {
+        return this.allowObservers
+          ? super.addListObservers(...arguments)
+          : Promise.resolve();
+      },
+      // This flag allows re-enabling the download directory logic for its tests.
+      _allowDirectories: false,
+      set allowDirectories(value) {
+        this._allowDirectories = value;
+        // We have to invalidate the previously computed directory path.
+        this._downloadsDirectory = null;
+      },
+      _getDirectory(name) {
+        return super._getDirectory(this._allowDirectories ? name : "TmpD");
+      },
+    };
+    Object.setPrototypeOf(override, base);
+    return override;
+  });
+
+  // Make sure that downloads started using nsIExternalHelperAppService are
+  // saved to disk without asking for a destination interactively.
+  let mock = {
+    QueryInterface: ChromeUtils.generateQI(["nsIHelperAppLauncherDialog"]),
+    promptForSaveToFileAsync(
+      aLauncher,
+      aWindowContext,
+      aDefaultFileName,
+      aSuggestedFileExtension,
+      aForcePrompt
+    ) {
+      // The dialog should create the empty placeholder file.
+      let file = getTempFile(TEST_TARGET_FILE_NAME);
+      file.create(Ci.nsIFile.NORMAL_FILE_TYPE, FileUtils.PERMS_FILE);
+      aLauncher.saveDestinationAvailable(file);
+    },
+  };
+
+  let cid = MockRegistrar.register(
+    "@mozilla.org/helperapplauncherdialog;1",
+    mock
+  );
+  registerCleanupFunction(() => {
+    MockRegistrar.unregister(cid);
+  });
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadBlockedTelemetry.js b/toolkit/components/downloads/test/unit/test_DownloadBlockedTelemetry.js
new file mode 100644
index 0000000000..2484e591a4
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadBlockedTelemetry.js
@@ -0,0 +1,113 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Services.prefs.setBoolPref(
+  "toolkit.telemetry.testing.overrideProductsCheck",
+  true
+);
+
+const errors = [
+  Downloads.Error.BLOCK_VERDICT_MALWARE,
+  Downloads.Error.BLOCK_VERDICT_POTENTIALLY_UNWANTED,
+  Downloads.Error.BLOCK_VERDICT_INSECURE,
+  Downloads.Error.BLOCK_VERDICT_UNCOMMON,
+];
+
+add_task(async function test_confirm_block_download() {
+  for (let error of errors) {
+    info(`Testing block ${error} download`);
+    let histogram = TelemetryTestUtils.getAndClearKeyedHistogram(
+      "DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD"
+    );
+
+    let download;
+    try {
+      info(`Create ${error} download`);
+      if (error == Downloads.Error.BLOCK_VERDICT_INSECURE) {
+        download = await promiseStartLegacyDownload(null, {
+          downloadClassification: Ci.nsITransfer.DOWNLOAD_POTENTIALLY_UNSAFE,
+        });
+      } else {
+        download = await promiseBlockedDownload({
+          keepPartialData: true,
+          keepBlockedData: true,
+          useLegacySaver: false,
+          verdict: error,
+        });
+      }
+      await download.start();
+      do_throw("The download should have failed.");
+    } catch (ex) {
+      if (!(ex instanceof Downloads.Error)) {
+        throw ex;
+      }
+    }
+
+    // Test blocked download is recorded
+    TelemetryTestUtils.assertKeyedHistogramValue(histogram, error, 0, 1);
+
+    // Test confirm block
+    histogram = TelemetryTestUtils.getAndClearKeyedHistogram(
+      "DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD"
+    );
+    info(`Block ${error} download`);
+    await download.confirmBlock();
+    TelemetryTestUtils.assertKeyedHistogramValue(histogram, error, 1, 1);
+  }
+});
+
+add_task(async function test_confirm_unblock_download() {
+  for (let error of errors) {
+    info(`Testing unblock ${error} download`);
+    let histogram = TelemetryTestUtils.getAndClearKeyedHistogram(
+      "DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD"
+    );
+
+    let download;
+    try {
+      info(`Create ${error} download`);
+      if (error == Downloads.Error.BLOCK_VERDICT_INSECURE) {
+        download = await promiseStartLegacyDownload(null, {
+          downloadClassification: Ci.nsITransfer.DOWNLOAD_POTENTIALLY_UNSAFE,
+        });
+      } else {
+        download = await promiseBlockedDownload({
+          keepPartialData: true,
+          keepBlockedData: true,
+          useLegacySaver: false,
+          verdict: error,
+        });
+      }
+      await download.start();
+      do_throw("The download should have failed.");
+    } catch (ex) {
+      if (!(ex instanceof Downloads.Error)) {
+        throw ex;
+      }
+    }
+
+    // Test blocked download is recorded
+    TelemetryTestUtils.assertKeyedHistogramValue(histogram, error, 0, 1);
+
+    // Test unblock
+    histogram = TelemetryTestUtils.getAndClearKeyedHistogram(
+      "DOWNLOADS_USER_ACTION_ON_BLOCKED_DOWNLOAD"
+    );
+    info(`Unblock ${error} download`);
+    let promise = new Promise(r => (download.onchange = r));
+    await download.unblock();
+    // The environment is not set up properly for performing a real download, cancel
+    // the unblocked download so it doesn't affect the next testcase.
+    await download.cancel();
+    await promise;
+    if (error == Downloads.Error.BLOCK_VERDICT_INSECURE) {
+      Assert.ok(!download.error, "Ensure we didn't set download.error");
+    }
+
+    TelemetryTestUtils.assertKeyedHistogramValue(histogram, error, 2, 1);
+  }
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadCore.js b/toolkit/components/downloads/test/unit/test_DownloadCore.js
new file mode 100644
index 0000000000..41aeaac33b
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadCore.js
@@ -0,0 +1,291 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests the main download interfaces using DownloadCopySaver.
+ */
+
+"use strict";
+
+ChromeUtils.defineESModuleGetters(this, {
+  DownloadError: "resource://gre/modules/DownloadCore.sys.mjs",
+});
+
+// Execution of common tests
+
+// This is used in common_test_Download.js
+// eslint-disable-next-line no-unused-vars
+var gUseLegacySaver = false;
+
+var scriptFile = do_get_file("common_test_Download.js");
+Services.scriptloader.loadSubScript(NetUtil.newURI(scriptFile).spec);
+
+// Tests
+
+/**
+ * The download should fail early if the source and the target are the same.
+ */
+add_task(async function test_error_target_downloadingToSameFile() {
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  targetFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, FileUtils.PERMS_FILE);
+
+  let download = await Downloads.createDownload({
+    source: NetUtil.newURI(targetFile),
+    target: targetFile,
+  });
+  await Assert.rejects(
+    download.start(),
+    ex => ex instanceof Downloads.Error && ex.becauseTargetFailed
+  );
+
+  Assert.ok(
+    await IOUtils.exists(download.target.path),
+    "The file should not have been deleted."
+  );
+});
+
+/**
+ * Tests allowHttpStatus allowing requests
+ */
+add_task(async function test_error_notfound() {
+  const targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  let called = false;
+  const download = await Downloads.createDownload({
+    source: {
+      url: httpUrl("notfound.gone"),
+      allowHttpStatus(aDownload, aStatusCode) {
+        Assert.strictEqual(download, aDownload, "Check Download objects");
+        Assert.strictEqual(aStatusCode, 404, "The status should be correct");
+        called = true;
+        return true;
+      },
+    },
+    target: targetFile,
+  });
+  await download.start();
+  Assert.ok(called, "allowHttpStatus should have been called");
+});
+
+/**
+ * Tests allowHttpStatus rejecting requests
+ */
+add_task(async function test_error_notfound_reject() {
+  const targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  let called = false;
+  const download = await Downloads.createDownload({
+    source: {
+      url: httpUrl("notfound.gone"),
+      allowHttpStatus(aDownload, aStatusCode) {
+        Assert.strictEqual(download, aDownload, "Check Download objects");
+        Assert.strictEqual(aStatusCode, 404, "The status should be correct");
+        called = true;
+        return false;
+      },
+    },
+    target: targetFile,
+  });
+  await Assert.rejects(
+    download.start(),
+    ex => ex instanceof Downloads.Error && ex.becauseSourceFailed,
+    "Download should have been rejected"
+  );
+  Assert.ok(called, "allowHttpStatus should have been called");
+});
+
+/**
+ * Tests allowHttpStatus rejecting requests other than 404
+ */
+add_task(async function test_error_busy_reject() {
+  const targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  let called = false;
+  const download = await Downloads.createDownload({
+    source: {
+      url: httpUrl("busy.txt"),
+      allowHttpStatus(aDownload, aStatusCode) {
+        Assert.strictEqual(download, aDownload, "Check Download objects");
+        Assert.strictEqual(aStatusCode, 504, "The status should be correct");
+        called = true;
+        return false;
+      },
+    },
+    target: targetFile,
+  });
+  await Assert.rejects(
+    download.start(),
+    ex => ex instanceof Downloads.Error && ex.becauseSourceFailed,
+    "Download should have been rejected"
+  );
+  Assert.ok(called, "allowHttpStatus should have been called");
+});
+
+/**
+ * Tests redirects are followed correctly, and the meta data corresponds
+ * to the correct, final response
+ */
+add_task(async function test_redirects() {
+  const targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  let called = false;
+  const download = await Downloads.createDownload({
+    source: {
+      url: httpUrl("redirect"),
+      allowHttpStatus(aDownload, aStatusCode) {
+        Assert.strictEqual(download, aDownload, "Check Download objects");
+        Assert.strictEqual(
+          aStatusCode,
+          504,
+          "The status should be correct after a redirect"
+        );
+        called = true;
+        return true;
+      },
+    },
+    target: targetFile,
+  });
+  await download.start();
+  Assert.equal(
+    download.contentType,
+    "text/plain",
+    "Content-Type is correct after redirect"
+  );
+  Assert.equal(
+    download.totalBytes,
+    TEST_DATA_SHORT.length,
+    "Content-Length is correct after redirect"
+  );
+  Assert.equal(download.target.size, TEST_DATA_SHORT.length);
+  Assert.ok(called, "allowHttpStatus should have been called");
+});
+
+/**
+ * Tests the DownloadError object.
+ */
+add_task(function test_DownloadError() {
+  let error = new DownloadError({
+    result: Cr.NS_ERROR_NOT_RESUMABLE,
+    message: "Not resumable.",
+  });
+  Assert.equal(error.result, Cr.NS_ERROR_NOT_RESUMABLE);
+  Assert.equal(error.message, "Not resumable.");
+  Assert.ok(!error.becauseSourceFailed);
+  Assert.ok(!error.becauseTargetFailed);
+  Assert.ok(!error.becauseBlocked);
+  Assert.ok(!error.becauseBlockedByParentalControls);
+
+  error = new DownloadError({ message: "Unknown error." });
+  Assert.equal(error.result, Cr.NS_ERROR_FAILURE);
+  Assert.equal(error.message, "Unknown error.");
+
+  error = new DownloadError({ result: Cr.NS_ERROR_NOT_RESUMABLE });
+  Assert.equal(error.result, Cr.NS_ERROR_NOT_RESUMABLE);
+  Assert.ok(error.message.indexOf("Exception") > 0);
+
+  // becauseSourceFailed will be set, but not the unknown property.
+  error = new DownloadError({
+    message: "Unknown error.",
+    becauseSourceFailed: true,
+    becauseUnknown: true,
+  });
+  Assert.ok(error.becauseSourceFailed);
+  Assert.equal(false, "becauseUnknown" in error);
+
+  error = new DownloadError({
+    result: Cr.NS_ERROR_MALFORMED_URI,
+    inferCause: true,
+  });
+  Assert.equal(error.result, Cr.NS_ERROR_MALFORMED_URI);
+  Assert.ok(error.becauseSourceFailed);
+  Assert.ok(!error.becauseTargetFailed);
+  Assert.ok(!error.becauseBlocked);
+  Assert.ok(!error.becauseBlockedByParentalControls);
+
+  // This test does not set inferCause, so becauseSourceFailed will not be set.
