Adding upstream version 115.7.0esr.upstream/115.7.0esr

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

34 files changed, 8083 insertions, 0 deletions

diff --git a/toolkit/components/bitsdownload/Bits.cpp b/toolkit/components/bitsdownload/Bits.cpp
new file mode 100644
index 0000000000..da16e40497
--- /dev/null
+++ b/toolkit/components/bitsdownload/Bits.cpp
@@ -0,0 +1,34 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/StaticPtr.h"
+#include "nsCOMPtr.h"
+#include "Bits.h"
+
+// This anonymous namespace prevents outside C++ code from improperly accessing
+// these implementation details.
+namespace {
+extern "C" {
+// Implemented in Rust.
+void new_bits_service(nsIBits** result);
+}
+
+static mozilla::StaticRefPtr<nsIBits> sBitsService;
+}  // namespace
+
+already_AddRefed<nsIBits> GetBitsService() {
+  nsCOMPtr<nsIBits> bitsService;
+
+  if (sBitsService) {
+    bitsService = sBitsService;
+  } else {
+    new_bits_service(getter_AddRefs(bitsService));
+    sBitsService = bitsService;
+    mozilla::ClearOnShutdown(&sBitsService);
+  }
+
+  return bitsService.forget();
+}
diff --git a/toolkit/components/bitsdownload/Bits.h b/toolkit/components/bitsdownload/Bits.h
new file mode 100644
index 0000000000..1c1e7ea996
--- /dev/null
+++ b/toolkit/components/bitsdownload/Bits.h
@@ -0,0 +1,9 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIBits.h"
+
+already_AddRefed<nsIBits> GetBitsService();
diff --git a/toolkit/components/bitsdownload/Bits.sys.mjs b/toolkit/components/bitsdownload/Bits.sys.mjs
new file mode 100644
index 0000000000..3248d2effa
--- /dev/null
+++ b/toolkit/components/bitsdownload/Bits.sys.mjs
@@ -0,0 +1,826 @@
+/* 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 module is used to interact with the Windows BITS component (Background
+ * Intelligent Transfer Service). This functionality cannot be used unless on
+ * Windows.
+ *
+ * The reason for this file's existence is that the interfaces in nsIBits.idl
+ * are asynchronous, but are unable to use Promises because they are implemented
+ * in Rust, which does not yet support Promises. This file functions as a layer
+ * between the Rust and the JS that provides access to the functionality
+ * provided by nsIBits via Promises rather than callbacks.
+ */
+
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+// This conditional prevents errors if this file is imported from operating
+// systems other than Windows. This is purely for convenient importing, because
+// attempting to use anything in this file on platforms other than Windows will
+// result in an error.
+if (AppConstants.MOZ_BITS_DOWNLOAD) {
+  XPCOMUtils.defineLazyServiceGetter(
+    lazy,
+    "gBits",
+    "@mozilla.org/bits;1",
+    "nsIBits"
+  );
+}
+
+// This value exists to mitigate a very unlikely problem: If a BITS method
+// catastrophically fails, it may never call its callback. This would result in
+// methods in this file returning promises that never resolve. This could, in
+// turn, lead to download code hanging altogether rather than being able to
+// report errors and utilize fallback mechanisms.
+// This problem is mitigated by giving these promises a timeout, the length of
+// which will be determined by this value.
+const kBitsMethodTimeoutMs = 10 * 60 * 1000; // 10 minutes
+
+/**
+ * This class will wrap the errors returned by the nsIBits interface to make
+ * them more uniform and more easily consumable.
+ *
+ * The values of stored by this error type are entirely numeric. This should
+ * make them easier to consume with JS and telemetry, but does make them fairly
+ * unreadable. nsIBits.idl will need to be referenced to look up what errors
+ * the values correspond to.
+ *
+ * The type of BitsError.code is dependent on the value of BitsError.codeType.
+ * It may be null, a number (corresponding to an nsresult or hresult value),
+ * a string, or an exception.
+ */
+export class BitsError extends Error {
+  // If codeType == "none", code may be unspecified.
+  constructor(type, action, stage, codeType, code) {
+    let message =
+      `${BitsError.name} {type: ${type}, action: ${action}, ` +
+      `stage: ${stage}`;
+    switch (codeType) {
+      case lazy.gBits.ERROR_CODE_TYPE_NONE:
+        code = null;
+        message += ", codeType: none}";
+        break;
+      case lazy.gBits.ERROR_CODE_TYPE_NSRESULT:
+        message += `, codeType: nsresult, code: ${code}}`;
+        break;
+      case lazy.gBits.ERROR_CODE_TYPE_HRESULT:
+        message += `, codeType: hresult, code: ${code}}`;
+        break;
+      case lazy.gBits.ERROR_CODE_TYPE_STRING:
+        message += `, codeType: string, code: ${JSON.stringify(code)}}`;
+        break;
+      case lazy.gBits.ERROR_CODE_TYPE_EXCEPTION:
+        message += `, codeType: exception, code: ${code}}`;
+        break;
+      default:
+        message += ", codeType: invalid}";
+        break;
+    }
+    super(message);
+
+    this.type = type;
+    this.action = action;
+    this.stage = stage;
+    this.codeType = codeType;
+    this.code = code;
+    this.name = this.constructor.name;
+    this.succeeded = false;
+  }
+}
+
+// These specializations exist to make them easier to construct since they may
+// need to be constructed outside of this file.
+export class BitsVerificationError extends BitsError {
+  constructor() {
+    super(
+      Ci.nsIBits.ERROR_TYPE_VERIFICATION_FAILURE,
+      Ci.nsIBits.ERROR_ACTION_NONE,
+      Ci.nsIBits.ERROR_STAGE_VERIFICATION,
+      Ci.nsIBits.ERROR_CODE_TYPE_NONE
+    );
+  }
+}
+
+export class BitsUnknownError extends BitsError {
+  constructor() {
+    super(
+      Ci.nsIBits.ERROR_TYPE_UNKNOWN,
+      Ci.nsIBits.ERROR_ACTION_UNKNOWN,
+      Ci.nsIBits.ERROR_STAGE_UNKNOWN,
+      Ci.nsIBits.ERROR_CODE_TYPE_NONE
+    );
+  }
+}
+
+/**
+ * Returns a timer object. If the timer expires, reject will be called with
+ * a BitsError error. The timer's cancel method should be called if the promise
+ * resolves or rejects without the timeout expiring.
+ */
+function makeTimeout(reject, errorAction) {
+  let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+  timer.initWithCallback(
+    () => {
+      let error = new BitsError(
+        lazy.gBits.ERROR_TYPE_METHOD_TIMEOUT,
+        errorAction,
+        lazy.gBits.ERROR_STAGE_UNKNOWN,
+        lazy.gBits.ERROR_CODE_TYPE_NONE
+      );
+      reject(error);
+    },
+    kBitsMethodTimeoutMs,
+    Ci.nsITimer.TYPE_ONE_SHOT
+  );
+  return timer;
+}
+
+/**
+ * This function does all of the wrapping and error handling for an async
+ * BitsRequest method. This allows the implementations for those methods to
+ * simply call this function with a closure that executes appropriate
+ * nsIBitsRequest method.
+ *
+ * Specifically, this function takes an nsBitsErrorAction and a function.
+ * The nsBitsErrorAction will be used when constructing a BitsError, if the
+ * wrapper encounters an error.
+ * The function will be passed the callback function that should be passed to
+ * the nsIBitsRequest method.
+ */
+async function requestPromise(errorAction, actionFn) {
+  return new Promise((resolve, reject) => {
+    let timer = makeTimeout(reject, errorAction);
+
+    let callback = {
+      QueryInterface: ChromeUtils.generateQI(["nsIBitsCallback"]),
+      success() {
+        timer.cancel();
+        resolve();
+      },
+      failure(type, action, stage) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_NONE
+        );
+        reject(error);
+      },
+      failureNsresult(type, action, stage, code) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_NSRESULT,
+          code
+        );
+        reject(error);
+      },
+      failureHresult(type, action, stage, code) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_HRESULT,
+          code
+        );
+        reject(error);
+      },
+      failureString(type, action, stage, message) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_STRING,
+          message
+        );
+        reject(error);
+      },
+    };
+
+    try {
+      actionFn(callback);
+    } catch (e) {
+      let error = new BitsError(
+        lazy.gBits.ERROR_TYPE_METHOD_THREW,
+        errorAction,
+        lazy.gBits.ERROR_STAGE_PRETASK,
+        lazy.gBits.ERROR_CODE_TYPE_EXCEPTION,
+        e
+      );
+      reject(error);
+    }
+  });
+}
+
+/**
+ * This class is a wrapper around nsIBitsRequest that converts functions taking
+ * callbacks to asynchronous functions. This class implements nsIRequest.
+ *
+ * Note that once the request has been shutdown, calling methods on it will
+ * cause an exception to be thrown. The request will be shutdown automatically
+ * when the BITS job is successfully completed or cancelled. If the request is
+ * no longer needed, but the transfer is still in progress, the shutdown method
+ * should be called manually to prevent memory leaks.
+ * Getter methods (except loadGroup and loadFlags) should continue to be
+ * accessible, even after shutdown.
+ */
+export class BitsRequest {
+  constructor(request) {
+    this._request = request;
+    this._request.QueryInterface(Ci.nsIBitsRequest);
+  }
+
+  /**
+   * This function releases the Rust request that backs this wrapper. Calling
+   * any method on this request after calling release will result in a BitsError
+   * being thrown.
+   *
+   * This step is important, because otherwise a cycle exists that the cycle
+   * collector doesn't know about. To break this cycle, either the Rust request
+   * needs to let go of the observer, or the JS request wrapper needs to let go
+   * of the Rust request (which is what we do here).
+   *
+   * Once there is support for cycle collection of cycles that extend through
+   * Rust, this function may no longer be necessary.
+   */
+  shutdown() {
+    if (this.hasShutdown) {
+      return;
+    }
+    // Cache some values before we shut down so they are still available
+    this._name = this._request.name;
+    this._status = this._request.status;
+    this._bitsId = this._request.bitsId;
+    this._transferError = this._request.transferError;
+
+    this._request = null;
+  }
+
+  /**
+   * Allows consumers to determine if this request has been shutdown.
+   */
+  get hasShutdown() {
+    return !this._request;
+  }
+
+  /**
+   * This is the nsIRequest implementation. Since this._request is an
+   * nsIRequest, these functions just call the corresponding method on it.
+   *
+   * Note that nsIBitsRequest does not yet properly implement load groups or
+   * load flags. This class will still forward those calls, but they will have
+   * not succeed.
+   */
+  get name() {
+    if (!this._request) {
+      return this._name;
+    }
+    return this._request.name;
+  }
+  isPending() {
+    if (!this._request) {
+      return false;
+    }
+    return this._request.isPending();
+  }
+  get status() {
+    if (!this._request) {
+      return this._status;
+    }
+    return this._request.status;
+  }
+  cancel(status) {
+    return this.cancelAsync(status);
+  }
+  suspend() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_SUSPEND,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    return this._request.suspend();
+  }
+  resume() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_RESUME,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    return this._request.resume();
+  }
+  get loadGroup() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_NONE,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    return this._request.loadGroup;
+  }
+  set loadGroup(group) {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_NONE,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    this._request.loadGroup = group;
+  }
+  get loadFlags() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_NONE,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    return this._request.loadFlags;
+  }
+  set loadFlags(flags) {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_NONE,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    this._request.loadFlags = flags;
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::bitsId.
+   */
+  get bitsId() {
+    if (!this._request) {
+      return this._bitsId;
+    }
+    return this._request.bitsId;
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::transferError.
+   *
+   * Instead of simply returning the nsBitsErrorType value, however, it returns
+   * a BitsError object, or null.
+   */
+  get transferError() {
+    let result;
+    if (this._request) {
+      result = this._request.transferError;
+    } else {
+      result = this._transferError;
+    }
+    if (result == Ci.nsIBits.ERROR_TYPE_SUCCESS) {
+      return null;
+    }
+    return new BitsError(
+      result,
+      Ci.nsIBits.ERROR_ACTION_NONE,
+      Ci.nsIBits.ERROR_STAGE_MONITOR,
+      Ci.nsIBits.ERROR_CODE_TYPE_NONE
+    );
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::changeMonitorInterval.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   */
+  async changeMonitorInterval(monitorIntervalMs) {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_CHANGE_MONITOR_INTERVAL,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    let action = lazy.gBits.ERROR_ACTION_CHANGE_MONITOR_INTERVAL;
+    return requestPromise(action, callback => {
+      this._request.changeMonitorInterval(monitorIntervalMs, callback);
+    });
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::cancelAsync.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   *
+   * Adds a default status of NS_ERROR_ABORT if one is not provided.