+  error = new DownloadError({ result: Cr.NS_ERROR_MALFORMED_URI });
+  Assert.equal(error.result, Cr.NS_ERROR_MALFORMED_URI);
+  Assert.ok(!error.becauseSourceFailed);
+
+  error = new DownloadError({
+    result: Cr.NS_ERROR_FILE_INVALID_PATH,
+    inferCause: true,
+  });
+  Assert.equal(error.result, Cr.NS_ERROR_FILE_INVALID_PATH);
+  Assert.ok(!error.becauseSourceFailed);
+  Assert.ok(error.becauseTargetFailed);
+  Assert.ok(!error.becauseBlocked);
+  Assert.ok(!error.becauseBlockedByParentalControls);
+
+  error = new DownloadError({ becauseBlocked: true });
+  Assert.equal(error.message, "Download blocked.");
+  Assert.ok(!error.becauseSourceFailed);
+  Assert.ok(!error.becauseTargetFailed);
+  Assert.ok(error.becauseBlocked);
+  Assert.ok(!error.becauseBlockedByParentalControls);
+
+  error = new DownloadError({ becauseBlockedByParentalControls: true });
+  Assert.equal(error.message, "Download blocked.");
+  Assert.ok(!error.becauseSourceFailed);
+  Assert.ok(!error.becauseTargetFailed);
+  Assert.ok(error.becauseBlocked);
+  Assert.ok(error.becauseBlockedByParentalControls);
+});
+
+add_task(async function test_cancel_interrupted_download() {
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+
+  let download = await Downloads.createDownload({
+    source: httpUrl("interruptible_resumable.txt"),
+    target: targetFile,
+  });
+
+  async function createAndCancelDownload() {
+    info("Create an interruptible download and cancel it midway");
+    mustInterruptResponses();
+    const promiseDownloaded = download.start();
+    await promiseDownloadMidway(download);
+    await download.cancel();
+
+    info("Unblock the interruptible download and wait for its annotation");
+    continueResponses();
+    await waitForAnnotation(
+      httpUrl("interruptible_resumable.txt"),
+      "downloads/destinationFileURI"
+    );
+
+    await Assert.rejects(
+      promiseDownloaded,
+      /DownloadError: Download canceled/,
+      "Got a download error as expected"
+    );
+  }
+
+  await new Promise(resolve => {
+    const DONE = "=== download xpcshell test console listener done ===";
+    const logDone = () => Services.console.logStringMessage(DONE);
+    const consoleListener = msg => {
+      if (msg == DONE) {
+        Services.console.unregisterListener(consoleListener);
+        resolve();
+      }
+    };
+    Services.console.reset();
+    Services.console.registerListener(consoleListener);
+
+    createAndCancelDownload().then(logDone);
+  });
+
+  info(
+    "Assert that nsIStreamListener.onDataAvailable has not been called after download.cancel"
+  );
+  let found = Services.console
+    .getMessageArray()
+    .map(m => m.message)
+    .filter(message => {
+      return message.includes("nsIStreamListener.onDataAvailable");
+    });
+  Assert.deepEqual(
+    found,
+    [],
+    "Expect no nsIStreamListener.onDataAvaialable error"
+  );
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadHistory.js b/toolkit/components/downloads/test/unit/test_DownloadHistory.js
new file mode 100644
index 0000000000..69eb1c4728
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadHistory.js
@@ -0,0 +1,273 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests the DownloadHistory module.
+ */
+
+"use strict";
+
+const { DownloadHistory } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadHistory.sys.mjs"
+);
+
+let baseDate = new Date("2000-01-01");
+
+/**
+ * Non-fatal assertion used to test whether the downloads in the list already
+ * match the expected state.
+ */
+function areEqual(a, b) {
+  if (a === b) {
+    Assert.equal(a, b);
+    return true;
+  }
+  info(a + " !== " + b);
+  return false;
+}
+
+/**
+ * This allows waiting for an expected list at various points during the test.
+ */
+class TestView {
+  constructor(expected) {
+    this.expected = [...expected];
+    this.downloads = [];
+    this.resolveWhenExpected = () => {};
+  }
+  onDownloadAdded(download, options = {}) {
+    if (options.insertBefore) {
+      let index = this.downloads.indexOf(options.insertBefore);
+      this.downloads.splice(index, 0, download);
+    } else {
+      this.downloads.push(download);
+    }
+    this.checkForExpectedDownloads();
+  }
+  onDownloadChanged(download) {
+    this.checkForExpectedDownloads();
+  }
+  onDownloadRemoved(download) {
+    let index = this.downloads.indexOf(download);
+    this.downloads.splice(index, 1);
+    this.checkForExpectedDownloads();
+  }
+  checkForExpectedDownloads() {
+    // Wait for all the expected downloads to be added or removed before doing
+    // the detailed tests. This is done to avoid creating irrelevant output.
+    if (this.downloads.length != this.expected.length) {
+      return;
+    }
+    for (let i = 0; i < this.downloads.length; i++) {
+      if (
+        this.downloads[i].source.url != this.expected[i].source.url ||
+        this.downloads[i].target.path != this.expected[i].target.path
+      ) {
+        return;
+      }
+    }
+    // Check and report the actual state of the downloads. Even if the items
+    // are in the expected order, the metadata for history downloads might not
+    // have been updated to the final state yet.
+    for (let i = 0; i < this.downloads.length; i++) {
+      let download = this.downloads[i];
+      let testDownload = this.expected[i];
+      info(
+        "Checking download source " +
+          download.source.url +
+          " with target " +
+          download.target.path
+      );
+      if (
+        !areEqual(download.succeeded, !!testDownload.succeeded) ||
+        !areEqual(download.canceled, !!testDownload.canceled) ||
+        !areEqual(download.hasPartialData, !!testDownload.hasPartialData) ||
+        !areEqual(!!download.error, !!testDownload.error)
+      ) {
+        return;
+      }
+      // If the above properties match, the error details should be correct.
+      if (download.error) {
+        if (testDownload.error.becauseSourceFailed) {
+          Assert.equal(download.error.message, "History download failed.");
+        }
+        Assert.equal(
+          download.error.becauseBlockedByParentalControls,
+          testDownload.error.becauseBlockedByParentalControls
+        );
+        Assert.equal(
+          download.error.becauseBlockedByReputationCheck,
+          testDownload.error.becauseBlockedByReputationCheck
+        );
+      }
+    }
+    this.resolveWhenExpected();
+  }
+  async waitForExpected() {
+    let promise = new Promise(resolve => (this.resolveWhenExpected = resolve));
+    this.checkForExpectedDownloads();
+    await promise;
+  }
+}
+
+/**
+ * Tests that various operations on session and history downloads are reflected
+ * by the DownloadHistoryList object, and that the order of results is correct.
+ */
+add_task(async function test_DownloadHistory() {
+  // Clean up at the beginning and at the end of the test.
+  async function cleanup() {
+    await PlacesUtils.history.clear();
+  }
+  registerCleanupFunction(cleanup);
+  await cleanup();
+
+  let testDownloads = [
+    // History downloads should appear in order at the beginning of the list.
+    { offset: 10, canceled: true },
+    { offset: 20, succeeded: true },
+    { offset: 30, error: { becauseSourceFailed: true } },
+    { offset: 40, error: { becauseBlockedByParentalControls: true } },
+    { offset: 50, error: { becauseBlockedByReputationCheck: true } },
+    // Session downloads should show up after all the history download, in the
+    // same order as they were added.
+    { offset: 45, canceled: true, inSession: true },
+    { offset: 35, canceled: true, hasPartialData: true, inSession: true },
+    { offset: 55, succeeded: true, inSession: true },
+  ];
+  const NEXT_OFFSET = 60;
+
+  let publicList = await promiseNewList();
+  let allList = await Downloads.getList(Downloads.ALL);
+
+  async function addTestDownload(properties) {
+    properties.source = {
+      url: httpUrl("source" + properties.offset),
+      isPrivate: properties.isPrivate,
+    };
+    let targetFile = getTempFile(TEST_TARGET_FILE_NAME + properties.offset);
+    properties.target = { path: targetFile.path };
+    properties.startTime = new Date(baseDate.getTime() + properties.offset);
+
+    let download = await Downloads.createDownload(properties);
+    if (properties.inSession) {
+      await allList.add(download);
+    }
+
+    if (properties.isPrivate) {
+      return;
+    }
+
+    // Add the download to history using the XPCOM service, then use the
+    // DownloadHistory module to save the associated metadata.
+    let promiseFileAnnotation = waitForAnnotation(
+      properties.source.url,
+      "downloads/destinationFileURI"
+    );
+    let promiseMetaAnnotation = waitForAnnotation(
+      properties.source.url,
+      "downloads/metaData"
+    );
+    let promiseVisit = promiseWaitForVisit(properties.source.url);
+    await DownloadHistory.addDownloadToHistory(download);
+    await promiseVisit;
+    await DownloadHistory.updateMetaData(download);
+    await Promise.all([promiseFileAnnotation, promiseMetaAnnotation]);
+  }
+
+  // Add all the test downloads to history.
+  for (let properties of testDownloads) {
+    await addTestDownload(properties);
+  }
+
+  // Initialize DownloadHistoryList only after having added the history and
+  // session downloads, and check that they are loaded in the correct order.
+  let historyList = await DownloadHistory.getList();
+  let view = new TestView(testDownloads);
+  await historyList.addView(view);
+  await view.waitForExpected();
+
+  // Remove a download from history and verify that the change is reflected.
+  let downloadToRemove = view.expected[1];
+  view.expected.splice(1, 1);
+  await PlacesUtils.history.remove(downloadToRemove.source.url);
+  await view.waitForExpected();
+
+  // Add a download to history and verify it's placed before session downloads,
+  // even if the start date is more recent.
+  let downloadToAdd = { offset: NEXT_OFFSET, canceled: true };
+  view.expected.splice(
+    view.expected.findIndex(d => d.inSession),
+    0,
+    downloadToAdd
+  );
+  await addTestDownload(downloadToAdd);
+  await view.waitForExpected();
+
+  // Add a session download and verify it's placed after all session downloads,
+  // even if the start date is less recent.
+  let sessionDownloadToAdd = { offset: 0, inSession: true, succeeded: true };
+  view.expected.push(sessionDownloadToAdd);
+  await addTestDownload(sessionDownloadToAdd);
+  await view.waitForExpected();
+
+  // Add a session download for the same URI without a history entry, and verify
+  // it's visible and placed after all session downloads.
+  view.expected.push(sessionDownloadToAdd);
+  await publicList.add(await Downloads.createDownload(sessionDownloadToAdd));
+  await view.waitForExpected();
+
+  // Create a new DownloadHistoryList that also shows private downloads. Since
+  // we only have public downloads, the two lists should contain the same items.
+  let allHistoryList = await DownloadHistory.getList({ type: Downloads.ALL });
+  let allView = new TestView(view.expected);
+  await allHistoryList.addView(allView);
+  await allView.waitForExpected();
+
+  // Add a new private download and verify it appears only on the complete list.
+  let privateDownloadToAdd = {
+    offset: NEXT_OFFSET + 10,
+    inSession: true,
+    succeeded: true,
+    isPrivate: true,
+  };
+  allView.expected.push(privateDownloadToAdd);
+  await addTestDownload(privateDownloadToAdd);
+  await view.waitForExpected();
+  await allView.waitForExpected();
+
+  // Now test the maxHistoryResults parameter.
+  let allHistoryList2 = await DownloadHistory.getList({
+    type: Downloads.ALL,
+    maxHistoryResults: 3,
+  });
+  // Prepare the set of downloads to contain fewer history downloads by removing
+  // the oldest ones.
+  let allView2 = new TestView(allView.expected.slice(3));
+  await allHistoryList2.addView(allView2);
+  await allView2.waitForExpected();
+
+  // Create a dummy list and view like the previous limited one to just add and
+  // remove its view to make sure it doesn't break other lists' updates.
+  let dummyList = await DownloadHistory.getList({
+    type: Downloads.ALL,
+    maxHistoryResults: 3,
+  });
+  let dummyView = new TestView([]);
+  await dummyList.addView(dummyView);
+  await dummyList.removeView(dummyView);
+
+  // Clear history and check that session downloads with partial data remain.
+  // Private downloads are also not cleared when clearing history.
+  view.expected = view.expected.filter(d => d.hasPartialData);
+  allView.expected = allView.expected.filter(
+    d => d.hasPartialData || d.isPrivate
+  );
+  await PlacesUtils.history.clear();
+  await view.waitForExpected();
+  await allView.waitForExpected();
+
+  // Check that the dummy view above did not prevent the limited from updating.
+  allView2.expected = allView.expected;
+  await allView2.waitForExpected();
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadHistory_initialization.js b/toolkit/components/downloads/test/unit/test_DownloadHistory_initialization.js
new file mode 100644
index 0000000000..ea41038bf1
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadHistory_initialization.js
@@ -0,0 +1,108 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { DownloadHistory } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadHistory.sys.mjs"
+);
+const { PlacesTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/PlacesTestUtils.sys.mjs"
+);
+
+let baseDate = new Date("2000-01-01");
+
+/**
+ * This test is designed to ensure the cache of download history is correctly
+ * loaded and initialized. We do this by having the test as the only test in
+ * this file, and injecting data into the places database before we start.
+ */
+add_task(async function test_DownloadHistory_initialization() {
+  // Clean up at the beginning and at the end of the test.
+  async function cleanup() {
+    await PlacesUtils.history.clear();
+  }
+  registerCleanupFunction(cleanup);
+  await cleanup();
+
+  let testDownloads = [];
+  for (let i = 10; i <= 30; i += 10) {
+    let targetFile = getTempFile(`${TEST_TARGET_FILE_NAME}${i}`);
+    let download = {
+      source: {
+        url: httpUrl(`source${i}`),
+        isPrivate: false,
+      },
+      target: { path: targetFile.path },
+      endTime: baseDate.getTime() + i,
+      fileSize: 100 + i,
+      state: i / 10,
+    };
+
+    await PlacesTestUtils.addVisits([
+      {
+        uri: download.source.url,
+        transition: PlacesUtils.history.TRANSITIONS.DOWNLOAD,
+      },
+    ]);
+
+    let targetUri = Services.io.newFileURI(
+      new FileUtils.File(download.target.path)
+    );
+
+    await PlacesUtils.history.update({
+      annotations: new Map([
+        ["downloads/destinationFileURI", targetUri.spec],
+        [
+          "downloads/metaData",
+          JSON.stringify({
+            state: download.state,
+            endTime: download.endTime,
+            fileSize: download.fileSize,
+          }),
+        ],
+      ]),
+      url: download.source.url,
+    });
+
+    testDownloads.push(download);
+  }
+
+  // Initialize DownloadHistoryList only after having added the history and
+  // session downloads.