+   */
+  async cancelAsync(status) {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_CANCEL,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    if (status === undefined) {
+      status = Cr.NS_ERROR_ABORT;
+    }
+    let action = lazy.gBits.ERROR_ACTION_CANCEL;
+    return requestPromise(action, callback => {
+      this._request.cancelAsync(status, callback);
+    }).then(() => this.shutdown());
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::setPriorityHigh.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   */
+  async setPriorityHigh() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_SET_PRIORITY,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    let action = lazy.gBits.ERROR_ACTION_SET_PRIORITY;
+    return requestPromise(action, callback => {
+      this._request.setPriorityHigh(callback);
+    });
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::setPriorityLow.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   */
+  async setPriorityLow() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_SET_PRIORITY,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    let action = lazy.gBits.ERROR_ACTION_SET_PRIORITY;
+    return requestPromise(action, callback => {
+      this._request.setPriorityLow(callback);
+    });
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::setNoProgressTimeout.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   */
+  async setNoProgressTimeout(timeoutSecs) {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_SET_NO_PROGRESS_TIMEOUT,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    let action = lazy.gBits.ERROR_ACTION_SET_NO_PROGRESS_TIMEOUT;
+    return requestPromise(action, callback => {
+      this._request.setNoProgressTimeout(timeoutSecs, callback);
+    });
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::complete.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   */
+  async complete() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_COMPLETE,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    let action = lazy.gBits.ERROR_ACTION_COMPLETE;
+    return requestPromise(action, callback => {
+      this._request.complete(callback);
+    }).then(() => this.shutdown());
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::suspendAsync.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   */
+  async suspendAsync() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_SUSPEND,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    let action = lazy.gBits.ERROR_ACTION_SUSPEND;
+    return requestPromise(action, callback => {
+      this._request.suspendAsync(callback);
+    });
+  }
+
+  /**
+   * This function wraps nsIBitsRequest::resumeAsync.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with no data, or rejects with a BitsError.
+   */
+  async resumeAsync() {
+    if (!this._request) {
+      throw new BitsError(
+        Ci.nsIBits.ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN,
+        Ci.nsIBits.ERROR_ACTION_RESUME,
+        Ci.nsIBits.ERROR_STAGE_PRETASK,
+        Ci.nsIBits.ERROR_CODE_TYPE_NONE
+      );
+    }
+    let action = lazy.gBits.ERROR_ACTION_RESUME;
+    return requestPromise(action, callback => {
+      this._request.resumeAsync(callback);
+    });
+  }
+}
+
+BitsRequest.prototype.QueryInterface = ChromeUtils.generateQI(["nsIRequest"]);
+
+/**
+ * This function does all of the wrapping and error handling for an async
+ * Bits Service method. This allows the implementations for those methods to
+ * simply call this function with a closure that executes appropriate
+ * nsIBits method.
+ *
+ * Specifically, this function takes an nsBitsErrorAction, an observer and a
+ * function.
+ * The nsBitsErrorAction will be used when constructing a BitsError, if the
+ * wrapper encounters an error.
+ * The observer should be the one that the caller passed to the Bits Interface
+ * method. It will be wrapped so that its methods are passed a BitsRequest
+ * rather than an nsIBitsRequest.
+ * The function will be passed the callback function and the wrapped observer,
+ * both of which should be passed to the nsIBitsRequest method.
+ */
+async function servicePromise(errorAction, observer, actionFn) {
+  return new Promise((resolve, reject) => {
+    if (!observer) {
+      let error = new BitsError(
+        lazy.gBits.ERROR_TYPE_NULL_ARGUMENT,
+        errorAction,
+        lazy.gBits.ERROR_STAGE_PRETASK,
+        lazy.gBits.ERROR_CODE_TYPE_NONE
+      );
+      reject(error);
+      return;
+    }
+    try {
+      observer.QueryInterface(Ci.nsIRequestObserver);
+    } catch (e) {
+      let error = new BitsError(
+        lazy.gBits.ERROR_TYPE_INVALID_ARGUMENT,
+        errorAction,
+        lazy.gBits.ERROR_STAGE_PRETASK,
+        lazy.gBits.ERROR_CODE_TYPE_EXCEPTION,
+        e
+      );
+      reject(error);
+      return;
+    }
+    let isProgressEventSink = false;
+    try {
+      observer.QueryInterface(Ci.nsIProgressEventSink);
+      isProgressEventSink = true;
+    } catch (e) {}
+
+    // Check if we are not late in creating new requests.
+    if (
+      Services &&
+      Services.startup &&
+      Services.startup.isInOrBeyondShutdownPhase(
+        Ci.nsIAppStartup.SHUTDOWN_PHASE_APPSHUTDOWNCONFIRMED
+      )
+    ) {
+      let error = new BitsError(
+        lazy.gBits.ERROR_TYPE_BROWSER_SHUTTING_DOWN,
+        errorAction,
+        lazy.gBits.ERROR_STAGE_PRETASK,
+        lazy.gBits.ERROR_CODE_TYPE_NONE
+      );
+      reject(error);
+      return;
+    }
+
+    // This will be set to the BitsRequest (wrapping the nsIBitsRequest), once
+    // it is available. This prevents a new wrapper from having to be made every
+    // time an observer function is called.
+    let wrappedRequest;
+
+    let wrappedObserver = {
+      onStartRequest: function wrappedObserver_onStartRequest(request) {
+        if (!wrappedRequest) {
+          wrappedRequest = new BitsRequest(request);
+        }
+        observer.onStartRequest(wrappedRequest);
+      },
+      onStopRequest: function wrappedObserver_onStopRequest(request, status) {
+        if (!wrappedRequest) {
+          wrappedRequest = new BitsRequest(request);
+        }
+        observer.onStopRequest(wrappedRequest, status);
+      },
+      onProgress: function wrappedObserver_onProgress(
+        request,
+        progress,
+        progressMax
+      ) {
+        if (isProgressEventSink) {
+          if (!wrappedRequest) {
+            wrappedRequest = new BitsRequest(request);
+          }
+          observer.onProgress(wrappedRequest, progress, progressMax);
+        }
+      },
+      onStatus: function wrappedObserver_onStatus(request, status, statusArg) {
+        if (isProgressEventSink) {
+          if (!wrappedRequest) {
+            wrappedRequest = new BitsRequest(request);
+          }
+          observer.onStatus(wrappedRequest, status, statusArg);
+        }
+      },
+      QueryInterface: ChromeUtils.generateQI([
+        "nsIRequestObserver",
+        "nsIProgressEventSink",
+      ]),
+    };
+
+    let timer = makeTimeout(reject, errorAction);
+    let callback = {
+      QueryInterface: ChromeUtils.generateQI(["nsIBitsNewRequestCallback"]),
+      success(request) {
+        timer.cancel();
+        if (!wrappedRequest) {
+          wrappedRequest = new BitsRequest(request);
+        }
+        resolve(wrappedRequest);
+      },
+      failure(type, action, stage) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_NONE
+        );
+        reject(error);
+      },
+      failureNsresult(type, action, stage, code) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_NSRESULT,
+          code
+        );
+        reject(error);
+      },
+      failureHresult(type, action, stage, code) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_HRESULT,
+          code
+        );
+        reject(error);
+      },
+      failureString(type, action, stage, message) {
+        timer.cancel();
+        let error = new BitsError(
+          type,
+          action,
+          stage,
+          lazy.gBits.ERROR_CODE_TYPE_STRING,
+          message
+        );
+        reject(error);
+      },
+    };
+
+    try {
+      actionFn(wrappedObserver, callback);
+    } catch (e) {
+      let error = new BitsError(
+        lazy.gBits.ERROR_TYPE_METHOD_THREW,
+        errorAction,
+        lazy.gBits.ERROR_STAGE_PRETASK,
+        lazy.gBits.ERROR_CODE_TYPE_EXCEPTION,
+        e
+      );
+      reject(error);
+    }
+  });
+}
+
+export var Bits = {
+  /**
+   * This function wraps nsIBits::initialized.
+   */
+  get initialized() {
+    return lazy.gBits.initialized;
+  },
+
+  /**
+   * This function wraps nsIBits::init.
+   */
+  init(jobName, savePathPrefix, monitorTimeoutMs) {
+    return lazy.gBits.init(jobName, savePathPrefix, monitorTimeoutMs);
+  },
+
+  /**
+   * This function wraps nsIBits::startDownload.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with a BitsRequest (which is also an
+   * nsIRequest), or rejects with a BitsError.
+   */
+  async startDownload(
+    downloadURL,
+    saveRelPath,
+    proxy,
+    noProgressTimeoutSecs,
+    monitorIntervalMs,
+    observer,
+    context
+  ) {
+    let action = lazy.gBits.ERROR_ACTION_START_DOWNLOAD;
+    return servicePromise(action, observer, (wrappedObserver, callback) => {
+      lazy.gBits.startDownload(
+        downloadURL,
+        saveRelPath,
+        proxy,
+        noProgressTimeoutSecs,
+        monitorIntervalMs,
+        wrappedObserver,
+        context,
+        callback
+      );
+    });
+  },
+
+  /**
+   * This function wraps nsIBits::monitorDownload.
+   *
+   * Instead of taking a callback, the function is asynchronous.
+   * This method either resolves with a BitsRequest (which is also an
+   * nsIRequest), or rejects with a BitsError.
+   */
+  async monitorDownload(id, monitorIntervalMs, observer, context) {
+    let action = lazy.gBits.ERROR_ACTION_MONITOR_DOWNLOAD;
+    return servicePromise(action, observer, (wrappedObserver, callback) => {
+      lazy.gBits.monitorDownload(
+        id,
+        monitorIntervalMs,
+        wrappedObserver,
+        context,
+        callback
+      );
+    });
+  },
+};
diff --git a/toolkit/components/bitsdownload/Cargo.toml b/toolkit/components/bitsdownload/Cargo.toml
new file mode 100644
index 0000000000..7aff42359e
--- /dev/null
+++ b/toolkit/components/bitsdownload/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "bitsdownload"
+version = "0.1.0"
+authors = ["Kirk Steuber <bytesized@mozilla.com>"]
+license = "MPL-2.0"
+
+[dependencies]
+bits_client = { path = "./bits_client" }
+comedy = "0.2.0"
+crossbeam-utils = "0.8"
+libc = "0.2"
+log = "0.4"
+moz_task = { path = "../../../xpcom/rust/moz_task" }
+nserror = { path = "../../../xpcom/rust/nserror" }
+nsstring = { path = "../../../xpcom/rust/nsstring" }
+xpcom = { path = "../../../xpcom/rust/xpcom" }
diff --git a/toolkit/components/bitsdownload/bits_client/.gitignore b/toolkit/components/bitsdownload/bits_client/.gitignore
new file mode 100644
index 0000000000..d78faf4575
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/.gitignore
@@ -0,0 +1,3 @@
+/target
+**/*.rs.bk
+**/.*.swp
diff --git a/toolkit/components/bitsdownload/bits_client/Cargo.toml b/toolkit/components/bitsdownload/bits_client/Cargo.toml
new file mode 100644
index 0000000000..eb0083d8e2
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/Cargo.toml
@@ -0,0 +1,19 @@
+[package]
+name = "bits_client"
+version = "0.2.0"
+authors = ["Adam Gashlin <agashlin@mozilla.com>"]
+license = "MPL-2.0"
+publish = false
+
+[dependencies]
+bits = { path = "./bits" }
+comedy = "0.2.0"
+guid_win = "0.2.0"
+thiserror = "1"
+
+[dev-dependencies]
+#ctrlc = "3.1.1"
+lazy_static = "1.0.1"
+rand = "0.8"
+regex = { version = "1", default_features = false, features = ["perf", "std"] }
+tempfile = "3"
diff --git a/toolkit/components/bitsdownload/bits_client/README.md b/toolkit/components/bitsdownload/bits_client/README.md
new file mode 100644
index 0000000000..0c21a5b68c
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/README.md
@@ -0,0 +1,23 @@
+bits\_client
+============
+
+Interfaces for BITS.
+
+bits\_client lib
+---------------
+
+`bits_client` is the primary target and provides `BitsClient`, an API for creating and monitoring BITS jobs.
+
+`bits_client::new()` creates a `BitsClient` that does all operations within the current process, as the current user.
+
+bits crate
+----------
+
+`bits` is a safe interface to BITS, providing connections to the
+Background Copy Manager, some basic operations on Background Copy Jobs, and
+methods for implementing `IBackgroundCopyCallback`s in Rust.