+  let historyList = await DownloadHistory.getList();
+  let downloads = await historyList.getAll();
+  Assert.equal(downloads.length, testDownloads.length);
+
+  for (let expected of testDownloads) {
+    let download = downloads.find(d => d.source.url == expected.source.url);
+
+    info(`Checking download ${expected.source.url}`);
+    Assert.ok(download, "Should have found the expected download");
+    Assert.equal(
+      download.endTime,
+      expected.endTime,
+      "Should have the correct end time"
+    );
+    Assert.equal(
+      download.target.size,
+      expected.fileSize,
+      "Should have the correct file size"
+    );
+    Assert.equal(
+      download.succeeded,
+      expected.state == 1,
+      "Should have the correct succeeded value"
+    );
+    Assert.equal(
+      download.canceled,
+      expected.state == 3,
+      "Should have the correct canceled value"
+    );
+    Assert.equal(
+      download.target.path,
+      expected.target.path,
+      "Should have the correct target path"
+    );
+  }
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadHistory_initialization2.js b/toolkit/components/downloads/test/unit/test_DownloadHistory_initialization2.js
new file mode 100644
index 0000000000..8b714a8fae
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadHistory_initialization2.js
@@ -0,0 +1,61 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { DownloadHistory } = ChromeUtils.importESModule(
+  "resource://gre/modules/DownloadHistory.sys.mjs"
+);
+
+/**
+ * This test is designed to ensure the cache of download history is correctly
+ * loaded and initialized via adding downloads. We do this by having the test as
+ * the only test in this file.
+ */
+add_task(async function test_initialization_via_addDownload() {
+  // Clean up at the beginning and at the end of the test.
+  async function cleanup() {
+    await PlacesUtils.history.clear();
+  }
+  registerCleanupFunction(cleanup);
+  await cleanup();
+
+  const download1FileLocation = getTempFile(`${TEST_TARGET_FILE_NAME}1`).path;
+  const download2FileLocation = getTempFile(`${TEST_TARGET_FILE_NAME}2`).path;
+  const download = {
+    source: {
+      url: httpUrl(`source1`),
+      isPrivate: false,
+    },
+    target: { path: download1FileLocation },
+  };
+
+  await DownloadHistory.addDownloadToHistory(download);
+
+  // Initialize DownloadHistoryList only after having added the history and
+  // session downloads.
+  let historyList = await DownloadHistory.getList();
+  let downloads = await historyList.getAll();
+  Assert.equal(downloads.length, 1, "Should have only one entry");
+
+  Assert.equal(
+    downloads[0].target.path,
+    download1FileLocation,
+    "Should have the correct target path"
+  );
+
+  // Now re-add the download but with a different target.
+  download.target.path = download2FileLocation;
+
+  await DownloadHistory.addDownloadToHistory(download);
+
+  historyList = await DownloadHistory.getList();
+  downloads = await historyList.getAll();
+  Assert.equal(downloads.length, 1, "Should still have only one entry");
+
+  Assert.equal(
+    downloads[0].target.path,
+    download2FileLocation,
+    "Should have the correct revised target path"
+  );
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadIntegration.js b/toolkit/components/downloads/test/unit/test_DownloadIntegration.js
new file mode 100644
index 0000000000..7ee34e8307
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadIntegration.js
@@ -0,0 +1,441 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests the DownloadIntegration object.
+ */
+
+"use strict";
+
+// Globals
+
+/**
+ * Notifies the prompt observers and verify the expected downloads count.
+ *
+ * @param aIsPrivate
+ *        Flag to know is test private observers.
+ * @param aExpectedCount
+ *        the expected downloads count for quit and offline observers.
+ * @param aExpectedPBCount
+ *        the expected downloads count for private browsing observer.
+ */
+function notifyPromptObservers(aIsPrivate, aExpectedCount, aExpectedPBCount) {
+  let cancelQuit = Cc["@mozilla.org/supports-PRBool;1"].createInstance(
+    Ci.nsISupportsPRBool
+  );
+
+  // Notify quit application requested observer.
+  DownloadIntegration._testPromptDownloads = -1;
+  Services.obs.notifyObservers(cancelQuit, "quit-application-requested");
+  Assert.equal(DownloadIntegration._testPromptDownloads, aExpectedCount);
+
+  // Notify offline requested observer.
+  DownloadIntegration._testPromptDownloads = -1;
+  Services.obs.notifyObservers(cancelQuit, "offline-requested");
+  Assert.equal(DownloadIntegration._testPromptDownloads, aExpectedCount);
+
+  if (aIsPrivate) {
+    // Notify last private browsing requested observer.
+    DownloadIntegration._testPromptDownloads = -1;
+    Services.obs.notifyObservers(cancelQuit, "last-pb-context-exiting");
+    Assert.equal(DownloadIntegration._testPromptDownloads, aExpectedPBCount);
+  }
+
+  delete DownloadIntegration._testPromptDownloads;
+}
+
+// Tests
+
+/**
+ * Allows re-enabling the real download directory logic during one test.
+ */
+function allowDirectoriesInTest() {
+  DownloadIntegration.allowDirectories = true;
+  function cleanup() {
+    DownloadIntegration.allowDirectories = false;
+  }
+  registerCleanupFunction(cleanup);
+  return cleanup;
+}
+
+ChromeUtils.defineLazyGetter(this, "gStringBundle", function () {
+  return Services.strings.createBundle(
+    "chrome://mozapps/locale/downloads/downloads.properties"
+  );
+});
+
+/**
+ * Tests that getSystemDownloadsDirectory returns an existing directory or
+ * creates a new directory depending on the platform. Instead of the real
+ * directory, this test is executed in the temporary directory so we can safely
+ * delete the created folder to check whether it is created again.
+ */
+add_task(async function test_getSystemDownloadsDirectory_exists_or_creates() {
+  let tempDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  let downloadDir;
+
+  // OSX / Linux / Windows but not XP/2k
+  if (
+    Services.appinfo.OS == "Darwin" ||
+    Services.appinfo.OS == "Linux" ||
+    (Services.appinfo.OS == "WINNT" &&
+      parseFloat(Services.sysinfo.getProperty("version")) >= 6)
+  ) {
+    downloadDir = await DownloadIntegration.getSystemDownloadsDirectory();
+    Assert.equal(downloadDir, tempDir.path);
+    Assert.ok(await IOUtils.exists(downloadDir));
+
+    let info = await IOUtils.stat(downloadDir);
+    Assert.equal(info.type, "directory");
+  } else {
+    let targetPath = PathUtils.join(
+      tempDir.path,
+      gStringBundle.GetStringFromName("downloadsFolder")
+    );
+    try {
+      await IOUtils.remove(targetPath);
+    } catch (e) {}
+    downloadDir = await DownloadIntegration.getSystemDownloadsDirectory();
+    Assert.equal(downloadDir, targetPath);
+    Assert.ok(await IOUtils.exists(downloadDir));
+
+    let info = await IOUtils.stat(downloadDir);
+    Assert.equal(info.type, "directory");
+    await IOUtils.remove(targetPath);
+  }
+});
+
+/**
+ * Tests that the real directory returned by getSystemDownloadsDirectory is not
+ * the one that is used during unit tests. Since this is the actual downloads
+ * directory of the operating system, we don't try to delete it afterwards.
+ */
+add_task(async function test_getSystemDownloadsDirectory_real() {
+  let fakeDownloadDir = await DownloadIntegration.getSystemDownloadsDirectory();
+
+  let cleanup = allowDirectoriesInTest();
+  let realDownloadDir = await DownloadIntegration.getSystemDownloadsDirectory();
+  cleanup();
+
+  Assert.notEqual(fakeDownloadDir, realDownloadDir);
+});
+
+/**
+ * Tests that the getPreferredDownloadsDirectory returns a valid download
+ * directory string path.
+ */
+add_task(async function test_getPreferredDownloadsDirectory() {
+  let cleanupDirectories = allowDirectoriesInTest();
+
+  let folderListPrefName = "browser.download.folderList";
+  let dirPrefName = "browser.download.dir";
+  function cleanupPrefs() {
+    Services.prefs.clearUserPref(folderListPrefName);
+    Services.prefs.clearUserPref(dirPrefName);
+  }
+  registerCleanupFunction(cleanupPrefs);
+
+  // For legacy cloudstorage users with folderListPrefName as 3,
+  // Should return the system downloads directory because the dir preference
+  // is not set.
+  Services.prefs.setIntPref(folderListPrefName, 3);
+  let systemDir = await DownloadIntegration.getSystemDownloadsDirectory();
+  let downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, systemDir);
+
+  // Should return the system downloads directory.
+  Services.prefs.setIntPref(folderListPrefName, 1);
+  systemDir = await DownloadIntegration.getSystemDownloadsDirectory();
+  downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, systemDir);
+
+  // Should return the desktop directory.
+  Services.prefs.setIntPref(folderListPrefName, 0);
+  downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, Services.dirsvc.get("Desk", Ci.nsIFile).path);
+
+  // Should return the system downloads directory because the dir preference
+  // is not set.
+  Services.prefs.setIntPref(folderListPrefName, 2);
+  downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, systemDir);
+
+  // Should return the directory which is listed in the dir preference.
+  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);
+  downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+  Assert.equal(downloadDir, tempDir.path);
+  Assert.ok(await IOUtils.exists(downloadDir));
+  await IOUtils.remove(tempDir.path);
+
+  // Should return the system downloads directory beacause the path is invalid
+  // in the dir preference.
+  tempDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  tempDir.append("dir_not_exist");
+  tempDir.append(time);
+  Services.prefs.setComplexValue("browser.download.dir", Ci.nsIFile, tempDir);
+  downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.equal(downloadDir, systemDir);
+
+  // Should return the system downloads directory because the folderList
+  // preference is invalid
+  Services.prefs.setIntPref(folderListPrefName, 999);
+  downloadDir = await DownloadIntegration.getPreferredDownloadsDirectory();
+  Assert.equal(downloadDir, systemDir);
+
+  cleanupPrefs();
+  cleanupDirectories();
+});
+
+/**
+ * Tests that the getTemporaryDownloadsDirectory returns a valid download
+ * directory string path.
+ */
+add_task(async function test_getTemporaryDownloadsDirectory() {
+  let cleanup = allowDirectoriesInTest();
+
+  let downloadDir = await DownloadIntegration.getTemporaryDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+
+  if ("nsILocalFileMac" in Ci) {
+    let preferredDownloadDir =
+      await DownloadIntegration.getPreferredDownloadsDirectory();
+    Assert.equal(downloadDir, preferredDownloadDir);
+  } else {
+    let tempDir = Services.dirsvc.get("TmpD", Ci.nsIFile);
+    Assert.equal(downloadDir, tempDir.path);
+  }
+
+  cleanup();
+});
+
+// Tests DownloadObserver
+
+/**
+ * Re-enables the default observers for the following tests.
+ *
+ * This takes effect the first time a DownloadList object is created, and lasts
+ * until this test file has completed.
+ */
+add_task(async function test_observers_setup() {
+  DownloadIntegration.allowObservers = true;
+  registerCleanupFunction(function () {
+    DownloadIntegration.allowObservers = false;
+  });
+});
+
+/**
+ * Tests notifications prompts when observers are notified if there are public
+ * and private active downloads.
+ */
+add_task(async function test_notifications() {
+  for (let isPrivate of [false, true]) {
+    mustInterruptResponses();
+
+    let list = await promiseNewList(isPrivate);
+    let download1 = await promiseNewDownload(httpUrl("interruptible.txt"));
+    let download2 = await promiseNewDownload(httpUrl("interruptible.txt"));
+    let download3 = await promiseNewDownload(httpUrl("interruptible.txt"));
+    let promiseAttempt1 = download1.start();
+    let promiseAttempt2 = download2.start();
+    download3.start().catch(() => {});
+
+    // Add downloads to list.
+    await list.add(download1);
+    await list.add(download2);
+    await list.add(download3);
+    // Cancel third download
+    await download3.cancel();
+
+    notifyPromptObservers(isPrivate, 2, 2);
+
+    // Allow the downloads to complete.
+    continueResponses();
+    await promiseAttempt1;
+    await promiseAttempt2;
+
+    // Clean up.
+    await list.remove(download1);
+    await list.remove(download2);
+    await list.remove(download3);
+  }
+});
+
+/**
+ * Tests that notifications prompts observers are not notified if there are no
+ * public or private active downloads.
+ */
+add_task(async function test_no_notifications() {
+  for (let isPrivate of [false, true]) {
+    let list = await promiseNewList(isPrivate);
+    let download1 = await promiseNewDownload(httpUrl("interruptible.txt"));
+    let download2 = await promiseNewDownload(httpUrl("interruptible.txt"));
+    download1.start().catch(() => {});
+    download2.start().catch(() => {});
+
+    // Add downloads to list.
+    await list.add(download1);
+    await list.add(download2);
+
+    await download1.cancel();
+    await download2.cancel();
+
+    notifyPromptObservers(isPrivate, 0, 0);
+
+    // Clean up.
+    await list.remove(download1);
+    await list.remove(download2);
+  }
+});
+
+/**
+ * Tests notifications prompts when observers are notified if there are public
+ * and private active downloads at the same time.
+ */
+add_task(async function test_mix_notifications() {
+  mustInterruptResponses();
+
+  let publicList = await promiseNewList();
+  let privateList = await Downloads.getList(Downloads.PRIVATE);
+  let download1 = await promiseNewDownload(httpUrl("interruptible.txt"));
+  let download2 = await promiseNewDownload(httpUrl("interruptible.txt"));
+  let promiseAttempt1 = download1.start();
+  let promiseAttempt2 = download2.start();
+
+  // Add downloads to lists.
+  await publicList.add(download1);
+  await privateList.add(download2);
+
+  notifyPromptObservers(true, 2, 1);
+
+  // Allow the downloads to complete.
+  continueResponses();
+  await promiseAttempt1;
+  await promiseAttempt2;
+
+  // Clean up.
+  await publicList.remove(download1);
+  await privateList.remove(download2);
+});
+
+/**
+ * Tests suspending and resuming as well as going offline and then online again.
+ * The downloads should stop when suspending and start again when resuming.
+ */
+add_task(async function test_suspend_resume() {
+  // The default wake delay is 10 seconds, so set the wake delay to be much
+  // faster for these tests.
+  Services.prefs.setIntPref("browser.download.manager.resumeOnWakeDelay", 5);
+
+  let addDownload = function (list) {
+    return (async function () {
+      let download = await promiseNewDownload(httpUrl("interruptible.txt"));
+      download.start().catch(() => {});
+      list.add(download);
+      return download;
+    })();
+  };
+
+  let publicList = await promiseNewList();
+  let privateList = await promiseNewList(true);
+
+  let download1 = await addDownload(publicList);
+  let download2 = await addDownload(publicList);
+  let download3 = await addDownload(privateList);
+  let download4 = await addDownload(privateList);
+  let download5 = await addDownload(publicList);
+
+  // First, check that the downloads are all canceled when going to sleep.
+  Services.obs.notifyObservers(null, "sleep_notification");
+  Assert.ok(download1.canceled);
+  Assert.ok(download2.canceled);
+  Assert.ok(download3.canceled);
+  Assert.ok(download4.canceled);
+  Assert.ok(download5.canceled);
+
+  // Remove a download. It should not be started again.
+  publicList.remove(download5);
+  Assert.ok(download5.canceled);
+
+  // When waking up again, the downloads start again after the wake delay. To be
+  // more robust, don't check after a delay but instead just wait for the
+  // downloads to finish.
+  Services.obs.notifyObservers(null, "wake_notification");
+  await download1.whenSucceeded();
+  await download2.whenSucceeded();
+  await download3.whenSucceeded();
+  await download4.whenSucceeded();
+
+  // Downloads should no longer be canceled. However, as download5 was removed
+  // from the public list, it will not be restarted.
+  Assert.ok(!download1.canceled);
+  Assert.ok(download5.canceled);
+
+  // Create four new downloads and check for going offline and then online again.
+
+  download1 = await addDownload(publicList);
+  download2 = await addDownload(publicList);
+  download3 = await addDownload(privateList);
+  download4 = await addDownload(privateList);
+
+  // Going offline should cancel the downloads.
+  Services.obs.notifyObservers(null, "network:offline-about-to-go-offline");
+  Assert.ok(download1.canceled);
+  Assert.ok(download2.canceled);
+  Assert.ok(download3.canceled);
+  Assert.ok(download4.canceled);
+
+  // Going back online should start the downloads again.
+  Services.obs.notifyObservers(
+    null,
+    "network:offline-status-changed",
+    "online"
+  );
+  await download1.whenSucceeded();
+  await download2.whenSucceeded();
+  await download3.whenSucceeded();
+  await download4.whenSucceeded();
+
+  Services.prefs.clearUserPref("browser.download.manager.resumeOnWakeDelay");
+});
+
+/**
+ * Tests both the downloads list and the in-progress downloads are clear when
+ * private browsing observer is notified.
+ */
+add_task(async function test_exit_private_browsing() {
+  mustInterruptResponses();
+
+  let privateList = await promiseNewList(true);
+  let download1 = await promiseNewDownload(httpUrl("source.txt"));
+  let download2 = await promiseNewDownload(httpUrl("interruptible.txt"));
+  let promiseAttempt1 = download1.start();
+  download2.start();
+
+  // Add downloads to list.
+  await privateList.add(download1);
+  await privateList.add(download2);
+
+  // Complete the download.
+  await promiseAttempt1;
+
+  Assert.equal((await privateList.getAll()).length, 2);
+
+  // Simulate exiting the private browsing.
+  await new Promise(resolve => {
+    DownloadIntegration._testResolveClearPrivateList = resolve;
+    Services.obs.notifyObservers(null, "last-pb-context-exited");
+  });
+  delete DownloadIntegration._testResolveClearPrivateList;
+
+  Assert.equal((await privateList.getAll()).length, 0);
+
+  continueResponses();
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadLegacy.js b/toolkit/components/downloads/test/unit/test_DownloadLegacy.js
new file mode 100644
index 0000000000..972820f29e
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadLegacy.js
@@ -0,0 +1,100 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests the integration with legacy interfaces for downloads.
+ */
+
+"use strict";
+
+// Execution of common tests
+
+// This is used in common_test_Download.js
+// eslint-disable-next-line no-unused-vars
+var gUseLegacySaver = true;
+
+var scriptFile = do_get_file("common_test_Download.js");
+Services.scriptloader.loadSubScript(NetUtil.newURI(scriptFile).spec);
+
+/**
+ * Checks the referrer for restart downloads.
+ * If the legacy download is stopped and restarted, the saving method
+ * is changed from DownloadLegacySaver to the DownloadCopySaver.
+ * The referrer header should be passed correctly.
+ */
+add_task(async function test_referrer_restart() {
+  let sourcePath = "/test_referrer_restart.txt";
+  let sourceUrl = httpUrl("test_referrer_restart.txt");
+
+  function cleanup() {
+    gHttpServer.registerPathHandler(sourcePath, null);
+  }
+  registerCleanupFunction(cleanup);
+
+  registerInterruptibleHandler(
+    sourcePath,
+    function firstPart(aRequest, aResponse) {
+      aResponse.setHeader("Content-Type", "text/plain", false);
+      Assert.ok(aRequest.hasHeader("Referer"));
+      Assert.equal(aRequest.getHeader("Referer"), TEST_REFERRER_URL);
+
+      aResponse.setHeader(
+        "Content-Length",
+        "" + TEST_DATA_SHORT.length * 2,
+        false
+      );
+      aResponse.write(TEST_DATA_SHORT);
+    },
+    function secondPart(aRequest, aResponse) {
+      Assert.ok(aRequest.hasHeader("Referer"));
+      Assert.equal(aRequest.getHeader("Referer"), TEST_REFERRER_URL);
+
+      aResponse.write(TEST_DATA_SHORT);
+    }
+  );
+
+  let referrerInfo = new ReferrerInfo(
+    Ci.nsIReferrerInfo.UNSAFE_URL,
+    true,
+    NetUtil.newURI(TEST_REFERRER_URL)
+  );
+
+  async function restart_and_check_referrer(download) {
+    let promiseSucceeded = download.whenSucceeded();
+
+    // Cancel the first download attempt.
+    await promiseDownloadMidway(download);
+    await download.cancel();
+
+    // The second request is allowed to complete.
+    continueResponses();
+    download.start().catch(() => {});
+
+    // Wait for the download to finish by waiting on the whenSucceeded promise.
+    await promiseSucceeded;
+
+    Assert.ok(download.stopped);
+    Assert.ok(download.succeeded);
+    Assert.ok(!download.canceled);
+
+    checkEqualReferrerInfos(download.source.referrerInfo, referrerInfo);
+  }
+
+  mustInterruptResponses();
+
+  let download = await promiseStartLegacyDownload(sourceUrl, {
+    referrerInfo,
+  });
+  await restart_and_check_referrer(download);
+
+  mustInterruptResponses();
+  download = await promiseStartLegacyDownload(sourceUrl, {
+    referrerInfo,
+    isPrivate: true,
+  });
+  await restart_and_check_referrer(download);
+
+  cleanup();
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadList.js b/toolkit/components/downloads/test/unit/test_DownloadList.js
new file mode 100644
index 0000000000..f7d03900af
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadList.js
@@ -0,0 +1,677 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests the DownloadList object.
+ */
+
+"use strict";
+
+// Globals
+
+const { TelemetryTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/TelemetryTestUtils.sys.mjs"
+);
+
+Services.prefs.setBoolPref(
+  "toolkit.telemetry.testing.overrideProductsCheck",
+  true
+);
+
+/**
+ * Returns a Date in the past usable to add expirable visits.
+ *
+ * @note Expiration ignores any visit added in the last 7 days, but it's
+ *       better be safe against DST issues, by going back one day more.
+ */
+function getExpirableDate() {
+  let dateObj = new Date();
+  // Normalize to midnight
+  dateObj.setHours(0);
+  dateObj.setMinutes(0);
+  dateObj.setSeconds(0);
+  dateObj.setMilliseconds(0);
+  return new Date(dateObj.getTime() - 8 * 86400000);
+}
+
+/**
+ * Adds an expirable history visit for a download.
+ *
+ * @param aSourceUrl
+ *        String containing the URI for the download source, or null to use
+ *        httpUrl("source.txt").
+ *
+ * @return {Promise}
+ * @rejects JavaScript exception.
+ */
+function promiseExpirableDownloadVisit(aSourceUrl) {
+  return PlacesUtils.history.insert({
+    url: aSourceUrl || httpUrl("source.txt"),
+    visits: [
+      {
+        transition: PlacesUtils.history.TRANSITIONS.DOWNLOAD,
+        date: getExpirableDate(),
+      },
+    ],
+  });
+}
+
+// Tests
+
+/**
+ * Checks the testing mechanism used to build different download lists.
+ */
+add_task(async function test_construction() {
+  let downloadListOne = await promiseNewList();
+  let downloadListTwo = await promiseNewList();
+  let privateDownloadListOne = await promiseNewList(true);
+  let privateDownloadListTwo = await promiseNewList(true);
+
+  Assert.notEqual(downloadListOne, downloadListTwo);
+  Assert.notEqual(privateDownloadListOne, privateDownloadListTwo);
+  Assert.notEqual(downloadListOne, privateDownloadListOne);
+});
+
+/**
+ * Checks the methods to add and retrieve items from the list.
+ */
+add_task(async function test_add_getAll() {
+  let list = await promiseNewList();
+
+  let downloadOne = await promiseNewDownload();
+  await list.add(downloadOne);
+
+  let itemsOne = await list.getAll();
+  Assert.equal(itemsOne.length, 1);
+  Assert.equal(itemsOne[0], downloadOne);
+
+  let downloadTwo = await promiseNewDownload();
+  await list.add(downloadTwo);
+
+  let itemsTwo = await list.getAll();
+  Assert.equal(itemsTwo.length, 2);
+  Assert.equal(itemsTwo[0], downloadOne);
+  Assert.equal(itemsTwo[1], downloadTwo);
+
+  // The first snapshot should not have been modified.
+  Assert.equal(itemsOne.length, 1);
+});
+
+/**
+ * Checks the method to remove items from the list.
+ */
+add_task(async function test_remove() {
+  let list = await promiseNewList();
+
+  await list.add(await promiseNewDownload());
+  await list.add(await promiseNewDownload());
+
+  let items = await list.getAll();
+  await list.remove(items[0]);
+
+  // Removing an item that was never added should not raise an error.
+  await list.remove(await promiseNewDownload());
+
+  items = await list.getAll();
+  Assert.equal(items.length, 1);
+});
+
+/**
+ * Tests that the "add", "remove", and "getAll" methods on the global
+ * DownloadCombinedList object combine the contents of the global DownloadList
+ * objects for public and private downloads.
+ */
+add_task(async function test_DownloadCombinedList_add_remove_getAll() {
+  let publicList = await promiseNewList();
+  let privateList = await Downloads.getList(Downloads.PRIVATE);
+  let combinedList = await Downloads.getList(Downloads.ALL);
+
+  let publicDownload = await promiseNewDownload();
+  let privateDownload = await Downloads.createDownload({
+    source: { url: httpUrl("source.txt"), isPrivate: true },
+    target: getTempFile(TEST_TARGET_FILE_NAME).path,
+  });
+
+  await publicList.add(publicDownload);
+  await privateList.add(privateDownload);
+
+  Assert.equal((await combinedList.getAll()).length, 2);
+
+  await combinedList.remove(publicDownload);
+  await combinedList.remove(privateDownload);
+
+  Assert.equal((await combinedList.getAll()).length, 0);
+
+  await combinedList.add(publicDownload);
+  await combinedList.add(privateDownload);
+
+  Assert.equal((await publicList.getAll()).length, 1);
+  Assert.equal((await privateList.getAll()).length, 1);
+  Assert.equal((await combinedList.getAll()).length, 2);
+
+  await publicList.remove(publicDownload);
+  await privateList.remove(privateDownload);
+
+  Assert.equal((await combinedList.getAll()).length, 0);
+});
+
+/**
+ * Checks that views receive the download add and remove notifications, and that
+ * adding and removing views works as expected, both for a normal and a combined
+ * list.