+
+test\_client example
+-------------------
+
+`examples/test_client.rs` shows how to use the API.
diff --git a/toolkit/components/bitsdownload/bits_client/bits/Cargo.toml b/toolkit/components/bitsdownload/bits_client/bits/Cargo.toml
new file mode 100644
index 0000000000..36b7b5ff89
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/bits/Cargo.toml
@@ -0,0 +1,31 @@
+[package]
+name = "bits"
+version = "0.2.0"
+authors = ["Adam Gashlin <agashlin@mozilla.com>"]
+license = "MIT/Apache-2.0"
+publish = false
+
+[features]
+status_serde = ["serde", "serde_derive"]
+
+[dependencies]
+comedy = "0.2.0"
+filetime_win = "0.2.0"
+guid_win = "0.2.0"
+serde = { version = "1.0.80", optional = true }
+serde_derive = { version = "1.0.80", optional = true }
+
+[dependencies.winapi]
+version = "0.3.7"
+features = ["basetsd",
+            "bits",
+            "bits2_5",
+            "bitsmsg",
+            "guiddef",
+            "minwindef",
+            "ntdef",
+            "rpcndr",
+            "unknwnbase",
+            "winerror",
+            "winnls",
+            ]
diff --git a/toolkit/components/bitsdownload/bits_client/bits/src/callback.rs b/toolkit/components/bitsdownload/bits_client/bits/src/callback.rs
new file mode 100644
index 0000000000..6dace83be8
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/bits/src/callback.rs
@@ -0,0 +1,205 @@
+// Licensed under the Apache License, Version 2.0
+// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// All files in the project carrying such notice may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::panic::{catch_unwind, RefUnwindSafe};
+use std::ptr::NonNull;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use comedy::{com::ComRef, HResult};
+use guid_win::Guid;
+use winapi::ctypes::c_void;
+use winapi::shared::guiddef::REFIID;
+use winapi::shared::minwindef::DWORD;
+use winapi::shared::ntdef::ULONG;
+use winapi::shared::winerror::{E_FAIL, E_NOINTERFACE, HRESULT, NOERROR, S_OK};
+use winapi::um::bits::{
+    IBackgroundCopyCallback, IBackgroundCopyCallbackVtbl, IBackgroundCopyError, IBackgroundCopyJob,
+};
+use winapi::um::unknwnbase::{IUnknown, IUnknownVtbl};
+use winapi::Interface;
+
+use BitsJob;
+
+/// The type of a notification callback.
+///
+/// The callbacks must be `Fn()` to be called arbitrarily many times, `RefUnwindSafe` to have a
+/// panic unwind safely caught, `Send`, `Sync` and `'static` to run on any thread COM invokes us on
+/// any time.
+///
+/// If the callback returns a non-success `HRESULT`, the notification may pass to other BITS
+/// mechanisms such as `IBackgroundCopyJob2::SetNotifyCmdLine`.
+pub type TransferredCallback =
+    dyn (Fn() -> Result<(), HRESULT>) + RefUnwindSafe + Send + Sync + 'static;
+pub type ErrorCallback = dyn (Fn() -> Result<(), HRESULT>) + RefUnwindSafe + Send + Sync + 'static;
+pub type ModificationCallback =
+    dyn (Fn() -> Result<(), HRESULT>) + RefUnwindSafe + Send + Sync + 'static;
+
+#[repr(C)]
+pub struct BackgroundCopyCallback {
+    // Everything assumes that the interface vtable is the first member of this struct.
+    interface: IBackgroundCopyCallback,
+    rc: AtomicUsize,
+    transferred_cb: Option<Box<TransferredCallback>>,
+    error_cb: Option<Box<ErrorCallback>>,
+    modification_cb: Option<Box<ModificationCallback>>,
+}
+
+impl BackgroundCopyCallback {
+    /// Construct the callback object and register it with a job.
+    ///
+    /// Only one notify interface can be present on a job at once, so this will release BITS'
+    /// ref to any previously registered interface.
+    pub fn register(
+        job: &mut BitsJob,
+        transferred_cb: Option<Box<TransferredCallback>>,
+        error_cb: Option<Box<ErrorCallback>>,
+        modification_cb: Option<Box<ModificationCallback>>,
+    ) -> Result<(), HResult> {
+        let cb = Box::new(BackgroundCopyCallback {
+            interface: IBackgroundCopyCallback { lpVtbl: &VTBL },
+            rc: AtomicUsize::new(1),
+            transferred_cb,
+            error_cb,
+            modification_cb,
+        });
+
+        // Leak the callback, it has no Rust owner until we need to drop it later.
+        // The ComRef will Release when it goes out of scope.
+        unsafe {
+            let cb = ComRef::from_raw(NonNull::new_unchecked(Box::into_raw(cb) as *mut IUnknown));
+
+            job.set_notify_interface(cb.as_raw_ptr())?;
+        }
+
+        Ok(())
+    }
+}
+
+extern "system" fn query_interface(
+    this: *mut IUnknown,
+    riid: REFIID,
+    obj: *mut *mut c_void,
+) -> HRESULT {
+    unsafe {
+        // `IBackgroundCopyCallback` is the first (currently only) interface on the
+        // `BackgroundCopyCallback` object, so we can return `this` either as
+        // `IUnknown` or `IBackgroundCopyCallback`.
+        if Guid(*riid) == Guid(IUnknown::uuidof())
+            || Guid(*riid) == Guid(IBackgroundCopyCallback::uuidof())
+        {
+            addref(this);
+            // Cast first to `IBackgroundCopyCallback` to be clear which `IUnknown`
+            // we are pointing at.
+            *obj = this as *mut IBackgroundCopyCallback as *mut c_void;
+            NOERROR
+        } else {
+            E_NOINTERFACE
+        }
+    }
+}
+
+extern "system" fn addref(raw_this: *mut IUnknown) -> ULONG {
+    unsafe {
+        let this = raw_this as *const BackgroundCopyCallback;
+
+        // Forge a reference for just this statement.
+        let old_rc = (*this).rc.fetch_add(1, Ordering::SeqCst);
+        (old_rc + 1) as ULONG
+    }
+}
+
+extern "system" fn release(raw_this: *mut IUnknown) -> ULONG {
+    unsafe {
+        {
+            let this = raw_this as *const BackgroundCopyCallback;
+
+            // Forge a reference for just this statement.
+            let old_rc = (*this).rc.fetch_sub(1, Ordering::SeqCst);
+
+            let rc = old_rc - 1;
+            if rc > 0 {
+                return rc as ULONG;
+            }
+        }
+
+        // rc will have been 0 for us to get here, and we're out of scope of the reference above,
+        // so there should be no references or pointers left (besides `this`).
+        // Re-Box and to drop immediately.
+        let _ = Box::from_raw(raw_this as *mut BackgroundCopyCallback);
+
+        0
+    }
+}
+
+extern "system" fn transferred_stub(
+    raw_this: *mut IBackgroundCopyCallback,
+    _job: *mut IBackgroundCopyJob,
+) -> HRESULT {
+    unsafe {
+        let this = raw_this as *const BackgroundCopyCallback;
+        // Forge a reference just for this statement.
+        if let Some(ref cb) = (*this).transferred_cb {
+            match catch_unwind(|| cb()) {
+                Ok(Ok(())) => S_OK,
+                Ok(Err(hr)) => hr,
+                Err(_) => E_FAIL,
+            }
+        } else {
+            S_OK
+        }
+    }
+}
+
+extern "system" fn error_stub(
+    raw_this: *mut IBackgroundCopyCallback,
+    _job: *mut IBackgroundCopyJob,
+    _error: *mut IBackgroundCopyError,
+) -> HRESULT {
+    unsafe {
+        let this = raw_this as *const BackgroundCopyCallback;
+        // Forge a reference just for this statement.
+        if let Some(ref cb) = (*this).error_cb {
+            match catch_unwind(|| cb()) {
+                Ok(Ok(())) => S_OK,
+                Ok(Err(hr)) => hr,
+                Err(_) => E_FAIL,
+            }
+        } else {
+            S_OK
+        }
+    }
+}
+
+extern "system" fn modification_stub(
+    raw_this: *mut IBackgroundCopyCallback,
+    _job: *mut IBackgroundCopyJob,
+    _reserved: DWORD,
+) -> HRESULT {
+    unsafe {
+        let this = raw_this as *const BackgroundCopyCallback;
+        // Forge a reference just for this statement.
+        if let Some(ref cb) = (*this).modification_cb {
+            match catch_unwind(|| cb()) {
+                Ok(Ok(())) => S_OK,
+                Ok(Err(hr)) => hr,
+                Err(_) => E_FAIL,
+            }
+        } else {
+            S_OK
+        }
+    }
+}
+
+pub static VTBL: IBackgroundCopyCallbackVtbl = IBackgroundCopyCallbackVtbl {
+    parent: IUnknownVtbl {
+        QueryInterface: query_interface,
+        AddRef: addref,
+        Release: release,
+    },
+    JobTransferred: transferred_stub,
+    JobError: error_stub,
+    JobModification: modification_stub,
+};
diff --git a/toolkit/components/bitsdownload/bits_client/bits/src/lib.rs b/toolkit/components/bitsdownload/bits_client/bits/src/lib.rs
new file mode 100644
index 0000000000..09da2b0349
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/bits/src/lib.rs
@@ -0,0 +1,592 @@
+// Licensed under the Apache License, Version 2.0
+// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// All files in the project carrying such notice may not be copied, modified, or distributed
+// except according to those terms.
+
+//! A safe interface for BITS
+//!
+//! The primary entry point into BITS is the
+//! [`BackgroundCopyManager`](struct.BackgroundCopyManager.html) struct.
+//!
+//! Functionality is only provided by this crate on an as-needed basis for
+//! [bits_client](../bits_client/index.html), so there are vast swathes of the BITS API
+//! unsupported.
+
+extern crate comedy;
+extern crate filetime_win;
+extern crate guid_win;
+extern crate winapi;
+
+#[cfg(feature = "status_serde")]
+extern crate serde;
+#[cfg(feature = "status_serde")]
+extern crate serde_derive;
+
+mod callback;
+pub mod status;
+mod wide;
+
+use std::ffi::{OsStr, OsString};
+use std::mem;
+use std::os::windows::ffi::OsStringExt;
+use std::ptr;
+use std::result;
+
+use comedy::com::{create_instance_local_server, CoTaskMem, ComRef, INIT_MTA};
+use comedy::error::{HResult, ResultExt};
+use comedy::{com_call, com_call_getter, com_call_taskmem_getter};
+use filetime_win::FileTime;
+use guid_win::Guid;
+use winapi::shared::minwindef::DWORD;
+use winapi::shared::ntdef::{HRESULT, LANGIDFROMLCID, ULONG};
+use winapi::shared::winerror::S_FALSE;
+use winapi::um::bits::{
+    IBackgroundCopyError, IBackgroundCopyFile, IBackgroundCopyJob, IBackgroundCopyManager,
+    IEnumBackgroundCopyFiles, IEnumBackgroundCopyJobs, BG_JOB_PRIORITY, BG_JOB_PRIORITY_FOREGROUND,
+    BG_JOB_PRIORITY_HIGH, BG_JOB_PRIORITY_LOW, BG_JOB_PRIORITY_NORMAL, BG_JOB_PROXY_USAGE,
+    BG_JOB_PROXY_USAGE_AUTODETECT, BG_JOB_PROXY_USAGE_NO_PROXY, BG_JOB_PROXY_USAGE_PRECONFIG,
+    BG_JOB_STATE_ERROR, BG_JOB_STATE_TRANSIENT_ERROR, BG_JOB_TYPE_DOWNLOAD, BG_NOTIFY_DISABLE,
+    BG_NOTIFY_JOB_ERROR, BG_NOTIFY_JOB_MODIFICATION, BG_NOTIFY_JOB_TRANSFERRED, BG_SIZE_UNKNOWN,
+};
+use winapi::um::bits2_5::{IBackgroundCopyJobHttpOptions, BG_HTTP_REDIRECT_POLICY_ALLOW_REPORT};
+use winapi::um::bitsmsg::BG_E_NOT_FOUND;
+use winapi::um::unknwnbase::IUnknown;
+use winapi::um::winnls::GetThreadLocale;
+
+pub use winapi::um::bits::{BG_ERROR_CONTEXT, BG_JOB_STATE};
+pub use winapi::um::bitsmsg::{BG_S_PARTIAL_COMPLETE, BG_S_UNABLE_TO_DELETE_FILES};
+
+pub use status::{
+    BitsErrorContext, BitsJobError, BitsJobProgress, BitsJobState, BitsJobStatus, BitsJobTimes,
+};
+use wide::ToWideNull;
+
+pub use winapi::shared::winerror::E_FAIL;
+
+#[repr(u32)]
+#[derive(Copy, Clone, Debug)]
+pub enum BitsJobPriority {
+    Foreground = BG_JOB_PRIORITY_FOREGROUND,
+    High = BG_JOB_PRIORITY_HIGH,
+    /// Default
+    Normal = BG_JOB_PRIORITY_NORMAL,
+    Low = BG_JOB_PRIORITY_LOW,
+}
+
+#[repr(u32)]
+#[derive(Copy, Clone, Debug)]
+pub enum BitsProxyUsage {
+    /// Directly access the network.
+    NoProxy = BG_JOB_PROXY_USAGE_NO_PROXY,
+    /// Use Internet Explorer proxy settings. This is the default.
+    Preconfig = BG_JOB_PROXY_USAGE_PRECONFIG,
+    /// Attempt to auto-detect the connection's proxy settings.
+    AutoDetect = BG_JOB_PROXY_USAGE_AUTODETECT,
+}
+
+type Result<T> = result::Result<T, HResult>;
+
+pub struct BackgroundCopyManager(ComRef<IBackgroundCopyManager>);
+
+impl BackgroundCopyManager {
+    /// Get access to the local BITS service.