+ */
+add_task(async function test_notifications_add_remove() {
+  for (let isCombined of [false, true]) {
+    // Force creating a new list for both the public and combined cases.
+    let list = await promiseNewList();
+    if (isCombined) {
+      list = await Downloads.getList(Downloads.ALL);
+    }
+
+    let downloadOne = await promiseNewDownload();
+    let downloadTwo = await Downloads.createDownload({
+      source: { url: httpUrl("source.txt"), isPrivate: true },
+      target: getTempFile(TEST_TARGET_FILE_NAME).path,
+    });
+    await list.add(downloadOne);
+    await list.add(downloadTwo);
+
+    // Check that we receive add notifications for existing elements.
+    let addNotifications = 0;
+    let viewOne = {
+      onDownloadAdded(aDownload) {
+        // The first download to be notified should be the first that was added.
+        if (addNotifications == 0) {
+          Assert.equal(aDownload, downloadOne);
+        } else if (addNotifications == 1) {
+          Assert.equal(aDownload, downloadTwo);
+        }
+        addNotifications++;
+      },
+    };
+    await list.addView(viewOne);
+    Assert.equal(addNotifications, 2);
+
+    // Check that we receive add notifications for new elements.
+    await list.add(await promiseNewDownload());
+    Assert.equal(addNotifications, 3);
+
+    // Check that we receive remove notifications.
+    let removeNotifications = 0;
+    let viewTwo = {
+      onDownloadRemoved(aDownload) {
+        Assert.equal(aDownload, downloadOne);
+        removeNotifications++;
+      },
+    };
+    await list.addView(viewTwo);
+    await list.remove(downloadOne);
+    Assert.equal(removeNotifications, 1);
+
+    // We should not receive remove notifications after the view is removed.
+    await list.removeView(viewTwo);
+    await list.remove(downloadTwo);
+    Assert.equal(removeNotifications, 1);
+
+    // We should not receive add notifications after the view is removed.
+    await list.removeView(viewOne);
+    await list.add(await promiseNewDownload());
+    Assert.equal(addNotifications, 3);
+  }
+});
+
+/**
+ * Checks that views receive the download change notifications, both for a
+ * normal and a combined list.
+ */
+add_task(async function test_notifications_change() {
+  for (let isCombined of [false, true]) {
+    // Force creating a new list for both the public and combined cases.
+    let list = await promiseNewList();
+    if (isCombined) {
+      list = await Downloads.getList(Downloads.ALL);
+    }
+
+    let downloadOne = await promiseNewDownload();
+    let downloadTwo = await Downloads.createDownload({
+      source: { url: httpUrl("source.txt"), isPrivate: true },
+      target: getTempFile(TEST_TARGET_FILE_NAME).path,
+    });
+    await list.add(downloadOne);
+    await list.add(downloadTwo);
+
+    // Check that we receive change notifications.
+    let receivedOnDownloadChanged = false;
+    await list.addView({
+      onDownloadChanged(aDownload) {
+        Assert.equal(aDownload, downloadOne);
+        receivedOnDownloadChanged = true;
+      },
+    });
+    await downloadOne.start();
+    Assert.ok(receivedOnDownloadChanged);
+
+    // We should not receive change notifications after a download is removed.
+    receivedOnDownloadChanged = false;
+    await list.remove(downloadTwo);
+    await downloadTwo.start();
+    Assert.ok(!receivedOnDownloadChanged);
+  }
+});
+
+/**
+ * Checks that the reference to "this" is correct in the view callbacks.
+ */
+add_task(async function test_notifications_this() {
+  let list = await promiseNewList();
+
+  // Check that we receive change notifications.
+  let receivedOnDownloadAdded = false;
+  let receivedOnDownloadChanged = false;
+  let receivedOnDownloadRemoved = false;
+  let view = {
+    onDownloadAdded() {
+      Assert.equal(this, view);
+      receivedOnDownloadAdded = true;
+    },
+    onDownloadChanged() {
+      // Only do this check once.
+      if (!receivedOnDownloadChanged) {
+        Assert.equal(this, view);
+        receivedOnDownloadChanged = true;
+      }
+    },
+    onDownloadRemoved() {
+      Assert.equal(this, view);
+      receivedOnDownloadRemoved = true;
+    },
+  };
+  await list.addView(view);
+
+  let download = await promiseNewDownload();
+  await list.add(download);
+  await download.start();
+  await list.remove(download);
+
+  // Verify that we executed the checks.
+  Assert.ok(receivedOnDownloadAdded);
+  Assert.ok(receivedOnDownloadChanged);
+  Assert.ok(receivedOnDownloadRemoved);
+});
+
+/**
+ * Checks that download is removed on history expiration.
+ */
+add_task(async function test_history_expiration() {
+  mustInterruptResponses();
+
+  function cleanup() {
+    Services.prefs.clearUserPref("places.history.expiration.max_pages");
+  }
+  registerCleanupFunction(cleanup);
+
+  // Set max pages to 0 to make the download expire.
+  Services.prefs.setIntPref("places.history.expiration.max_pages", 0);
+
+  let list = await promiseNewList();
+  let downloadOne = await promiseNewDownload();
+  let downloadTwo = await promiseNewDownload(httpUrl("interruptible.txt"));
+
+  let deferred = Promise.withResolvers();
+  let removeNotifications = 0;
+  let downloadView = {
+    onDownloadRemoved(aDownload) {
+      if (++removeNotifications == 2) {
+        deferred.resolve();
+      }
+    },
+  };
+  await list.addView(downloadView);
+
+  // Work with one finished download and one canceled download.
+  await downloadOne.start();
+  downloadTwo.start().catch(() => {});
+  await downloadTwo.cancel();
+
+  // We must replace the visits added while executing the downloads with visits
+  // that are older than 7 days, otherwise they will not be expired.
+  await PlacesUtils.history.clear();
+  await promiseExpirableDownloadVisit();
+  await promiseExpirableDownloadVisit(httpUrl("interruptible.txt"));
+
+  // After clearing history, we can add the downloads to be removed to the list.
+  await list.add(downloadOne);
+  await list.add(downloadTwo);
+
+  // Force a history expiration.
+  Cc["@mozilla.org/places/expiration;1"]
+    .getService(Ci.nsIObserver)
+    .observe(null, "places-debug-start-expiration", -1);
+
+  // Wait for both downloads to be removed.
+  await deferred.promise;
+
+  cleanup();
+});
+
+/**
+ * Checks all downloads are removed after clearing history.
+ */
+add_task(async function test_history_clear() {
+  let list = await promiseNewList();
+  let downloadOne = await promiseNewDownload();
+  let downloadTwo = await promiseNewDownload();
+  await list.add(downloadOne);
+  await list.add(downloadTwo);
+
+  let deferred = Promise.withResolvers();
+  let removeNotifications = 0;
+  let downloadView = {
+    onDownloadRemoved(aDownload) {
+      if (++removeNotifications == 2) {
+        deferred.resolve();
+      }
+    },
+  };
+  await list.addView(downloadView);
+
+  await downloadOne.start();
+  await downloadTwo.start();
+
+  await PlacesUtils.history.clear();
+
+  // Wait for the removal notifications that may still be pending.
+  await deferred.promise;
+});
+
+/**
+ * Tests the removeFinished method to ensure that it only removes
+ * finished downloads.
+ */
+add_task(async function test_removeFinished() {
+  let list = await promiseNewList();
+  let downloadOne = await promiseNewDownload();
+  let downloadTwo = await promiseNewDownload();
+  let downloadThree = await promiseNewDownload();
+  let downloadFour = await promiseNewDownload();
+  await list.add(downloadOne);
+  await list.add(downloadTwo);
+  await list.add(downloadThree);
+  await list.add(downloadFour);
+
+  let deferred = Promise.withResolvers();
+  let removeNotifications = 0;
+  let downloadView = {
+    onDownloadRemoved(aDownload) {
+      Assert.ok(
+        aDownload == downloadOne ||
+          aDownload == downloadTwo ||
+          aDownload == downloadThree
+      );
+      Assert.ok(removeNotifications < 3);
+      if (++removeNotifications == 3) {
+        deferred.resolve();
+      }
+    },
+  };
+  await list.addView(downloadView);
+
+  // Start three of the downloads, but don't start downloadTwo, then set
+  // downloadFour to have partial data. All downloads except downloadFour
+  // should be removed.
+  await downloadOne.start();
+  await downloadThree.start();
+  await downloadFour.start();
+  downloadFour.hasPartialData = true;
+
+  list.removeFinished();
+  await deferred.promise;
+
+  let downloads = await list.getAll();
+  Assert.equal(downloads.length, 1);
+});
+
+/**
+ * Tests that removeFinished method keeps the file that is currently downloading,
+ * even if it needs to remove failed download of the same file.
+ */
+add_task(async function test_removeFinished_keepsDownloadingFile() {
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+
+  let oneDownload = await Downloads.createDownload({
+    source: httpUrl("empty.txt"),
+    target: targetFile.path,
+  });
+
+  let otherDownload = await Downloads.createDownload({
+    source: httpUrl("empty.txt"),
+    target: targetFile.path,
+  });
+
+  let list = await promiseNewList();
+  await list.add(oneDownload);
+  await list.add(otherDownload);
+
+  let deferred = Promise.withResolvers();
+  let downloadView = {
+    async onDownloadRemoved(aDownload) {
+      Assert.equal(aDownload, oneDownload);
+      await TestUtils.waitForCondition(() => oneDownload._finalizeExecuted);
+      deferred.resolve();
+    },
+  };
+  await list.addView(downloadView);
+
+  await oneDownload.start();
+  await otherDownload.start();
+
+  oneDownload.hasPartialData = otherDownload.hasPartialData = true;
+  oneDownload.error = "Download failed";
+
+  list.removeFinished();
+  await deferred.promise;
+
+  let downloads = await list.getAll();
+  Assert.equal(
+    downloads.length,
+    1,
+    "Failed download should be removed, active download should be kept"
+  );
+
+  Assert.ok(
+    await IOUtils.exists(otherDownload.target.path),
+    "The file should not have been deleted."
+  );
+});
+
+/**
+ * Tests the global DownloadSummary objects for the public, private, and
+ * combined download lists.
+ */
+add_task(async function test_DownloadSummary() {
+  mustInterruptResponses();
+
+  let publicList = await promiseNewList();
+  let privateList = await Downloads.getList(Downloads.PRIVATE);
+
+  let publicSummary = await Downloads.getSummary(Downloads.PUBLIC);
+  let privateSummary = await Downloads.getSummary(Downloads.PRIVATE);
+  let combinedSummary = await Downloads.getSummary(Downloads.ALL);
+
+  // Add a public download that has succeeded.
+  let succeededPublicDownload = await promiseNewDownload();
+  await succeededPublicDownload.start();
+  await publicList.add(succeededPublicDownload);
+
+  // Add a public download that has been canceled midway.
+  let canceledPublicDownload = await promiseNewDownload(
+    httpUrl("interruptible.txt")
+  );
+  canceledPublicDownload.start().catch(() => {});
+  await promiseDownloadMidway(canceledPublicDownload);
+  await canceledPublicDownload.cancel();
+  await publicList.add(canceledPublicDownload);
+
+  // Add a public download that is in progress.
+  let inProgressPublicDownload = await promiseNewDownload(
+    httpUrl("interruptible.txt")
+  );
+  inProgressPublicDownload.start().catch(() => {});
+  await promiseDownloadMidway(inProgressPublicDownload);
+  await publicList.add(inProgressPublicDownload);
+
+  // Add a public download of unknown size that is in progress.
+  let inProgressSizelessPublicDownload = await promiseNewDownload(
+    httpUrl("interruptible_nosize.txt")
+  );
+  inProgressSizelessPublicDownload.start().catch(() => {});
+  await promiseDownloadStarted(inProgressSizelessPublicDownload);
+  await publicList.add(inProgressSizelessPublicDownload);
+
+  // Add a private download that is in progress.
+  let inProgressPrivateDownload = await Downloads.createDownload({
+    source: { url: httpUrl("interruptible.txt"), isPrivate: true },
+    target: getTempFile(TEST_TARGET_FILE_NAME).path,
+  });
+  inProgressPrivateDownload.start().catch(() => {});
+  await promiseDownloadMidway(inProgressPrivateDownload);
+  await privateList.add(inProgressPrivateDownload);
+
+  // Verify that the summary includes the total number of bytes and the
+  // currently transferred bytes only for the downloads that are not stopped.
+  // For simplicity, we assume that after a download is added to the list, its
+  // current state is immediately propagated to the summary object, which is
+  // true in the current implementation, though it is not guaranteed as all the
+  // download operations may happen asynchronously.