+    ///
+    /// # COM Initialization and Threading Model #
+    ///
+    /// This method uses a thread local variable to initialize COM with a multithreaded apartment
+    /// model for this thread, and leaves it this way until the thread local is dropped.
+    /// If the thread was in a single-threaded apartment, `connect()` will fail gracefully.
+    ///
+    /// # Safety #
+    ///
+    /// If there are mismatched `CoUninitialize` calls on this thread which lead to COM shutting
+    /// down before this thread ends, unsafe behavior may result.
+    pub fn connect() -> Result<BackgroundCopyManager> {
+        INIT_MTA.with(|com| {
+            if let Err(e) = com {
+                return Err(e.clone());
+            }
+            Ok(())
+        })?;
+
+        // Assuming no mismatched CoUninitialize calls, methods do not have to check for
+        // successfully initialized COM once the object is constructed: `BackgroundCopyManager`
+        // is not `Send` or `Sync` so it must be used on the thread it was constructed on,
+        // which has now successfully inited MTA for the lifetime of thread local `INIT_MTA`.
+        // This also holds for any functions using pointers only derived from these methods, like
+        // the `BitsJob` methods.
+
+        Ok(BackgroundCopyManager(create_instance_local_server::<
+            winapi::um::bits::BackgroundCopyManager,
+            IBackgroundCopyManager,
+        >()?))
+    }
+
+    /// Create a new download job with the given name.
+    pub fn create_job(&self, display_name: &OsStr) -> Result<BitsJob> {
+        unsafe {
+            let mut guid = mem::zeroed();
+            Ok(BitsJob(com_call_getter!(
+                |job| self.0,
+                IBackgroundCopyManager::CreateJob(
+                    display_name.to_wide_null().as_ptr(),
+                    BG_JOB_TYPE_DOWNLOAD,
+                    &mut guid,
+                    job,
+                )
+            )?))
+        }
+    }
+
+    /// Cancel all jobs with the given name.
+    ///
+    /// This only attempts to cancel jobs owned by the current user.
+    /// No errors are returned for jobs that failed to cancel.
+    pub fn cancel_jobs_by_name(&self, match_name: &OsStr) -> Result<()> {
+        let jobs =
+            unsafe { com_call_getter!(|jobs| self.0, IBackgroundCopyManager::EnumJobs(0, jobs))? };
+
+        loop {
+            let result = unsafe {
+                com_call_getter!(
+                    |job| jobs,
+                    IEnumBackgroundCopyJobs::Next(1, job, ptr::null_mut())
+                )
+            };
+            match result {
+                Ok(job) => {
+                    if job_name_eq(&job, match_name)? {
+                        unsafe {
+                            let _ = com_call!(job, IBackgroundCopyJob::Cancel());
+                        }
+                    }
+                }
+                Err(e) => {
+                    if e.code() == S_FALSE {
+                        // Ran out of jobs to enumerate
+                        return Ok(());
+                    } else {
+                        return Err(e);
+                    }
+                }
+            }
+        }
+    }
+
+    /// Get the job with the given GUID.
+    ///
+    /// Returns Err if the job was not found.
+    pub fn get_job_by_guid(&self, guid: &Guid) -> Result<BitsJob> {
+        unsafe { com_call_getter!(|job| self.0, IBackgroundCopyManager::GetJob(&guid.0, job)) }
+            .map(BitsJob)
+    }
+
+    /// Try to find a job with a given GUID.
+    ///
+    /// Returns Ok(None) if the job was not found but there was no other error.
+    pub fn find_job_by_guid(&self, guid: &Guid) -> Result<Option<BitsJob>> {
+        Ok(self
+            .get_job_by_guid(guid)
+            .map(Some)
+            .allow_err(BG_E_NOT_FOUND as i32, None)?)
+    }
+
+    /// Try to find a job with a given GUID and name.
+    ///
+    /// Returns Ok(None) if the job was not found, or if it had the wrong name, as long as there
+    /// was no other error.
+    pub fn find_job_by_guid_and_name(
+        &self,
+        guid: &Guid,
+        match_name: &OsStr,
+    ) -> Result<Option<BitsJob>> {
+        Ok(match self.find_job_by_guid(guid)? {
+            None => None,
+            Some(BitsJob(ref job)) if !job_name_eq(job, match_name)? => None,
+            result => result,
+        })
+    }
+
+    /// Translate a BITS `HRESULT` to a textual description.
+    ///
+    /// This uses the current thread's locale to look up the message associated with a BITS
+    /// error. It should only be used for `HRESULT`s returned from BITS COM interfaces.
+    pub fn get_error_description(&self, hr: HRESULT) -> Result<String> {
+        unsafe {
+            let language_id = DWORD::from(LANGIDFROMLCID(GetThreadLocale()));
+
+            Ok(taskmem_into_lossy_string(com_call_taskmem_getter!(
+                |desc| self.0,
+                IBackgroundCopyManager::GetErrorDescription(hr, language_id, desc)
+            )?))
+        }
+    }
+}
+
+unsafe fn taskmem_into_lossy_string(taskmem: CoTaskMem<u16>) -> String {
+    OsString::from_wide(taskmem.as_slice_until_null())
+        .to_string_lossy()
+        .into_owned()
+}
+
+fn job_name_eq(job: &ComRef<IBackgroundCopyJob>, match_name: &OsStr) -> Result<bool> {
+    let job_name = unsafe {
+        OsString::from_wide(
+            com_call_taskmem_getter!(|name| job, IBackgroundCopyJob::GetDisplayName(name))?
+                .as_slice_until_null(),
+        )
+    };
+
+    Ok(job_name == match_name)
+}
+
+pub struct BitsJob(ComRef<IBackgroundCopyJob>);
+
+impl BitsJob {
+    /// Get the job's GUID.
+    pub fn guid(&self) -> Result<Guid> {
+        // TODO: cache on create or retrieved by GUID?
+        unsafe {
+            let mut guid = mem::zeroed();
+            com_call!(self.0, IBackgroundCopyJob::GetId(&mut guid))?;
+            Ok(Guid(guid))
+        }
+    }
+
+    /// Add a file to the job.
+    pub fn add_file(&mut self, remote_url: &OsStr, local_file: &OsStr) -> Result<()> {
+        unsafe {
+            com_call!(
+                self.0,
+                IBackgroundCopyJob::AddFile(
+                    remote_url.to_wide_null().as_ptr(),
+                    local_file.to_wide_null().as_ptr(),
+                )
+            )
+        }?;
+        Ok(())
+    }
+
+    /// Get the first file in the job.
+    ///
+    /// This is provided for collecting the redirected remote name of single file jobs.
+    pub fn get_first_file(&mut self) -> Result<BitsFile> {
+        let files = unsafe { com_call_getter!(|e| self.0, IBackgroundCopyJob::EnumFiles(e))? };
+
+        let file = unsafe {
+            com_call_getter!(
+                |file| files,
+                IEnumBackgroundCopyFiles::Next(1, file, ptr::null_mut())
+            )?
+        };
+
+        Ok(BitsFile(file))
+    }
+
+    /// Set the job's description string.
+    ///
+    /// This is different from the display name set when creating the job.
+    pub fn set_description(&mut self, description: &OsStr) -> Result<()> {
+        unsafe {
+            com_call!(
+                self.0,
+                IBackgroundCopyJob::SetDescription(description.to_wide_null().as_ptr())
+            )
+        }?;
+        Ok(())
+    }
+
+    /// Change the job's proxy usage setting.
+    ///
+    /// The default is `BitsProxyUsage::Preconfig`.
+    pub fn set_proxy_usage(&mut self, usage: BitsProxyUsage) -> Result<()> {
+        use BitsProxyUsage::*;
+
+        match usage {
+            Preconfig | NoProxy | AutoDetect => {
+                unsafe {
+                    com_call!(
+                        self.0,
+                        IBackgroundCopyJob::SetProxySettings(
+                            usage as BG_JOB_PROXY_USAGE,
+                            ptr::null_mut(),
+                            ptr::null_mut(),
+                        )
+                    )
+                }?;
+                Ok(())
+            }
+        }
+    }
+
+    /// Change the job's priority.
+    ///
+    /// The default is `BitsJobPriority::Normal`.
+    pub fn set_priority(&mut self, priority: BitsJobPriority) -> Result<()> {
+        unsafe {
+            com_call!(
+                self.0,
+                IBackgroundCopyJob::SetPriority(priority as BG_JOB_PRIORITY)
+            )
+        }?;
+        Ok(())
+    }
+
+    pub fn set_minimum_retry_delay(&mut self, seconds: ULONG) -> Result<()> {
+        unsafe { com_call!(self.0, IBackgroundCopyJob::SetMinimumRetryDelay(seconds)) }?;
+        Ok(())
+    }
+
+    pub fn set_no_progress_timeout(&mut self, seconds: ULONG) -> Result<()> {
+        unsafe { com_call!(self.0, IBackgroundCopyJob::SetNoProgressTimeout(seconds)) }?;
+        Ok(())
+    }
+
+    /// Enable HTTP redirect reporting.
+    ///
+    /// The default setting is to allow HTTP redirects, but to not report them in any way. With
+    /// this setting enabled, the remote name of a file will be updated to reflect the redirect.
+    ///
+    /// # Compatibility #
+    ///
+    /// First available in Windows Vista.
+    pub fn set_redirect_report(&mut self) -> Result<()> {
+        unsafe {
+            com_call!(
+                self.0.cast()?,
+                IBackgroundCopyJobHttpOptions::SetSecurityFlags(
+                    BG_HTTP_REDIRECT_POLICY_ALLOW_REPORT
+                )
+            )
+        }?;
+
+        Ok(())
+    }
+
+    /// Resume the job. This must be done at least once to initially enqueue the job.
+    pub fn resume(&mut self) -> Result<()> {
+        unsafe { com_call!(self.0, IBackgroundCopyJob::Resume()) }?;
+        Ok(())
+    }
+
+    pub fn suspend(&mut self) -> Result<()> {
+        unsafe { com_call!(self.0, IBackgroundCopyJob::Suspend()) }?;
+        Ok(())
+    }
+
+    /// Complete the job, moving the local files to their final names.
+    ///
+    /// Has two interesting success `HRESULT`s: `BG_S_PARTIAL_COMPLETE` and
+    /// `BG_S_UNABLE_TO_DELETE_FILES`.
+    pub fn complete(&mut self) -> Result<HRESULT> {
+        unsafe { com_call!(self.0, IBackgroundCopyJob::Complete()) }
+    }
+
+    /// Cancel the job, deleting any temporary files.
+    ///
+    /// Has an interesting success `HRESULT`: `BG_S_UNABLE_TO_DELETE_FILES`.
+    pub fn cancel(&mut self) -> Result<HRESULT> {
+        unsafe { com_call!(self.0, IBackgroundCopyJob::Cancel()) }
+    }
+
+    /// Set the notification callbacks to use with this job.
+    ///
+    /// This will replace any previously set callbacks.
+    pub fn register_callbacks(
+        &mut self,
+        transferred_cb: Option<Box<callback::TransferredCallback>>,
+        error_cb: Option<Box<callback::ErrorCallback>>,
+        modification_cb: Option<Box<callback::ModificationCallback>>,
+    ) -> Result<()> {
+        let mut flags = 0;
+        if transferred_cb.is_some() {
+            flags |= BG_NOTIFY_JOB_TRANSFERRED;
+        }
+        if error_cb.is_some() {
+            flags |= BG_NOTIFY_JOB_ERROR;
+        }
+        if modification_cb.is_some() {
+            flags |= BG_NOTIFY_JOB_MODIFICATION;
+        }
+
+        callback::BackgroundCopyCallback::register(
+            self,
+            transferred_cb,
+            error_cb,
+            modification_cb,
+        )?;
+
+        unsafe { com_call!(self.0, IBackgroundCopyJob::SetNotifyFlags(flags)) }?;
+
+        Ok(())
+    }
+
+    fn _clear_callbacks(&mut self) -> Result<()> {
+        unsafe {
+            com_call!(
+                self.0,
+                IBackgroundCopyJob::SetNotifyFlags(BG_NOTIFY_DISABLE)
+            )?;
+
+            self.set_notify_interface(ptr::null_mut() as *mut IUnknown)
+        }
+    }
+
+    /// Collect the current status of the job, including errors.
+    pub fn get_status(&self) -> Result<BitsJobStatus> {
+        let mut state = 0;
+        let mut progress = unsafe { mem::zeroed() };
+        let mut error_count = 0;
+        let mut times = unsafe { mem::zeroed() };
+
+        unsafe {
+            com_call!(self.0, IBackgroundCopyJob::GetState(&mut state))?;
+            com_call!(self.0, IBackgroundCopyJob::GetProgress(&mut progress))?;
+            com_call!(self.0, IBackgroundCopyJob::GetErrorCount(&mut error_count))?;
+            com_call!(self.0, IBackgroundCopyJob::GetTimes(&mut times))?;
+        }
+
+        Ok(BitsJobStatus {
+            state: BitsJobState::from(state),
+            progress: BitsJobProgress {
+                total_bytes: if progress.BytesTotal == BG_SIZE_UNKNOWN {
+                    None
+                } else {
+                    Some(progress.BytesTotal)
+                },
+                transferred_bytes: progress.BytesTransferred,
+                total_files: progress.FilesTotal,
+                transferred_files: progress.FilesTransferred,
+            },
+            error_count,
+            error: if state == BG_JOB_STATE_ERROR || state == BG_JOB_STATE_TRANSIENT_ERROR {
+                let error_obj =
+                    unsafe { com_call_getter!(|e| self.0, IBackgroundCopyJob::GetError(e)) }?;
+
+                Some(BitsJob::get_error(error_obj)?)