+  Assert.ok(!publicSummary.allHaveStopped);
+  Assert.ok(!publicSummary.allUnknownSize);
+  Assert.equal(publicSummary.progressTotalBytes, TEST_DATA_SHORT.length * 3);
+  Assert.equal(publicSummary.progressCurrentBytes, TEST_DATA_SHORT.length * 2);
+
+  Assert.ok(!privateSummary.allHaveStopped);
+  Assert.ok(!privateSummary.allUnknownSize);
+  Assert.equal(privateSummary.progressTotalBytes, TEST_DATA_SHORT.length * 2);
+  Assert.equal(privateSummary.progressCurrentBytes, TEST_DATA_SHORT.length);
+
+  Assert.ok(!combinedSummary.allHaveStopped);
+  Assert.ok(!combinedSummary.allUnknownSize);
+  Assert.equal(combinedSummary.progressTotalBytes, TEST_DATA_SHORT.length * 5);
+  Assert.equal(
+    combinedSummary.progressCurrentBytes,
+    TEST_DATA_SHORT.length * 3
+  );
+
+  await inProgressPublicDownload.cancel();
+
+  // Stopping the download should have excluded it from the summary, but we
+  // should still have one public download (with unknown size) and also one
+  // private download remaining.
+  Assert.ok(!publicSummary.allHaveStopped);
+  Assert.ok(publicSummary.allUnknownSize);
+  Assert.equal(publicSummary.progressTotalBytes, TEST_DATA_SHORT.length);
+  Assert.equal(publicSummary.progressCurrentBytes, TEST_DATA_SHORT.length);
+
+  Assert.ok(!privateSummary.allHaveStopped);
+  Assert.ok(!privateSummary.allUnknownSize);
+  Assert.equal(privateSummary.progressTotalBytes, TEST_DATA_SHORT.length * 2);
+  Assert.equal(privateSummary.progressCurrentBytes, TEST_DATA_SHORT.length);
+
+  Assert.ok(!combinedSummary.allHaveStopped);
+  Assert.ok(!combinedSummary.allUnknownSize);
+  Assert.equal(combinedSummary.progressTotalBytes, TEST_DATA_SHORT.length * 3);
+  Assert.equal(
+    combinedSummary.progressCurrentBytes,
+    TEST_DATA_SHORT.length * 2
+  );
+
+  await inProgressPrivateDownload.cancel();
+
+  // Stopping the private download should have excluded it from the summary, so
+  // now only the unknown size public download should remain.
+  Assert.ok(!publicSummary.allHaveStopped);
+  Assert.ok(publicSummary.allUnknownSize);
+  Assert.equal(publicSummary.progressTotalBytes, TEST_DATA_SHORT.length);
+  Assert.equal(publicSummary.progressCurrentBytes, TEST_DATA_SHORT.length);
+
+  Assert.ok(privateSummary.allHaveStopped);
+  Assert.ok(privateSummary.allUnknownSize);
+  Assert.equal(privateSummary.progressTotalBytes, 0);
+  Assert.equal(privateSummary.progressCurrentBytes, 0);
+
+  Assert.ok(!combinedSummary.allHaveStopped);
+  Assert.ok(combinedSummary.allUnknownSize);
+  Assert.equal(combinedSummary.progressTotalBytes, TEST_DATA_SHORT.length);
+  Assert.equal(combinedSummary.progressCurrentBytes, TEST_DATA_SHORT.length);
+
+  await inProgressSizelessPublicDownload.cancel();
+
+  // All the downloads should be stopped now.
+  Assert.ok(publicSummary.allHaveStopped);
+  Assert.ok(publicSummary.allUnknownSize);
+  Assert.equal(publicSummary.progressTotalBytes, 0);
+  Assert.equal(publicSummary.progressCurrentBytes, 0);
+
+  Assert.ok(privateSummary.allHaveStopped);
+  Assert.ok(privateSummary.allUnknownSize);
+  Assert.equal(privateSummary.progressTotalBytes, 0);
+  Assert.equal(privateSummary.progressCurrentBytes, 0);
+
+  Assert.ok(combinedSummary.allHaveStopped);
+  Assert.ok(combinedSummary.allUnknownSize);
+  Assert.equal(combinedSummary.progressTotalBytes, 0);
+  Assert.equal(combinedSummary.progressCurrentBytes, 0);
+});
+
+/**
+ * Checks that views receive the summary change notification.  This is tested on
+ * the combined summary when adding a public download, as we assume that if we
+ * pass the test in this case we will also pass it in the others.
+ */
+add_task(async function test_DownloadSummary_notifications() {
+  let list = await promiseNewList();
+  let summary = await Downloads.getSummary(Downloads.ALL);
+
+  let download = await promiseNewDownload();
+  await list.add(download);
+
+  // Check that we receive change notifications.
+  let receivedOnSummaryChanged = false;
+  await summary.addView({
+    onSummaryChanged() {
+      receivedOnSummaryChanged = true;
+    },
+  });
+  await download.start();
+  Assert.ok(receivedOnSummaryChanged);
+});
+
+/**
+ * Tests that if a new download is added, telemetry records the event and type of the file.
+ */
+add_task(async function test_downloadAddedTelemetry() {
+  Services.telemetry.clearEvents();
+  Services.telemetry.setEventRecordingEnabled("downloads", true);
+
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+
+  let download = await Downloads.createDownload({
+    source: httpUrl("empty.txt"),
+    target: targetFile.path,
+  });
+
+  let list = await Downloads.getList(Downloads.ALL);
+  await list.add(download);
+  download.start();
+  await promiseDownloadFinished(download);
+
+  TelemetryTestUtils.assertEvents([
+    {
+      category: "downloads",
+      method: "added",
+      object: "fileExtension",
+      value: "txt",
+    },
+  ]);
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadPaths.js b/toolkit/components/downloads/test/unit/test_DownloadPaths.js
new file mode 100644
index 0000000000..00fb070669
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadPaths.js
@@ -0,0 +1,189 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests for the "DownloadPaths.sys.mjs" JavaScript module.
+ */
+
+function testSanitize(leafName, expectedLeafName, options = {}) {
+  Assert.equal(DownloadPaths.sanitize(leafName, options), expectedLeafName);
+}
+
+function testSplitBaseNameAndExtension(aLeafName, [aBase, aExt]) {
+  var [base, ext] = DownloadPaths.splitBaseNameAndExtension(aLeafName);
+  Assert.equal(base, aBase);
+  Assert.equal(ext, aExt);
+
+  // If we modify the base name and concatenate it with the extension again,
+  // another roundtrip through the function should give a consistent result.
+  // The only exception is when we introduce an extension in a file name that
+  // didn't have one or that ended with one of the special cases like ".gz". If
+  // we avoid using a dot and we introduce at least another special character,
+  // the results are always consistent.
+  [base, ext] = DownloadPaths.splitBaseNameAndExtension("(" + base + ")" + ext);
+  Assert.equal(base, "(" + aBase + ")");
+  Assert.equal(ext, aExt);
+}
+
+function testCreateNiceUniqueFile(aTempFile, aExpectedLeafName) {
+  var createdFile = DownloadPaths.createNiceUniqueFile(aTempFile);
+  Assert.equal(createdFile.leafName, aExpectedLeafName);
+}
+
+add_task(async function test_sanitize() {
+  // Platform-dependent conversion of special characters to spaces.
+  const kSpecialChars = 'A:*?|""<<>>;,+=[]B][=+,;>><<""|?*:C';
+  if (AppConstants.platform == "android") {
+    testSanitize(kSpecialChars, "A B C");
+    testSanitize(" :: Website :: ", "Website");
+    testSanitize("* Website!", "Website!");
+    testSanitize("Website | Page!", "Website Page!");
+    testSanitize("Directory Listing: /a/b/", "Directory Listing _a_b_");
+  } else if (AppConstants.platform == "win") {
+    testSanitize(kSpecialChars, "A ;,+=[]B][=+,; C");
+    testSanitize(" :: Website :: ", "Website");
+    testSanitize("* Website!", "Website!");
+    testSanitize("Website | Page!", "Website Page!");
+    testSanitize("Directory Listing: /a/b/", "Directory Listing _a_b_");
+  } else if (AppConstants.platform == "macosx") {
+    testSanitize(kSpecialChars, "A ;,+=[]B][=+,; C");
+    testSanitize(" :: Website :: ", "Website");
+    testSanitize("* Website!", "Website!");
+    testSanitize("Website | Page!", "Website Page!");
+    testSanitize("Directory Listing: /a/b/", "Directory Listing _a_b_");
+  } else {
+    testSanitize(kSpecialChars, "A ;,+=[]B][=+,; C");
+    testSanitize(" :: Website :: ", "Website");
+    testSanitize("* Website!", "Website!");
+    testSanitize("Website | Page!", "Website Page!");
+    testSanitize("Directory Listing: /a/b/", "Directory Listing _a_b_");
+  }
+
+  // Conversion of consecutive runs of slashes and backslashes to underscores.
+  testSanitize("\\ \\\\Website\\/Page// /", "_ __Website__Page__ _");
+
+  // Removal of leading and trailing whitespace and dots after conversion.
+  testSanitize("  Website  ", "Website");
+  testSanitize(". . Website . Page . .", "Website . Page");
+  testSanitize(" File . txt ", "File . txt");
+  testSanitize("\f\n\r\t\v\x00\x1f\x7f\x80\x9f\xa0 . txt", "txt");
+  testSanitize("\u1680\u180e\u2000\u2008\u200a . txt", "txt");
+  testSanitize("\u2028\u2029\u202f\u205f\u3000\ufeff . txt", "txt");
+
+  // Strings with whitespace and dots only.
+  testSanitize(".", "");
+  testSanitize("..", "");
+  testSanitize(" ", "");
+  testSanitize(" . ", "");
+
+  // Stripping of BIDI formatting characters.
+  testSanitize("\u200e \u202b\u202c\u202d\u202etest\x7f\u200f", "_ ____test _");
+  testSanitize("AB\x7f\u202a\x7f\u202a\x7fCD", "AB _ _ CD");
+
+  // Stripping of colons:
+  testSanitize("foo:bar", "foo bar");
+
+  // not compressing whitespaces.
+  testSanitize("foo : bar", "foo   bar", { compressWhitespaces: false });
+
+  testSanitize("thing.lnk", "thing.lnk.download");
+  testSanitize("thing.lnk\n", "thing.lnk.download");
+  testSanitize("thing.lnk", "thing.lnk", {
+    allowInvalidFilenames: true,
+  });
+  testSanitize("thing.lnk\n", "thing.lnk", {
+    allowInvalidFilenames: true,
+  });
+  testSanitize("thing.URl", "thing.URl.download");
+  testSanitize("thing.URl  \n", "thing.URl", {
+    allowInvalidFilenames: true,
+  });
+
+  testSanitize("thing  and more   .URl", "thing and more .URl", {
+    allowInvalidFilenames: true,
+  });
+  testSanitize("thing  and more   .URl ", "thing  and more   .URl", {
+    compressWhitespaces: false,
+    allowInvalidFilenames: true,
+  });
+
+  testSanitize("thing.local|", "thing.local.download");
+  testSanitize("thing.lo|cal", "thing.lo cal");
+  testSanitize('thing.local/*"', "thing.local_");
+
+  testSanitize("thing.desktoP", "thing.desktoP.download");
+  testSanitize("thing.desktoP  \n", "thing.desktoP", {
+    allowInvalidFilenames: true,
+  });
+});
+
+add_task(async function test_splitBaseNameAndExtension() {
+  // Usual file names.
+  testSplitBaseNameAndExtension("base", ["base", ""]);
+  testSplitBaseNameAndExtension("base.ext", ["base", ".ext"]);
+  testSplitBaseNameAndExtension("base.application", ["base", ".application"]);
+  testSplitBaseNameAndExtension("base.x.Z", ["base", ".x.Z"]);
+  testSplitBaseNameAndExtension("base.ext.Z", ["base", ".ext.Z"]);
+  testSplitBaseNameAndExtension("base.ext.gz", ["base", ".ext.gz"]);
+  testSplitBaseNameAndExtension("base.ext.Bz2", ["base", ".ext.Bz2"]);
+  testSplitBaseNameAndExtension("base..ext", ["base.", ".ext"]);
+  testSplitBaseNameAndExtension("base..Z", ["base.", ".Z"]);
+  testSplitBaseNameAndExtension("base. .Z", ["base. ", ".Z"]);
+  testSplitBaseNameAndExtension("base.base.Bz2", ["base.base", ".Bz2"]);
+  testSplitBaseNameAndExtension("base  .ext", ["base  ", ".ext"]);
+
+  // Corner cases. A name ending with a dot technically has no extension, but
+  // we consider the ending dot separately from the base name so that modifying
+  // the latter never results in an extension being introduced accidentally.
+  // Names beginning with a dot are hidden files on Unix-like platforms and if
+  // their name doesn't contain another dot they should have no extension, but
+  // on Windows the whole name is considered as an extension.
+  testSplitBaseNameAndExtension("base.", ["base", "."]);
+  testSplitBaseNameAndExtension(".ext", ["", ".ext"]);
+
+  // Unusual file names (not recommended as input to the function).