+            } else {
+                None
+            },
+            times: BitsJobTimes {
+                creation: FileTime(times.CreationTime),
+                modification: FileTime(times.ModificationTime),
+                transfer_completion: if times.TransferCompletionTime.dwLowDateTime == 0
+                    && times.TransferCompletionTime.dwHighDateTime == 0
+                {
+                    None
+                } else {
+                    Some(FileTime(times.TransferCompletionTime))
+                },
+            },
+        })
+    }
+
+    fn get_error(error_obj: ComRef<IBackgroundCopyError>) -> Result<BitsJobError> {
+        let mut context = 0;
+        let mut hresult = 0;
+        unsafe {
+            com_call!(
+                error_obj,
+                IBackgroundCopyError::GetError(&mut context, &mut hresult)
+            )?;
+
+            let language_id = DWORD::from(LANGIDFROMLCID(GetThreadLocale()));
+
+            let context = BitsErrorContext::from(context);
+            let context_str = com_call_taskmem_getter!(
+                |desc| error_obj,
+                IBackgroundCopyError::GetErrorContextDescription(language_id, desc)
+            )
+            .map(|s| taskmem_into_lossy_string(s))
+            .unwrap_or_else(|_| format!("{:?}", context));
+            let error_str = com_call_taskmem_getter!(
+                |desc| error_obj,
+                IBackgroundCopyError::GetErrorDescription(language_id, desc)
+            )
+            .map(|s| taskmem_into_lossy_string(s))
+            .unwrap_or_else(|_| format!("{:#08x}", hresult));
+
+            Ok(BitsJobError {
+                context,
+                context_str,
+                error: hresult,
+                error_str,
+            })
+        }
+    }
+
+    unsafe fn set_notify_interface(&self, interface: *mut IUnknown) -> Result<()> {
+        com_call!(self.0, IBackgroundCopyJob::SetNotifyInterface(interface))?;
+        Ok(())
+    }
+}
+
+pub struct BitsFile(ComRef<IBackgroundCopyFile>);
+
+/// A single file in a BITS job.
+///
+/// This is provided for collecting the redirected remote name.
+impl BitsFile {
+    /// Get the remote name from which the file is being downloaded.
+    ///
+    /// If [`BitsJob::set_redirect_report()`](struct.BitsJob.html#method.set_redirect_report)
+    /// hasn't been called on the job, this won't be
+    /// updated as HTTP redirects are processed.
+    pub fn get_remote_name(&self) -> Result<OsString> {
+        unsafe {
+            Ok(OsString::from_wide(
+                com_call_taskmem_getter!(|name| self.0, IBackgroundCopyFile::GetRemoteName(name))?
+                    .as_slice_until_null(),
+            ))
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::BackgroundCopyManager;
+    use std::ffi::OsString;
+    use std::mem;
+
+    #[test]
+    #[ignore]
+    fn test_find_job() {
+        let bcm = BackgroundCopyManager::connect().unwrap();
+        let name = OsString::from("bits test job");
+        let wrong_name = OsString::from("bits test jobbo");
+
+        let mut job = bcm.create_job(&name).unwrap();
+        let guid = job.guid().unwrap();
+
+        assert_eq!(
+            bcm.find_job_by_guid(&guid)
+                .unwrap()
+                .unwrap()
+                .guid()
+                .unwrap(),
+            guid
+        );
+        assert_eq!(
+            bcm.find_job_by_guid_and_name(&guid, &name)
+                .unwrap()
+                .unwrap()
+                .guid()
+                .unwrap(),
+            guid
+        );
+        assert!(bcm
+            .find_job_by_guid_and_name(&guid, &wrong_name)
+            .unwrap()
+            .is_none());
+
+        job.cancel().unwrap();
+        mem::drop(job);
+
+        assert!(bcm.find_job_by_guid(&guid).unwrap().is_none());
+        assert!(bcm
+            .find_job_by_guid_and_name(&guid, &name)
+            .unwrap()
+            .is_none());
+    }
+}
diff --git a/toolkit/components/bitsdownload/bits_client/bits/src/status.rs b/toolkit/components/bitsdownload/bits_client/bits/src/status.rs
new file mode 100644
index 0000000000..648a26866e
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/bits/src/status.rs
@@ -0,0 +1,118 @@
+// Licensed under the Apache License, Version 2.0
+// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// All files in the project carrying such notice may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Data types for reporting a job's status
+
+use filetime_win::FileTime;
+use winapi::shared::winerror::HRESULT;
+use winapi::um::bits::{BG_ERROR_CONTEXT, BG_JOB_STATE};
+
+#[cfg(feature = "status_serde")]
+use serde_derive::{Deserialize, Serialize};
+
+#[derive(Clone, Debug)]
+#[cfg_attr(feature = "status_serde", derive(Serialize, Deserialize))]
+pub struct BitsJobStatus {
+    pub state: BitsJobState,
+    pub progress: BitsJobProgress,
+    pub error_count: u32,
+    pub error: Option<BitsJobError>,
+    pub times: BitsJobTimes,
+}
+
+#[derive(Clone, Debug)]
+#[cfg_attr(feature = "status_serde", derive(Serialize, Deserialize))]
+pub struct BitsJobError {
+    pub context: BitsErrorContext,
+    pub context_str: String,
+    pub error: HRESULT,
+    pub error_str: String,
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[cfg_attr(feature = "status_serde", derive(Serialize, Deserialize))]
+pub enum BitsErrorContext {
+    None,
+    Unknown,
+    GeneralQueueManager,
+    QueueManagerNotification,
+    LocalFile,
+    RemoteFile,
+    GeneralTransport,
+    RemoteApplication,
+    /// No other values are documented
+    Other(BG_ERROR_CONTEXT),
+}
+
+impl From<BG_ERROR_CONTEXT> for BitsErrorContext {
+    fn from(ec: BG_ERROR_CONTEXT) -> BitsErrorContext {
+        use self::BitsErrorContext::*;
+        use winapi::um::bits;
+        match ec {
+            bits::BG_ERROR_CONTEXT_NONE => None,
+            bits::BG_ERROR_CONTEXT_UNKNOWN => Unknown,
+            bits::BG_ERROR_CONTEXT_GENERAL_QUEUE_MANAGER => GeneralQueueManager,
+            bits::BG_ERROR_CONTEXT_QUEUE_MANAGER_NOTIFICATION => QueueManagerNotification,
+            bits::BG_ERROR_CONTEXT_LOCAL_FILE => LocalFile,
+            bits::BG_ERROR_CONTEXT_REMOTE_FILE => RemoteFile,
+            bits::BG_ERROR_CONTEXT_GENERAL_TRANSPORT => GeneralTransport,
+            bits::BG_ERROR_CONTEXT_REMOTE_APPLICATION => RemoteApplication,
+            ec => Other(ec),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[cfg_attr(feature = "status_serde", derive(Serialize, Deserialize))]
+pub enum BitsJobState {
+    Queued,
+    Connecting,
+    Transferring,
+    Suspended,
+    Error,
+    TransientError,
+    Transferred,
+    Acknowledged,
+    Cancelled,
+    /// No other values are documented
+    Other(BG_JOB_STATE),
+}
+
+impl From<BG_JOB_STATE> for BitsJobState {
+    fn from(s: BG_JOB_STATE) -> BitsJobState {
+        use self::BitsJobState::*;
+        use winapi::um::bits;
+        match s {
+            bits::BG_JOB_STATE_QUEUED => Queued,
+            bits::BG_JOB_STATE_CONNECTING => Connecting,
+            bits::BG_JOB_STATE_TRANSFERRING => Transferring,
+            bits::BG_JOB_STATE_SUSPENDED => Suspended,
+            bits::BG_JOB_STATE_ERROR => Error,
+            bits::BG_JOB_STATE_TRANSIENT_ERROR => TransientError,
+            bits::BG_JOB_STATE_TRANSFERRED => Transferred,
+            bits::BG_JOB_STATE_ACKNOWLEDGED => Acknowledged,
+            bits::BG_JOB_STATE_CANCELLED => Cancelled,
+            s => Other(s),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+#[cfg_attr(feature = "status_serde", derive(Serialize, Deserialize))]
+pub struct BitsJobProgress {
+    pub total_bytes: Option<u64>,
+    pub transferred_bytes: u64,
+    pub total_files: u32,
+    pub transferred_files: u32,
+}
+
+#[derive(Copy, Clone, Debug)]
+#[cfg_attr(feature = "status_serde", derive(Serialize, Deserialize))]
+pub struct BitsJobTimes {
+    pub creation: FileTime,
+    pub modification: FileTime,
+    pub transfer_completion: Option<FileTime>,
+}
diff --git a/toolkit/components/bitsdownload/bits_client/bits/src/wide.rs b/toolkit/components/bitsdownload/bits_client/bits/src/wide.rs
new file mode 100644
index 0000000000..c108f8d629
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/bits/src/wide.rs
@@ -0,0 +1,38 @@
+// Licensed under the Apache License, Version 2.0
+// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// All files in the project carrying such notice may not be copied, modified, or distributed
+// except according to those terms.
+
+// Minimal null-terminated wide string support from wio.
+
+use std::ffi::{OsStr, OsString};
+use std::os::windows::ffi::{OsStrExt, OsStringExt};
+use std::slice;
+
+pub trait ToWideNull {
+    fn to_wide_null(&self) -> Vec<u16>;
+}
+
+impl<T: AsRef<OsStr>> ToWideNull for T {
+    fn to_wide_null(&self) -> Vec<u16> {
+        self.as_ref().encode_wide().chain(Some(0)).collect()
+    }
+}
+
+pub trait FromWidePtrNull {
+    unsafe fn from_wide_ptr_null(wide: *const u16) -> Self;
+}
+
+impl FromWidePtrNull for OsString {
+    unsafe fn from_wide_ptr_null(wide: *const u16) -> Self {
+        assert!(!wide.is_null());
+
+        for i in 0.. {
+            if *wide.offset(i) == 0 {
+                return Self::from_wide(&slice::from_raw_parts(wide, i as usize));
+            }
+        }
+        unreachable!()
+    }
+}
diff --git a/toolkit/components/bitsdownload/bits_client/examples/test_client.rs b/toolkit/components/bitsdownload/bits_client/examples/test_client.rs
new file mode 100644
index 0000000000..25d7a0d82e
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/examples/test_client.rs
@@ -0,0 +1,285 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+extern crate bits_client;
+extern crate comedy;
+//extern crate ctrlc;
+extern crate guid_win;
+extern crate thiserror;
+
+use std::env;
+use std::ffi::{OsStr, OsString};
+use std::process;
+use std::str::FromStr;
+use std::sync::{Arc, Mutex};
+
+use thiserror::Error;
+
+use bits_client::bits_protocol::HResultMessage;
+use bits_client::{BitsClient, BitsJobState, BitsMonitorClient, BitsProxyUsage, Guid, PipeError};
+
+#[derive(Debug, Error)]
+enum MyError {
+    #[error("{0}")]
+    Msg(String),
+    #[error("HResult")]
+    HResult(#[from] comedy::HResult),
+    #[error("Win32Error")]
+    Win32Error(#[from] comedy::Win32Error),
+    #[error("PipeError")]
+    PipeError(#[from] PipeError),
+    #[error("HResultMessage")]
+    HResultMessage(#[from] HResultMessage),
+}
+
+macro_rules! bail {
+    ($e:expr) => {
+        return Err($crate::MyError::Msg($e.to_string()))
+    };
+    ($fmt:expr, $($arg:tt)*) => {
+        return Err($crate::MyError::Msg(format!($fmt, $($arg)*)))
+    };
+}
+
+type Result = std::result::Result<(), MyError>;
+
+pub fn main() {
+    if let Err(err) = entry() {
+        eprintln!("{}", err);
+        let mut err: &dyn std::error::Error = &err;
+        while let Some(source) = err.source() {
+            eprintln!("caused by {}", source);
+            err = source;
+        }
+
+        process::exit(1);
+    } else {
+        println!("OK");
+    }
+}
+
+const EXE_NAME: &'static str = "test_client";
+
+fn usage() -> String {
+    format!(
+        concat!(
+            "Usage {0} <command> ",
+            "[local-service] ",
+            "[args...]\n",
+            "Commands:\n",
+            "  bits-start <URL> <local file>\n",
+            "  bits-monitor <GUID>\n",
+            "  bits-bg <GUID>\n",
+            "  bits-fg <GUID>\n",
+            "  bits-suspend <GUID>\n",
+            "  bits-resume <GUID>\n",
+            "  bits-complete <GUID>\n",
+            "  bits-cancel <GUID> ...\n"
+        ),
+        EXE_NAME
+    )
+}
+
+fn entry() -> Result {
+    let args: Vec<_> = env::args_os().collect();
+
+    let mut client = BitsClient::new(
+        OsString::from("bits_client test"),
+        OsString::from("C:\\ProgramData"),
+    )?;
+
+    if args.len() < 2 {
+        eprintln!("{}", usage());
+        bail!("not enough arguments");
+    }
+
+    let cmd = &*args[1].to_string_lossy();
+    let cmd_args = &args[2..];
+
+    match cmd {
+        // command line client for testing
+        "bits-start" if cmd_args.len() == 2 => bits_start(
+            Arc::new(Mutex::new(client)),
+            cmd_args[0].clone(),
+            cmd_args[1].clone(),
+            BitsProxyUsage::Preconfig,
+        ),
+        "bits-monitor" if cmd_args.len() == 1 => {
+            bits_monitor(Arc::new(Mutex::new(client)), &cmd_args[0])
+        }
+        // TODO: some way of testing set update interval
+        "bits-bg" if cmd_args.len() == 1 => bits_bg(&mut client, &cmd_args[0]),
+        "bits-fg" if cmd_args.len() == 1 => bits_fg(&mut client, &cmd_args[0]),
+        "bits-suspend" if cmd_args.len() == 1 => bits_suspend(&mut client, &cmd_args[0]),
+        "bits-resume" if cmd_args.len() == 1 => bits_resume(&mut client, &cmd_args[0]),
+        "bits-complete" if cmd_args.len() == 1 => bits_complete(&mut client, &cmd_args[0]),
+        "bits-cancel" if cmd_args.len() >= 1 => {
+            for guid in cmd_args {
+                bits_cancel(&mut client, guid)?;
+            }
+            Ok(())
+        }
+        _ => {
+            eprintln!("{}", usage());
+            bail!("usage error");
+        }
+    }
+}
+
+fn bits_start(
+    client: Arc<Mutex<BitsClient>>,
+    url: OsString,
+    save_path: OsString,
+    proxy_usage: BitsProxyUsage,
+) -> Result {
+    //let interval = 10 * 60 * 1000;
+    let no_progress_timeout_secs = 60;
+    let interval = 1000;
+
+    let result = client.lock().unwrap().start_job(
+        url,
+        save_path,
+        proxy_usage,
+        no_progress_timeout_secs,
+        interval,
+    )?;
+
+    match result {
+        Ok((r, monitor_client)) => {
+            println!("start success, guid = {}", r.guid);
+            client
+                .lock()
+                .unwrap()
+                .set_update_interval(r.guid.clone(), interval)?
+                .unwrap();
+            monitor_loop(client, monitor_client, r.guid.clone(), interval)?;
+            Ok(())
+        }
+        Err(e) => {
+            let _ = e.clone();
+            bail!("error from server {}", e)
+        }
+    }
+}
+
+fn bits_monitor(client: Arc<Mutex<BitsClient>>, guid: &OsStr) -> Result {
+    let guid = Guid::from_str(&guid.to_string_lossy())?;
+    let result = client.lock().unwrap().monitor_job(guid.clone(), 1000)?;
+    match result {
+        Ok(monitor_client) => {
+            println!("monitor success");
+            monitor_loop(client, monitor_client, guid, 1000)?;
+            Ok(())
+        }
+        Err(e) => bail!("error from server {}", e),
+    }
+}
+
+fn _check_client_send()
+where
+    BitsClient: Send,
+{
+}
+fn _check_monitor_send()
+where
+    BitsMonitorClient: Send,
+{
+}
+
+fn monitor_loop(
+    _client: Arc<Mutex<BitsClient>>,
+    mut monitor_client: BitsMonitorClient,
+    _guid: Guid,
+    wait_millis: u32,
+) -> Result {
+    /*
+    // Commented out to avoid vendoring ctrlc.
+    // This could also possibly be done with `exclude` in the mozilla-central `Cargo.toml`.
+    let client_for_handler = _client.clone();
+    ctrlc::set_handler(move || {
+        eprintln!("Ctrl-C!");
+        let _ = client_for_handler.lock().unwrap().stop_update(_guid.clone());
+    })
+    .expect("Error setting Ctrl-C handler");
+    */
+
+    loop {
+        let status = monitor_client.get_status(wait_millis * 10)??;
+
+        println!("{:?} {:?}", BitsJobState::from(status.state), status);
+
+        //println!("{}", job.get_first_file()?.get_remote_name()?.into_string().unwrap());
+        let transfer_completion_time = if let Some(ft) = status.times.transfer_completion {
+            format!("Some({})", ft.to_system_time_utc()?)
+        } else {
+            String::from("None")
+        };
+        println!(
+            "creation: {}, modification: {}, transfer completion: {}",
+            status.times.creation.to_system_time_utc()?,
+            status.times.modification.to_system_time_utc()?,
+            transfer_completion_time
+        );
+
+        match BitsJobState::from(status.state) {
+            BitsJobState::Connecting
+            | BitsJobState::Transferring
+            | BitsJobState::TransientError => {}
+            _ => break,
+        }
+    }
+    println!("monitor loop ending");
+    println!("sleeping...");
+    std::thread::sleep(std::time::Duration::from_secs(1));
+
+    Ok(())
+}
+
+fn bits_bg(client: &mut BitsClient, guid: &OsStr) -> Result {
+    bits_set_priority(client, guid, false)
+}
+
+fn bits_fg(client: &mut BitsClient, guid: &OsStr) -> Result {
+    bits_set_priority(client, guid, true)
+}
+
+fn bits_set_priority(client: &mut BitsClient, guid: &OsStr, foreground: bool) -> Result {
+    let guid = Guid::from_str(&guid.to_string_lossy())?;
+    match client.set_job_priority(guid, foreground)? {
+        Ok(()) => Ok(()),
+        Err(e) => bail!("error from server {}", e),
+    }
+}
+
+fn bits_suspend(client: &mut BitsClient, guid: &OsStr) -> Result {
+    let guid = Guid::from_str(&guid.to_string_lossy())?;
+    match client.suspend_job(guid)? {
+        Ok(()) => Ok(()),
+        Err(e) => bail!("error from server {}", e),
+    }
+}
+
+fn bits_resume(client: &mut BitsClient, guid: &OsStr) -> Result {
+    let guid = Guid::from_str(&guid.to_string_lossy())?;
+    match client.resume_job(guid)? {
+        Ok(()) => Ok(()),
+        Err(e) => bail!("error from server {}", e),
+    }
+}
+
+fn bits_complete(client: &mut BitsClient, guid: &OsStr) -> Result {
+    let guid = Guid::from_str(&guid.to_string_lossy())?;
+    match client.complete_job(guid)? {
+        Ok(()) => Ok(()),
+        Err(e) => bail!("error from server {}", e),
+    }
+}
+
+fn bits_cancel(client: &mut BitsClient, guid: &OsStr) -> Result {
+    let guid = Guid::from_str(&guid.to_string_lossy())?;
+    match client.cancel_job(guid)? {
+        Ok(()) => Ok(()),
+        Err(e) => bail!("error from server {}", e),
+    }
+}
diff --git a/toolkit/components/bitsdownload/bits_client/src/bits_protocol.rs b/toolkit/components/bitsdownload/bits_client/src/bits_protocol.rs
new file mode 100644
index 0000000000..6f19f5ef23
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/src/bits_protocol.rs
@@ -0,0 +1,380 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! Command, response, and status types.
+
+use std::error::Error as StdError;
+use std::ffi::OsString;
+use std::fmt;
+use std::result;
+
+use guid_win::Guid;
+use thiserror::Error;
+
+use super::{BitsErrorContext, BitsJobProgress, BitsJobState, BitsJobTimes, BitsProxyUsage};
+
+type HRESULT = i32;
+
+/// An HRESULT with a descriptive message
+#[derive(Clone, Debug)]
+pub struct HResultMessage {
+    pub hr: HRESULT,
+    pub message: String,
+}
+
+impl fmt::Display for HResultMessage {
+    fn fmt(&self, f: &mut fmt::Formatter) -> result::Result<(), fmt::Error> {
+        self.message.fmt(f)
+    }
+}
+
+impl StdError for HResultMessage {}
+
+/// Commands which can be sent to the server.
+///
+/// This is currently unused as the out-of-process Local Service server is not finished.
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub enum Command {
+    StartJob(StartJobCommand),
+    MonitorJob(MonitorJobCommand),
+    SuspendJob(SuspendJobCommand),
+    ResumeJob(ResumeJobCommand),
+    SetJobPriority(SetJobPriorityCommand),
+    SetNoProgressTimeout(SetNoProgressTimeoutCommand),
+    SetUpdateInterval(SetUpdateIntervalCommand),
+    CompleteJob(CompleteJobCommand),
+    CancelJob(CancelJobCommand),
+}
+
+/// Combine a [`Command`](enum.Command.html) with its success and failure result types.
+#[doc(hidden)]
+pub trait CommandType {
+    type Success;
+    type Failure: StdError;
+    fn wrap(command: Self) -> Command;
+}
+
+// Start Job
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct StartJobCommand {
+    pub url: OsString,
+    pub save_path: OsString,
+    pub proxy_usage: BitsProxyUsage,
+    pub no_progress_timeout_secs: u32,
+    pub monitor: Option<MonitorConfig>,
+}
+
+impl CommandType for StartJobCommand {
+    type Success = StartJobSuccess;
+    type Failure = StartJobFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::StartJob(cmd)
+    }
+}
+
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct MonitorConfig {
+    pub pipe_name: OsString,
+    pub interval_millis: u32,
+}
+
+#[derive(Clone, Debug)]
+pub struct StartJobSuccess {
+    pub guid: Guid,
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum StartJobFailure {
+    #[error("Argument validation failed: {0}")]
+    ArgumentValidation(String),
+    #[error("Create job: {0}")]
+    Create(HResultMessage),
+    #[error("Add file to job: {0}")]
+    AddFile(HResultMessage),
+    #[error("Apply settings to job: {0}")]
+    ApplySettings(HResultMessage),
+    #[error("Resume job: {0}")]
+    Resume(HResultMessage),
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Monitor Job
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct MonitorJobCommand {
+    pub guid: Guid,
+    pub monitor: MonitorConfig,
+}
+
+impl CommandType for MonitorJobCommand {
+    type Success = ();
+    type Failure = MonitorJobFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::MonitorJob(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum MonitorJobFailure {
+    #[error("Argument validation failed: {0}")]
+    ArgumentValidation(String),
+    #[error("Job not found")]
+    NotFound,
+    #[error("Get job: {0}")]
+    GetJob(HResultMessage),
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Suspend Job
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct SuspendJobCommand {
+    pub guid: Guid,
+}
+
+impl CommandType for SuspendJobCommand {
+    type Success = ();
+    type Failure = SuspendJobFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::SuspendJob(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum SuspendJobFailure {
+    #[error("Job not found")]
+    NotFound,
+    #[error("Get job: {0}")]
+    GetJob(HResultMessage),
+    #[error("Suspend job: {0}")]
+    SuspendJob(HResultMessage),
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Resume Job
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct ResumeJobCommand {
+    pub guid: Guid,
+}
+
+impl CommandType for ResumeJobCommand {
+    type Success = ();
+    type Failure = ResumeJobFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::ResumeJob(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum ResumeJobFailure {
+    #[error("Job not found")]
+    NotFound,
+    #[error("Get job: {0}")]
+    GetJob(HResultMessage),
+    #[error("Resume job: {0}")]
+    ResumeJob(HResultMessage),
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Set Job Priority
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct SetJobPriorityCommand {
+    pub guid: Guid,
+    pub foreground: bool,
+}
+
+impl CommandType for SetJobPriorityCommand {
+    type Success = ();
+    type Failure = SetJobPriorityFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::SetJobPriority(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum SetJobPriorityFailure {
+    #[error("Job not found")]
+    NotFound,
+    #[error("Get job: {0}")]
+    GetJob(HResultMessage),
+    #[error("Apply settings to job: {0}")]
+    ApplySettings(HResultMessage),
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Set No Progress Timeout
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct SetNoProgressTimeoutCommand {
+    pub guid: Guid,
+    pub timeout_secs: u32,
+}
+
+impl CommandType for SetNoProgressTimeoutCommand {
+    type Success = ();
+    type Failure = SetNoProgressTimeoutFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::SetNoProgressTimeout(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum SetNoProgressTimeoutFailure {
+    #[error("Job not found")]
+    NotFound,
+    #[error("Get job: {0}")]
+    GetJob(HResultMessage),
+    #[error("Apply settings to job: {0}")]
+    ApplySettings(HResultMessage),
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Set Update Interval
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct SetUpdateIntervalCommand {
+    pub guid: Guid,
+    pub interval_millis: u32,
+}
+
+impl CommandType for SetUpdateIntervalCommand {
+    type Success = ();
+    type Failure = SetUpdateIntervalFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::SetUpdateInterval(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum SetUpdateIntervalFailure {
+    #[error("Argument validation: {0}")]
+    ArgumentValidation(String),
+    #[error("Monitor not found")]
+    NotFound,
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Complete Job
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct CompleteJobCommand {
+    pub guid: Guid,
+}
+
+impl CommandType for CompleteJobCommand {
+    type Success = ();
+    type Failure = CompleteJobFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::CompleteJob(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum CompleteJobFailure {
+    #[error("Job not found")]
+    NotFound,
+    #[error("Get job: {0}")]
+    GetJob(HResultMessage),
+    #[error("Complete job: {0}")]
+    CompleteJob(HResultMessage),
+    #[error("Job only partially completed")]
+    PartialComplete,
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+// Cancel Job
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct CancelJobCommand {
+    pub guid: Guid,
+}
+
+impl CommandType for CancelJobCommand {
+    type Success = ();
+    type Failure = CancelJobFailure;
+    fn wrap(cmd: Self) -> Command {
+        Command::CancelJob(cmd)
+    }
+}
+
+#[derive(Clone, Debug, Error)]
+pub enum CancelJobFailure {
+    #[error("Job not found")]
+    NotFound,
+    #[error("Get job: {0}")]
+    GetJob(HResultMessage),
+    #[error("Cancel job: {0}")]
+    CancelJob(HResultMessage),
+    #[error("Connect to BackgroundCopyManager: {0}")]
+    ConnectBcm(HResultMessage),
+    #[error("BITS error: {0}")]
+    OtherBITS(HResultMessage),
+    #[error("Other failure: {0}")]
+    Other(String),
+}
+
+/// Job status report
+///
+/// This includes a URL which updates with redirect but is otherwise the same as
+/// `bits::status::BitsJobStatus`.