+  testSplitBaseNameAndExtension("base. ", ["base", ". "]);
+  testSplitBaseNameAndExtension("base ", ["base ", ""]);
+  testSplitBaseNameAndExtension("", ["", ""]);
+  testSplitBaseNameAndExtension(" ", [" ", ""]);
+  testSplitBaseNameAndExtension(" . ", [" ", ". "]);
+  testSplitBaseNameAndExtension(" .. ", [" .", ". "]);
+  testSplitBaseNameAndExtension(" .ext", [" ", ".ext"]);
+  testSplitBaseNameAndExtension(" .ext. ", [" .ext", ". "]);
+  testSplitBaseNameAndExtension(" .ext.gz ", [" .ext", ".gz "]);
+});
+
+add_task(async function test_createNiceUniqueFile() {
+  var destDir = FileTestUtils.getTempFile("destdir");
+  destDir.create(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+
+  // Single extension.
+  var tempFile = destDir.clone();
+  tempFile.append("test.txt");
+  testCreateNiceUniqueFile(tempFile, "test.txt");
+  testCreateNiceUniqueFile(tempFile, "test(1).txt");
+  testCreateNiceUniqueFile(tempFile, "test(2).txt");
+
+  // Double extension.
+  tempFile.leafName = "test.tar.gz";
+  testCreateNiceUniqueFile(tempFile, "test.tar.gz");
+  testCreateNiceUniqueFile(tempFile, "test(1).tar.gz");
+  testCreateNiceUniqueFile(tempFile, "test(2).tar.gz");
+
+  // Test automatic shortening of long file names. We don't know exactly how
+  // many characters are removed, because it depends on the name of the folder
+  // where the file is located.
+  tempFile.leafName = new Array(256).join("T") + ".txt";
+  var newFile = DownloadPaths.createNiceUniqueFile(tempFile);
+  Assert.ok(newFile.leafName.length < tempFile.leafName.length);
+  Assert.equal(newFile.leafName.slice(-4), ".txt");
+
+  // Creating a valid file name from an invalid one is not always possible.
+  tempFile.append("file-under-long-directory.txt");
+  try {
+    DownloadPaths.createNiceUniqueFile(tempFile);
+    do_throw("Exception expected with a long parent directory name.");
+  } catch (e) {
+    // An exception is expected, but we don't know which one exactly.
+  }
+});
diff --git a/toolkit/components/downloads/test/unit/test_DownloadStore.js b/toolkit/components/downloads/test/unit/test_DownloadStore.js
new file mode 100644
index 0000000000..9353b63060
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_DownloadStore.js
@@ -0,0 +1,458 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests the DownloadStore object.
+ */
+
+"use strict";
+
+// Globals
+
+ChromeUtils.defineESModuleGetters(this, {
+  DownloadError: "resource://gre/modules/DownloadCore.sys.mjs",
+  DownloadStore: "resource://gre/modules/DownloadStore.sys.mjs",
+});
+
+/**
+ * Returns a new DownloadList object with an associated DownloadStore.
+ *
+ * @param aStorePath
+ *        String pointing to the file to be associated with the DownloadStore,
+ *        or undefined to use a non-existing temporary file.  In this case, the
+ *        temporary file is deleted when the test file execution finishes.
+ *
+ * @return {Promise}
+ * @resolves Array [ Newly created DownloadList , associated DownloadStore ].
+ * @rejects JavaScript exception.
+ */
+function promiseNewListAndStore(aStorePath) {
+  return promiseNewList().then(function (aList) {
+    let path = aStorePath || getTempFile(TEST_STORE_FILE_NAME).path;
+    let store = new DownloadStore(aList, path);
+    return [aList, store];
+  });
+}
+
+// Tests
+
+/**
+ * Saves downloads to a file, then reloads them.
+ */
+add_task(async function test_save_reload() {
+  let [listForSave, storeForSave] = await promiseNewListAndStore();
+  let [listForLoad, storeForLoad] = await promiseNewListAndStore(
+    storeForSave.path
+  );
+  let referrerInfo = new ReferrerInfo(
+    Ci.nsIReferrerInfo.EMPTY,
+    true,
+    NetUtil.newURI(TEST_REFERRER_URL)
+  );
+
+  listForSave.add(await promiseNewDownload(httpUrl("source.txt")));
+  listForSave.add(
+    await Downloads.createDownload({
+      source: { url: httpUrl("empty.txt"), referrerInfo },
+      target: getTempFile(TEST_TARGET_FILE_NAME),
+    })
+  );
+
+  // If we used a callback to adjust the channel, the download should
+  // not be serialized because we can't recreate it across sessions.
+  let adjustedDownload = await Downloads.createDownload({
+    source: {
+      url: httpUrl("empty.txt"),
+      adjustChannel: () => Promise.resolve(),
+    },
+    target: getTempFile(TEST_TARGET_FILE_NAME),
+  });
+  listForSave.add(adjustedDownload);
+
+  let legacyDownload = await promiseStartLegacyDownload();
+  await legacyDownload.cancel();
+  listForSave.add(legacyDownload);
+
+  await storeForSave.save();
+  await storeForLoad.load();
+
+  // Remove the adjusted download because it should not appear here.
+  listForSave.remove(adjustedDownload);
+
+  let itemsForSave = await listForSave.getAll();
+  let itemsForLoad = await listForLoad.getAll();
+
+  Assert.equal(itemsForSave.length, itemsForLoad.length);
+
+  // Downloads should be reloaded in the same order.
+  for (let i = 0; i < itemsForSave.length; i++) {
+    // The reloaded downloads are different objects.
+    Assert.notEqual(itemsForSave[i], itemsForLoad[i]);
+
+    // The reloaded downloads have the same properties.
+    Assert.equal(itemsForSave[i].source.url, itemsForLoad[i].source.url);
+    Assert.equal(
+      !!itemsForSave[i].source.referrerInfo,
+      !!itemsForLoad[i].source.referrerInfo
+    );
+    if (
+      itemsForSave[i].source.referrerInfo &&
+      itemsForLoad[i].source.referrerInfo
+    ) {
+      Assert.ok(
+        itemsForSave[i].source.referrerInfo.equals(
+          itemsForLoad[i].source.referrerInfo
+        )
+      );
+    }
+    Assert.equal(itemsForSave[i].target.path, itemsForLoad[i].target.path);
+    Assert.equal(
+      itemsForSave[i].saver.toSerializable(),
+      itemsForLoad[i].saver.toSerializable()
+    );
+  }
+});
+
+/**
+ * Checks that saving an empty list deletes any existing file.
+ */
+add_task(async function test_save_empty() {
+  let [, store] = await promiseNewListAndStore();
+
+  await IOUtils.write(store.path, new Uint8Array());
+
+  await store.save();
+
+  let successful;
+  try {
+    await IOUtils.read(store.path);
+    successful = true;
+  } catch (ex) {
+    successful = ex.name != "NotFoundError";
+  }
+
+  ok(!successful, "File should not exist");
+
+  // If the file does not exist, saving should not generate exceptions.
+  await store.save();
+});
+
+/**
+ * Checks that loading from a missing file results in an empty list.
+ */
+add_task(async function test_load_empty() {
+  let [list, store] = await promiseNewListAndStore();
+
+  let succeesful;
+  try {
+    await IOUtils.read(store.path);
+    succeesful = true;
+  } catch (ex) {
+    succeesful = ex.name != "NotFoundError";
+  }
+
+  ok(!succeesful, "File should not exist");
+
+  let items = await list.getAll();
+  Assert.equal(items.length, 0);
+});
+
+/**
+ * Loads downloads from a string in a predefined format.  The purpose of this
+ * test is to verify that the JSON format used in previous versions can be
+ * loaded, assuming the file is reloaded on the same platform.
+ */
+add_task(async function test_load_string_predefined() {
+  let [list, store] = await promiseNewListAndStore();
+
+  // The platform-dependent file name should be generated dynamically.
+  let targetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  let filePathLiteral = JSON.stringify(targetPath);
+  let sourceUriLiteral = JSON.stringify(httpUrl("source.txt"));
+  let emptyUriLiteral = JSON.stringify(httpUrl("empty.txt"));
+  let referrerInfo = new ReferrerInfo(
+    Ci.nsIReferrerInfo.EMPTY,
+    true,
+    NetUtil.newURI(TEST_REFERRER_URL)
+  );
+  let referrerInfoLiteral = JSON.stringify(
+    E10SUtils.serializeReferrerInfo(referrerInfo)
+  );
+
+  let string =
+    '{"list":[{"source":' +
+    sourceUriLiteral +
+    "," +
+    '"target":' +
+    filePathLiteral +
+    "}," +
+    '{"source":{"url":' +
+    emptyUriLiteral +
+    "," +
+    '"referrerInfo":' +
+    referrerInfoLiteral +
+    "}," +
+    '"target":' +
+    filePathLiteral +
+    "}]}";
+
+  await IOUtils.write(store.path, new TextEncoder().encode(string), {
+    tmpPath: store.path + ".tmp",
+  });
+
+  await store.load();
+
+  let items = await list.getAll();
+
+  Assert.equal(items.length, 2);
+
+  Assert.equal(items[0].source.url, httpUrl("source.txt"));
+  Assert.equal(items[0].target.path, targetPath);
+
+  Assert.equal(items[1].source.url, httpUrl("empty.txt"));
+
+  checkEqualReferrerInfos(items[1].source.referrerInfo, referrerInfo);
+  Assert.equal(items[1].target.path, targetPath);
+});
+
+/**
+ * Loads downloads from a well-formed JSON string containing unrecognized data.
+ */
+add_task(async function test_load_string_unrecognized() {
+  let [list, store] = await promiseNewListAndStore();
+
+  // The platform-dependent file name should be generated dynamically.
+  let targetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  let filePathLiteral = JSON.stringify(targetPath);
+  let sourceUriLiteral = JSON.stringify(httpUrl("source.txt"));
+
+  let string =
+    '{"list":[{"source":null,' +
+    '"target":null},' +
+    '{"source":{"url":' +
+    sourceUriLiteral +
+    "}," +
+    '"target":{"path":' +
+    filePathLiteral +
+    "}," +
+    '"saver":{"type":"copy"}}]}';
+
+  await IOUtils.write(store.path, new TextEncoder().encode(string), {
+    tmpPath: store.path + ".tmp",
+  });
+
+  await store.load();
+
+  let items = await list.getAll();
+
+  Assert.equal(items.length, 1);
+
+  Assert.equal(items[0].source.url, httpUrl("source.txt"));
+  Assert.equal(items[0].target.path, targetPath);
+});
+
+/**
+ * Loads downloads from a malformed JSON string.
+ */
+add_task(async function test_load_string_malformed() {
+  let [list, store] = await promiseNewListAndStore();
+
+  let string =
+    '{"list":[{"source":null,"target":null},' +
+    '{"source":{"url":"about:blank"}}}';
+
+  await IOUtils.write(store.path, new TextEncoder().encode(string), {
+    tmpPath: store.path + ".tmp",
+  });
+
+  try {
+    await store.load();
+    do_throw("Exception expected when JSON data is malformed.");
+  } catch (ex) {
+    if (ex.name != "SyntaxError") {
+      throw ex;
+    }
+    info("The expected SyntaxError exception was thrown.");
+  }
+
+  let items = await list.getAll();
+
+  Assert.equal(items.length, 0);
+});
+
+/**
+ * Saves downloads with unknown properties to a file and then reloads
+ * them to ensure that these properties are preserved.