+#[derive(Clone, Debug)]
+pub struct JobStatus {
+    pub state: BitsJobState,
+    pub progress: BitsJobProgress,
+    pub error_count: u32,
+    pub error: Option<JobError>,
+    pub times: BitsJobTimes,
+    /// None means same as last time
+    pub url: Option<OsString>,
+}
+
+/// Job error report
+#[derive(Clone, Debug, Error)]
+#[error("Job error in context {context_str}: {error}")]
+pub struct JobError {
+    pub context: BitsErrorContext,
+    pub context_str: String,
+    pub error: HResultMessage,
+}
diff --git a/toolkit/components/bitsdownload/bits_client/src/in_process/mod.rs b/toolkit/components/bitsdownload/bits_client/src/in_process/mod.rs
new file mode 100644
index 0000000000..c7f6869167
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/src/in_process/mod.rs
@@ -0,0 +1,504 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use std::cmp;
+use std::collections::{hash_map, HashMap};
+use std::ffi;
+use std::path;
+use std::sync::{Arc, Condvar, Mutex, Weak};
+use std::time::{Duration, Instant};
+
+use bits::{
+    BackgroundCopyManager, BitsJob, BitsJobPriority, BitsProxyUsage, BG_S_PARTIAL_COMPLETE, E_FAIL,
+};
+use guid_win::Guid;
+
+use bits_protocol::*;
+
+use super::Error;
+
+// This is a macro in order to use the NotFound and GetJob variants from whatever enum is in scope.
+macro_rules! get_job {
+    ($bcm:ident, $guid:expr, $name:expr) => {{
+        $bcm = BackgroundCopyManager::connect().map_err(|e| {
+            ConnectBcm(HResultMessage {
+                hr: e.code(),
+                message: e.to_string(),
+            })
+        })?;
+        $bcm.find_job_by_guid_and_name($guid, $name)
+            .map_err(|e| GetJob($crate::in_process::format_error(&$bcm, e)))?
+            .ok_or(NotFound)?
+    }};
+}
+
+fn format_error(bcm: &BackgroundCopyManager, error: comedy::HResult) -> HResultMessage {
+    let bits_description = bcm.get_error_description(error.code()).ok();
+
+    HResultMessage {
+        hr: error.code(),
+        message: if let Some(desc) = bits_description {
+            format!("{}: {}", error, desc)
+        } else {
+            format!("{}", error)
+        },
+    }
+}
+
+// The in-process client uses direct BITS calls via the `bits` crate.
+// See the corresponding functions in BitsClient.
+pub struct InProcessClient {
+    job_name: ffi::OsString,
+    save_path_prefix: path::PathBuf,
+    monitors: HashMap<Guid, InProcessMonitorControl>,
+}
+
+impl InProcessClient {
+    pub fn new(
+        job_name: ffi::OsString,
+        save_path_prefix: ffi::OsString,
+    ) -> Result<InProcessClient, Error> {
+        Ok(InProcessClient {
+            job_name,
+            save_path_prefix: path::PathBuf::from(save_path_prefix),
+            monitors: HashMap::new(),
+        })
+    }
+
+    pub fn start_job(
+        &mut self,
+        url: ffi::OsString,
+        save_path: ffi::OsString,
+        proxy_usage: BitsProxyUsage,
+        no_progress_timeout_secs: u32,
+        monitor_interval_millis: u32,
+    ) -> Result<(StartJobSuccess, InProcessMonitor), StartJobFailure> {
+        use StartJobFailure::*;
+
+        let full_path = self.save_path_prefix.join(save_path);
+
+        // Verify that `full_path` is under the directory called `save_path_prefix`.
+        {
+            let canonical_prefix = self.save_path_prefix.canonicalize().map_err(|e| {
+                ArgumentValidation(format!("save_path_prefix.canonicalize(): {}", e))
+            })?;
+            // Full path minus file name, canonicalize() fails with nonexistent files, but the
+            // parent directory ought to exist.
+            let canonical_full_path = full_path
+                .parent()
+                .ok_or_else(|| ArgumentValidation("full_path.parent(): None".into()))?
+                .canonicalize()
+                .map_err(|e| {
+                    ArgumentValidation(format!("full_path.parent().canonicalize(): {}", e))
+                })?;
+
+            if !canonical_full_path.starts_with(&canonical_prefix) {
+                return Err(ArgumentValidation(format!(
+                    "{:?} is not within {:?}",
+                    canonical_full_path, canonical_prefix
+                )));
+            }
+        }
+
+        // TODO: Should the job be explicitly cleaned up if this fn can't return success?
+        // If the job is dropped before `AddFile` succeeds, I think it automatically gets
+        // deleted from the queue. There is only one fallible call after that (`Resume`).
+
+        let bcm = BackgroundCopyManager::connect().map_err(|e| {
+            ConnectBcm(HResultMessage {
+                hr: e.code(),
+                message: e.to_string(),
+            })
+        })?;
+        let mut job = bcm
+            .create_job(&self.job_name)
+            .map_err(|e| Create(format_error(&bcm, e)))?;
+
+        let guid = job.guid().map_err(|e| OtherBITS(format_error(&bcm, e)))?;
+
+        (|| {
+            job.set_proxy_usage(proxy_usage)?;
+            job.set_minimum_retry_delay(60)?;
+            job.set_no_progress_timeout(no_progress_timeout_secs)?;
+            job.set_redirect_report()?;
+
+            job.set_priority(BitsJobPriority::Foreground)?;
+
+            Ok(())
+        })()
+        .map_err(|e| ApplySettings(format_error(&bcm, e)))?;
+
+        let (client, control) = InProcessMonitor::new(&mut job, monitor_interval_millis)
+            .map_err(|e| OtherBITS(format_error(&bcm, e)))?;
+
+        job.add_file(&url, &full_path.into_os_string())
+            .map_err(|e| AddFile(format_error(&bcm, e)))?;
+
+        job.resume().map_err(|e| Resume(format_error(&bcm, e)))?;
+
+        self.monitors.insert(guid.clone(), control);
+
+        Ok((StartJobSuccess { guid }, client))
+    }
+
+    pub fn monitor_job(
+        &mut self,
+        guid: Guid,
+        interval_millis: u32,
+    ) -> Result<InProcessMonitor, MonitorJobFailure> {
+        use MonitorJobFailure::*;
+
+        // Stop any preexisting monitor for the same guid.
+        let _ = self.stop_update(guid.clone());
+
+        let bcm;
+        let (client, control) =
+            InProcessMonitor::new(&mut get_job!(bcm, &guid, &self.job_name), interval_millis)
+                .map_err(|e| OtherBITS(format_error(&bcm, e)))?;
+
+        self.monitors.insert(guid, control);
+
+        Ok(client)
+    }
+
+    pub fn suspend_job(&mut self, guid: Guid) -> Result<(), SuspendJobFailure> {
+        use SuspendJobFailure::*;
+
+        let bcm;
+        get_job!(bcm, &guid, &self.job_name)
+            .suspend()
+            .map_err(|e| SuspendJob(format_error(&bcm, e)))?;
+
+        Ok(())
+    }
+
+    pub fn resume_job(&mut self, guid: Guid) -> Result<(), ResumeJobFailure> {
+        use ResumeJobFailure::*;
+
+        let bcm;
+        get_job!(bcm, &guid, &self.job_name)
+            .resume()
+            .map_err(|e| ResumeJob(format_error(&bcm, e)))?;
+
+        Ok(())
+    }
+
+    pub fn set_job_priority(
+        &mut self,
+        guid: Guid,
+        foreground: bool,
+    ) -> Result<(), SetJobPriorityFailure> {
+        use SetJobPriorityFailure::*;
+
+        let priority = if foreground {
+            BitsJobPriority::Foreground
+        } else {
+            BitsJobPriority::Normal
+        };
+
+        let bcm;
+        get_job!(bcm, &guid, &self.job_name)
+            .set_priority(priority)
+            .map_err(|e| ApplySettings(format_error(&bcm, e)))?;
+
+        Ok(())
+    }
+
+    pub fn set_no_progress_timeout(
+        &mut self,
+        guid: Guid,
+        timeout_secs: u32,
+    ) -> Result<(), SetNoProgressTimeoutFailure> {
+        use SetNoProgressTimeoutFailure::*;
+
+        let bcm;
+        get_job!(bcm, &guid, &self.job_name)
+            .set_no_progress_timeout(timeout_secs)
+            .map_err(|e| ApplySettings(format_error(&bcm, e)))?;
+
+        Ok(())
+    }
+
+    fn get_monitor_control_sender(&mut self, guid: Guid) -> Option<Arc<ControlPair>> {
+        if let hash_map::Entry::Occupied(occ) = self.monitors.entry(guid) {
+            if let Some(sender) = occ.get().0.upgrade() {
+                Some(sender)
+            } else {
+                // Remove dangling Weak
+                occ.remove_entry();
+                None
+            }
+        } else {
+            None
+        }
+    }
+
+    pub fn set_update_interval(
+        &mut self,
+        guid: Guid,
+        interval_millis: u32,
+    ) -> Result<(), SetUpdateIntervalFailure> {
+        use SetUpdateIntervalFailure::*;
+
+        if let Some(sender) = self.get_monitor_control_sender(guid) {
+            let mut s = sender.1.lock().unwrap();
+            s.interval_millis = interval_millis;
+            sender.0.notify_all();
+            Ok(())
+        } else {
+            Err(NotFound)
+        }
+    }
+
+    pub fn stop_update(&mut self, guid: Guid) -> Result<(), SetUpdateIntervalFailure> {
+        use SetUpdateIntervalFailure::*;
+
+        if let Some(sender) = self.get_monitor_control_sender(guid) {
+            sender.1.lock().unwrap().shutdown = true;
+            sender.0.notify_all();
+            Ok(())
+        } else {
+            Err(NotFound)
+        }
+    }
+
+    pub fn complete_job(&mut self, guid: Guid) -> Result<(), CompleteJobFailure> {
+        use CompleteJobFailure::*;
+
+        let bcm;
+        get_job!(bcm, &guid, &self.job_name)
+            .complete()
+            .map_err(|e| CompleteJob(format_error(&bcm, e)))
+            .and_then(|hr| {
+                if hr == BG_S_PARTIAL_COMPLETE as i32 {
+                    Err(PartialComplete)
+                } else {
+                    Ok(())
+                }
+            })?;
+
+        let _ = self.stop_update(guid);
+
+        Ok(())
+    }
+
+    pub fn cancel_job(&mut self, guid: Guid) -> Result<(), CancelJobFailure> {
+        use CancelJobFailure::*;
+
+        let bcm;
+        get_job!(bcm, &guid, &self.job_name)
+            .cancel()
+            .map_err(|e| CancelJob(format_error(&bcm, e)))?;
+
+        let _ = self.stop_update(guid);
+
+        Ok(())
+    }
+}
+
+// InProcessMonitor can be used on any thread, and `ControlPair` can be synchronously modified to
+// control a blocked `get_status` call from another thread.
+pub struct InProcessMonitor {
+    vars: Arc<ControlPair>,
+    guid: Guid,
+    last_status_time: Option<Instant>,
+    last_url: Option<ffi::OsString>,
+}
+
+// The `Condvar` is notified when `InProcessMonitorVars` changes.