+ */
+add_task(async function test_save_reload_unknownProperties() {
+  let [listForSave, storeForSave] = await promiseNewListAndStore();
+  let [listForLoad, storeForLoad] = await promiseNewListAndStore(
+    storeForSave.path
+  );
+
+  let download1 = await promiseNewDownload(httpUrl("source.txt"));
+  // startTime should be ignored as it is a known property, and error
+  // is ignored by serialization
+  download1._unknownProperties = {
+    peanut: "butter",
+    orange: "marmalade",
+    startTime: 77,
+    error: { message: "Passed" },
+  };
+  listForSave.add(download1);
+
+  let download2 = await promiseStartLegacyDownload();
+  await download2.cancel();
+  download2._unknownProperties = { number: 5, object: { test: "string" } };
+  listForSave.add(download2);
+
+  let referrerInfo = new ReferrerInfo(
+    Ci.nsIReferrerInfo.EMPTY,
+    true,
+    NetUtil.newURI(TEST_REFERRER_URL)
+  );
+  let download3 = await Downloads.createDownload({
+    source: {
+      url: httpUrl("empty.txt"),
+      referrerInfo,
+      source1: "download3source1",
+      source2: "download3source2",
+    },
+    target: {
+      path: getTempFile(TEST_TARGET_FILE_NAME).path,
+      target1: "download3target1",
+      target2: "download3target2",
+    },
+    saver: {
+      type: "copy",
+      saver1: "download3saver1",
+      saver2: "download3saver2",
+    },
+  });
+  listForSave.add(download3);
+
+  await storeForSave.save();
+  await storeForLoad.load();
+
+  let itemsForSave = await listForSave.getAll();
+  let itemsForLoad = await listForLoad.getAll();
+
+  Assert.equal(itemsForSave.length, itemsForLoad.length);
+
+  Assert.equal(Object.keys(itemsForLoad[0]._unknownProperties).length, 2);
+  Assert.equal(itemsForLoad[0]._unknownProperties.peanut, "butter");
+  Assert.equal(itemsForLoad[0]._unknownProperties.orange, "marmalade");
+  Assert.equal(false, "startTime" in itemsForLoad[0]._unknownProperties);
+  Assert.equal(false, "error" in itemsForLoad[0]._unknownProperties);
+
+  Assert.equal(Object.keys(itemsForLoad[1]._unknownProperties).length, 2);
+  Assert.equal(itemsForLoad[1]._unknownProperties.number, 5);
+  Assert.equal(itemsForLoad[1]._unknownProperties.object.test, "string");
+
+  Assert.equal(
+    Object.keys(itemsForLoad[2].source._unknownProperties).length,
+    2
+  );
+  Assert.equal(
+    itemsForLoad[2].source._unknownProperties.source1,
+    "download3source1"
+  );
+  Assert.equal(
+    itemsForLoad[2].source._unknownProperties.source2,
+    "download3source2"
+  );
+
+  Assert.equal(
+    Object.keys(itemsForLoad[2].target._unknownProperties).length,
+    2
+  );
+  Assert.equal(
+    itemsForLoad[2].target._unknownProperties.target1,
+    "download3target1"
+  );
+  Assert.equal(
+    itemsForLoad[2].target._unknownProperties.target2,
+    "download3target2"
+  );
+
+  Assert.equal(Object.keys(itemsForLoad[2].saver._unknownProperties).length, 2);
+  Assert.equal(
+    itemsForLoad[2].saver._unknownProperties.saver1,
+    "download3saver1"
+  );
+  Assert.equal(
+    itemsForLoad[2].saver._unknownProperties.saver2,
+    "download3saver2"
+  );
+});
+
+/**
+ * Saves insecure downloads to a file, then reloads the file and checks if they
+ * are still there.
+ */
+add_task(async function test_insecure_download_deletion() {
+  let [listForSave, storeForSave] = await promiseNewListAndStore();
+  let [listForLoad, storeForLoad] = await promiseNewListAndStore(
+    storeForSave.path
+  );
+  let referrerInfo = new ReferrerInfo(
+    Ci.nsIReferrerInfo.EMPTY,
+    true,
+    NetUtil.newURI(TEST_REFERRER_URL)
+  );
+
+  const createTestDownload = async startTime => {
+    // Create a valid test download and start it so it creates a file
+    let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+    let download = await Downloads.createDownload({
+      source: { url: httpUrl("empty.txt"), referrerInfo },
+      target: targetFile.path,
+      startTime: new Date().toISOString(),
+      contentType: "application/zip",
+    });
+    await download.start();
+
+    // Add an "Insecure Download" error and overwrite the start time for
+    // serialization
+    download.hasBlockedData = true;
+    download.error = DownloadError.fromSerializable({
+      becauseBlockedByReputationCheck: true,
+      reputationCheckVerdict: "Insecure",
+    });
+    download.startTime = startTime;
+
+    let targetPath = download.target.path;
+
+    // Add download to store, save, load and retrieve deserialized download list
+    listForSave.add(download);
+    await storeForSave.save();
+    await storeForLoad.load();
+    let loadedDownloadList = await listForLoad.getAll();
+
+    return [loadedDownloadList, targetPath];
+  };
+
+  // Insecure downloads that are older than 5 minutes should get removed from
+  // the download-store and the file should get deleted. (360000 = 6 minutes)
+  let [loadedDownloadList1, targetPath1] = await createTestDownload(
+    new Date(Date.now() - 360000)
+  );
+
+  Assert.equal(loadedDownloadList1.length, 0, "Download should be removed");
+  Assert.ok(
+    !(await IOUtils.exists(targetPath1)),
+    "The file should have been deleted."
+  );
+
+  // Insecure downloads that are newer than 5 minutes should stay in the
+  // download store and the file should remain.
+  let [loadedDownloadList2, targetPath2] = await createTestDownload(new Date());
+
+  Assert.equal(loadedDownloadList2.length, 1, "Download should be kept");
+  Assert.ok(
+    await IOUtils.exists(targetPath2),
+    "The file should have not been deleted."
+  );
+});
diff --git a/toolkit/components/downloads/test/unit/test_Download_noext_win.js b/toolkit/components/downloads/test/unit/test_Download_noext_win.js
new file mode 100644
index 0000000000..0e226229f6
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_Download_noext_win.js
@@ -0,0 +1,59 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function () {
+  info("Get a file without extension");
+  let noExtFile = getTempFile("test_bug_1661365");
+  Assert.ok(!noExtFile.leafName.includes("."), "Sanity check the filename");
+  info("Create an exe file with the same name");
+  await IOUtils.remove(noExtFile.path + ".exe", { ignoreAbsent: true });
+  let exeFile = new FileUtils.File(noExtFile.path + ".exe");
+  exeFile.create(Ci.nsIFile.NORMAL_FILE_TYPE, FileUtils.PERMS_FILE);
+  Assert.ok(await fileExists(exeFile.path), "Sanity check the exe exists.");
+  Assert.equal(
+    exeFile.leafName,
+    noExtFile.leafName + ".exe",
+    "Sanity check the file names."
+  );
+  registerCleanupFunction(async function () {
+    await IOUtils.remove(noExtFile.path, { ignoreAbsent: true });
+    await IOUtils.remove(exeFile.path, { ignoreAbsent: true });
+  });
+
+  info("Download to the no-extension file");
+  let download = await Downloads.createDownload({
+    source: httpUrl("source.txt"),
+    target: noExtFile,
+  });
+  await download.start();
+
+  Assert.ok(
+    await fileExists(download.target.path),
+    "The file should have been created."
+  );
+  Assert.ok(await fileExists(exeFile.path), "Sanity check the exe exists.");
+
+  info("Launch should open the containing folder");
+  let promiseShowInFolder = waitForDirectoryShown();
+  download.launch();
+  Assert.equal(await promiseShowInFolder, noExtFile.path);
+});
+
+/**
+ * Waits for an attempt to show the directory where a file is located, and
+ * returns the path of the file.
+ */
+function waitForDirectoryShown() {
+  return new Promise(resolve => {
+    let waitFn = base => ({
+      showContainingDirectory(path) {
+        Integration.downloads.unregister(waitFn);
+        resolve(path);
+        return Promise.resolve();
+      },
+    });
+    Integration.downloads.register(waitFn);
+  });
+}
diff --git a/toolkit/components/downloads/test/unit/test_Downloads.js b/toolkit/components/downloads/test/unit/test_Downloads.js
new file mode 100644
index 0000000000..b99f823008
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/test_Downloads.js
@@ -0,0 +1,153 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Tests the functions located directly in the "Downloads" object.
+ */
+
+"use strict";
+
+// Tests
+
+/**
+ * Tests that the createDownload function exists and can be called.  More
+ * detailed tests are implemented separately for the DownloadCore module.
+ */
+add_task(async function test_createDownload() {
+  // Creates a simple Download object without starting the download.
+  await Downloads.createDownload({
+    source: { url: "about:blank" },
+    target: { path: getTempFile(TEST_TARGET_FILE_NAME).path },
+    saver: { type: "copy" },
+  });
+});
+
+/**
+ * Tests createDownload for private download.
+ */
+add_task(async function test_createDownload_private() {
+  let download = await Downloads.createDownload({
+    source: { url: "about:blank", isPrivate: true },
+    target: { path: getTempFile(TEST_TARGET_FILE_NAME).path },
+    saver: { type: "copy" },
+  });
+  Assert.ok(download.source.isPrivate);
+});
+
+/**
+ * Tests createDownload for normal (public) download.
+ */
+add_task(async function test_createDownload_public() {
+  let tempPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  let download = await Downloads.createDownload({
+    source: { url: "about:blank", isPrivate: false },
+    target: { path: tempPath },
+    saver: { type: "copy" },
+  });
+  Assert.ok(!download.source.isPrivate);
+
+  download = await Downloads.createDownload({
+    source: { url: "about:blank" },
+    target: { path: tempPath },
+    saver: { type: "copy" },
+  });
+  Assert.ok(!download.source.isPrivate);
+});
+
+/**
+ * Tests "fetch" with nsIURI and nsIFile as arguments.
+ */
+add_task(async function test_fetch_uri_file_arguments() {
+  let targetFile = getTempFile(TEST_TARGET_FILE_NAME);
+  await Downloads.fetch(NetUtil.newURI(httpUrl("source.txt")), targetFile);
+  await promiseVerifyContents(targetFile.path, TEST_DATA_SHORT);
+});
+
+/**
+ * Tests "fetch" with DownloadSource and DownloadTarget as arguments.
+ */
+add_task(async function test_fetch_object_arguments() {
+  let targetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  await Downloads.fetch({ url: httpUrl("source.txt") }, { path: targetPath });
+  await promiseVerifyContents(targetPath, TEST_DATA_SHORT);
+});
+
+/**
+ * Tests "fetch" with string arguments.
+ */
+add_task(async function test_fetch_string_arguments() {
+  let targetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  await Downloads.fetch(httpUrl("source.txt"), targetPath);
+  await promiseVerifyContents(targetPath, TEST_DATA_SHORT);
+
+  targetPath = getTempFile(TEST_TARGET_FILE_NAME).path;
+  await Downloads.fetch(httpUrl("source.txt"), targetPath);
+  await promiseVerifyContents(targetPath, TEST_DATA_SHORT);
+});
+
+/**
+ * Tests that the getList function returns the same list when called multiple
+ * times with the same argument, but returns different lists when called with
+ * different arguments.  More detailed tests are implemented separately for the
+ * DownloadList module.
+ */
+add_task(async function test_getList() {
+  let publicListOne = await Downloads.getList(Downloads.PUBLIC);
+  let privateListOne = await Downloads.getList(Downloads.PRIVATE);
+
+  let publicListTwo = await Downloads.getList(Downloads.PUBLIC);
+  let privateListTwo = await Downloads.getList(Downloads.PRIVATE);
+
+  Assert.equal(publicListOne, publicListTwo);
+  Assert.equal(privateListOne, privateListTwo);
+
+  Assert.notEqual(publicListOne, privateListOne);
+});
+
+/**
+ * Tests that the getSummary function returns the same summary when called
+ * multiple times with the same argument, but returns different summaries when
+ * called with different arguments.  More detailed tests are implemented
+ * separately for the DownloadSummary object in the DownloadList module.
+ */
+add_task(async function test_getSummary() {
+  let publicSummaryOne = await Downloads.getSummary(Downloads.PUBLIC);
+  let privateSummaryOne = await Downloads.getSummary(Downloads.PRIVATE);
+
+  let publicSummaryTwo = await Downloads.getSummary(Downloads.PUBLIC);
+  let privateSummaryTwo = await Downloads.getSummary(Downloads.PRIVATE);
+
+  Assert.equal(publicSummaryOne, publicSummaryTwo);
+  Assert.equal(privateSummaryOne, privateSummaryTwo);
+
+  Assert.notEqual(publicSummaryOne, privateSummaryOne);
+});
+
+/**
+ * Tests that the getSystemDownloadsDirectory returns a non-empty download
+ * directory string.
+ */
+add_task(async function test_getSystemDownloadsDirectory() {
+  let downloadDir = await Downloads.getSystemDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+});
+
+/**
+ * Tests that the getPreferredDownloadsDirectory returns a non-empty download
+ * directory string.
+ */
+add_task(async function test_getPreferredDownloadsDirectory() {
+  let downloadDir = await Downloads.getPreferredDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+});
+
+/**
+ * Tests that the getTemporaryDownloadsDirectory returns a non-empty download
+ * directory string.
+ */
+add_task(async function test_getTemporaryDownloadsDirectory() {
+  let downloadDir = await Downloads.getTemporaryDownloadsDirectory();
+  Assert.notEqual(downloadDir, "");
+});
diff --git a/toolkit/components/downloads/test/unit/xpcshell.toml b/toolkit/components/downloads/test/unit/xpcshell.toml
new file mode 100644
index 0000000000..073a4a8eeb
--- /dev/null
+++ b/toolkit/components/downloads/test/unit/xpcshell.toml
@@ -0,0 +1,33 @@
+[DEFAULT]
+head = "head.js"
+skip-if = ["os == 'android'"]
+
+# Note: The "tail.js" file is not defined in the "tail" key because it calls
+#       the "add_test_task" function, that does not work properly in tail files.
+support-files = ["common_test_Download.js"]
+
+["test_DownloadBlockedTelemetry.js"]
+
+["test_DownloadCore.js"]
+
+["test_DownloadHistory.js"]
+
+["test_DownloadHistory_initialization.js"]
+
+["test_DownloadHistory_initialization2.js"]
+
+["test_DownloadIntegration.js"]
+
+["test_DownloadLegacy.js"]
+run-sequentially = "very high failure rate in parallel"
+
+["test_DownloadList.js"]
+
+["test_DownloadPaths.js"]
+
+["test_DownloadStore.js"]
+
+["test_Download_noext_win.js"]
+run-if = ["os == 'win'"] # Windows-specific test.
+
+["test_Downloads.js"]