+type ControlPair = (Condvar, Mutex<InProcessMonitorVars>);
+struct InProcessMonitorControl(Weak<ControlPair>);
+
+// RefUnwindSafe is not impl'd for Condvar but likely should be,
+// see https://github.com/rust-lang/rust/issues/54768
+impl std::panic::RefUnwindSafe for InProcessMonitorControl {}
+
+struct InProcessMonitorVars {
+    interval_millis: u32,
+    notified: bool,
+    shutdown: bool,
+}
+
+impl InProcessMonitor {
+    fn new(
+        job: &mut BitsJob,
+        interval_millis: u32,
+    ) -> Result<(InProcessMonitor, InProcessMonitorControl), comedy::HResult> {
+        let guid = job.guid()?;
+
+        let vars = Arc::new((
+            Condvar::new(),
+            Mutex::new(InProcessMonitorVars {
+                interval_millis,
+                notified: false,
+                shutdown: false,
+            }),
+        ));
+
+        let transferred_control = InProcessMonitorControl(Arc::downgrade(&vars));
+        let transferred_cb = Box::new(move || {
+            if let Some(control) = transferred_control.0.upgrade() {
+                if let Ok(mut vars) = control.1.lock() {
+                    vars.notified = true;
+                    control.0.notify_all();
+                    return Ok(());
+                }
+            }
+            Err(E_FAIL)
+        });
+
+        let error_control = InProcessMonitorControl(Arc::downgrade(&vars));
+        let error_cb = Box::new(move || {
+            if let Some(control) = error_control.0.upgrade() {
+                if let Ok(mut vars) = control.1.lock() {
+                    vars.notified = true;
+                    control.0.notify_all();
+                    return Ok(());
+                }
+            }
+            Err(E_FAIL)
+        });
+
+        // Note: These callbacks are never explicitly cleared. They will be freed when the
+        // job is deleted from BITS, and they will be cleared if an attempt is made to call them
+        // when they are no longer valid (e.g. after the process exits). This is done mostly for
+        // simplicity and should be safe.
+
+        job.register_callbacks(Some(transferred_cb), Some(error_cb), None)?;
+
+        let control = InProcessMonitorControl(Arc::downgrade(&vars));
+
+        let monitor = InProcessMonitor {
+            guid,
+            vars,
+            last_status_time: None,
+            last_url: None,
+        };
+
+        Ok((monitor, control))
+    }
+
+    pub fn get_status(
+        &mut self,
+        timeout_millis: u32,
+    ) -> Result<Result<JobStatus, HResultMessage>, Error> {
+        let timeout = Duration::from_millis(u64::from(timeout_millis));
+
+        let started = Instant::now();
+        let timeout_end = started + timeout;
+
+        {
+            let mut s = self.vars.1.lock().unwrap();
+            loop {
+                let wait_start = Instant::now();
+
+                if s.shutdown {
+                    // Disconnected, immediately return error.
+                    // Note: Shutdown takes priority over simultaneous notification.
+                    return Err(Error::NotConnected);
+                }
+
+                if wait_start >= timeout_end {
+                    // Timed out, immediately return timeout error.
+                    // This should not normally happen with the in-process monitor, but the
+                    // monitor interval could be longer than the timeout.
+                    s.shutdown = true;
+                    return Err(Error::Timeout);
+                }
+
+                // Get the interval every pass through the loop, in case it has changed.
+                let interval = Duration::from_millis(u64::from(s.interval_millis));
+
+                let wait_until = self
+                    .last_status_time
+                    .map(|last_status_time| cmp::min(last_status_time + interval, timeout_end));
+
+                if s.notified {
+                    // Notified, exit loop to get status.
+                    s.notified = false;
+                    break;
+                }
+
+                if wait_until.is_none() {
+                    // First status report, no waiting, exit loop to get status.
+                    break;
+                }
+
+                let wait_until = wait_until.unwrap();
+
+                if wait_until <= wait_start {
+                    // No time left to wait. This can't be due to timeout because
+                    // `wait_until <= wait_start < timeout_end`.
+                    // Status report due, exit loop to get status.
+                    break;
+                }
+
+                // Wait.
+                // Do not attempt to recover from poisoned Mutex.
+                s = self
+                    .vars
+                    .0
+                    .wait_timeout(s, wait_until - wait_start)
+                    .unwrap()
+                    .0;
+
+                // Mutex re-acquired, loop.
+            }
+        }
+
+        // No error yet, start getting status now.
+        self.last_status_time = Some(Instant::now());
+
+        let bcm = match BackgroundCopyManager::connect() {
+            Ok(bcm) => bcm,
+            Err(e) => {
+                // On any error, disconnect.
+                self.vars.1.lock().unwrap().shutdown = true;
+
+                // Errors below can use the BCM to do `format_error()`, but this one just gets the
+                // basic `comedy::HResult` treatment.
+                return Ok(Err(HResultMessage {
+                    hr: e.code(),
+                    message: format!("{}", e),
+                }));
+            }
+        };
+
+        Ok((|| {
+            let mut job = bcm.get_job_by_guid(&self.guid)?;
+
+            let status = job.get_status()?;
+            let url = job.get_first_file()?.get_remote_name()?;
+
+            Ok(JobStatus {
+                state: status.state,
+                progress: status.progress,
+                error_count: status.error_count,
+                error: status.error.map(|e| JobError {
+                    context: e.context,
+                    context_str: e.context_str,
+                    error: HResultMessage {
+                        hr: e.error,
+                        message: e.error_str,
+                    },
+                }),
+                times: status.times,
+                url: if self.last_url.is_some() && *self.last_url.as_ref().unwrap() == url {
+                    None
+                } else {
+                    self.last_url = Some(url);
+                    self.last_url.clone()
+                },
+            })
+        })()
+        .map_err(|e| {
+            // On any error, disconnect.
+            self.vars.1.lock().unwrap().shutdown = true;
+            format_error(&bcm, e)
+        }))
+    }
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/toolkit/components/bitsdownload/bits_client/src/in_process/tests.rs b/toolkit/components/bitsdownload/bits_client/src/in_process/tests.rs
new file mode 100644
index 0000000000..5545f6bd74
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/src/in_process/tests.rs
@@ -0,0 +1,628 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+// These are full integration tests that use the BITS service.
+
+// TODO
+// It may make sense to restrict how many tests can run at once. BITS is only supposed to support
+// four simultaneous notifications per user, it is not impossible that this test suite could
+// exceed that.
+
+#![cfg(test)]
+extern crate bits;
+extern crate lazy_static;
+extern crate rand;
+extern crate regex;
+extern crate tempfile;
+
+use std::ffi::{OsStr, OsString};
+use std::fs::{self, File};
+use std::io::{Read, Write};
+use std::net::{SocketAddr, TcpListener, TcpStream};
+use std::panic;
+use std::sync::{Arc, Condvar, Mutex};
+use std::thread::{self, JoinHandle};
+use std::time::{Duration, Instant};
+
+use self::{
+    bits::BackgroundCopyManager,
+    lazy_static::lazy_static,
+    rand::{thread_rng, Rng},
+    regex::bytes::Regex,
+    tempfile::{Builder, TempDir},
+};
+use super::{
+    super::{BitsJobState, Error},
+    BitsProxyUsage, InProcessClient, StartJobSuccess,
+};
+
+static SERVER_ADDRESS: [u8; 4] = [127, 0, 0, 1];
+
+lazy_static! {
+    static ref TEST_MUTEX: Mutex<()> = Mutex::new(());
+}
+
+fn format_job_name(name: &str) -> OsString {
+    format!("InProcessClient Test {}", name).into()
+}
+
+fn format_dir_prefix(tmp_dir: &TempDir) -> OsString {
+    let mut dir = tmp_dir.path().to_path_buf().into_os_string();
+    dir.push("\\");
+    dir
+}
+
+fn cancel_jobs(name: &OsStr) {
+    BackgroundCopyManager::connect()
+        .unwrap()
+        .cancel_jobs_by_name(name)
+        .unwrap();
+}
+
+struct HttpServerResponses {
+    body: Box<[u8]>,
+    delay: u64,
+    //error: Box<[u8]>,
+}
+
+struct MockHttpServerHandle {
+    port: u16,
+    join: Option<JoinHandle<Result<(), ()>>>,
+    shutdown: Arc<(Mutex<bool>, Condvar)>,
+}
+
+impl MockHttpServerHandle {
+    fn shutdown(&mut self) {
+        if self.join.is_none() {
+            return;
+        }
+
+        {
+            let &(ref lock, ref cvar) = &*self.shutdown;
+            let mut shutdown = lock.lock().unwrap();
+
+            if !*shutdown {
+                *shutdown = true;
+                cvar.notify_all();
+            }
+        }
+        // Wake up the server from `accept()`. Will fail if the server wasn't listening.
+        let _ = TcpStream::connect_timeout(
+            &(SERVER_ADDRESS, self.port).into(),
+            Duration::from_millis(10_000),
+        );
+
+        match self.join.take().unwrap().join() {
+            Ok(_) => {}
+            Err(p) => panic::resume_unwind(p),
+        }
+    }
+
+    fn format_url(&self, name: &str) -> OsString {
+        format!(
+            "http://{}/{}",
+            SocketAddr::from((SERVER_ADDRESS, self.port)),
+            name
+        )
+        .into()
+    }
+}
+
+fn mock_http_server(name: &'static str, responses: HttpServerResponses) -> MockHttpServerHandle {
+    let mut bind_retries = 10;
+    let shutdown = Arc::new((Mutex::new(false), Condvar::new()));
+    let caller_shutdown = shutdown.clone();
+
+    let (listener, port) = loop {
+        let port = thread_rng().gen_range(1024..0x1_0000u32) as u16;
+        match TcpListener::bind(SocketAddr::from((SERVER_ADDRESS, port))) {
+            Ok(listener) => {
+                break (listener, port);
+            }
+            r @ Err(_) => {
+                if bind_retries == 0 {
+                    r.unwrap();
+                }
+                bind_retries -= 1;
+                continue;
+            }
+        }
+    };
+
+    let join = thread::Builder::new()
+        .name(format!("mock_http_server {}", name))
+        .spawn(move || {
+            // returns Err(()) if server should shut down immediately
+            fn check_shutdown(shutdown: &Arc<(Mutex<bool>, Condvar)>) -> Result<(), ()> {
+                if *shutdown.0.lock().unwrap() {
+                    Err(())
+                } else {
+                    Ok(())
+                }
+            }
+            fn sleep(shutdown: &Arc<(Mutex<bool>, Condvar)>, delay_millis: u64) -> Result<(), ()> {
+                let sleep_start = Instant::now();
+                let sleep_end = sleep_start + Duration::from_millis(delay_millis);
+
+                let (ref lock, ref cvar) = **shutdown;
+                let mut shutdown_requested = lock.lock().unwrap();
+                loop {
+                    if *shutdown_requested {
+                        return Err(());
+                    }
+
+                    let before_wait = Instant::now();
+                    if before_wait >= sleep_end {
+                        return Ok(());
+                    }
+                    let wait_dur = sleep_end - before_wait;
+                    shutdown_requested = cvar.wait_timeout(shutdown_requested, wait_dur).unwrap().0;
+                }
+            }
+
+            let error_404 = Regex::new(r"^((GET)|(HEAD)) [[:print:]]*/error_404 ").unwrap();
+            let error_500 = Regex::new(r"^((GET)|(HEAD)) [[:print:]]*/error_500 ").unwrap();
+
+            loop {
+                let (mut socket, _addr) = listener.accept().expect("accept should succeed");
+
+                socket
+                    .set_read_timeout(Some(Duration::from_millis(10_000)))
+                    .unwrap();
+                let mut s = Vec::new();
+                for b in Read::by_ref(&mut socket).bytes() {
+                    if b.is_err() {
+                        eprintln!("read error {:?}", b);
+                        break;
+                    }
+                    let b = b.unwrap();
+                    s.push(b);
+                    if s.ends_with(b"\r\n\r\n") {
+                        break;
+                    }
+                    check_shutdown(&shutdown)?;
+                }
+
+                // request received
+
+                check_shutdown(&shutdown)?;
+
+                // Special error URIs
+                if error_404.is_match(&s) {
+                    sleep(&shutdown, responses.delay)?;
+                    let result = socket
+                        .write(b"HTTP/1.1 404 Not Found\r\n\r\n")
+                        .and_then(|_| socket.flush());
+                    if let Err(e) = result {
+                        eprintln!("error writing 404 header {:?}", e);
+                    }
+                    continue;
+                }
+
+                if error_500.is_match(&s) {
+                    sleep(&shutdown, responses.delay)?;
+                    let result = socket
+                        .write(b"HTTP/1.1 500 Internal Server Error\r\n\r\n")
+                        .and_then(|_| socket.flush());
+                    if let Err(e) = result {
+                        eprintln!("error writing 500 header {:?}", e);
+                    }
+                    continue;
+                }
+
+                // Response with a body.
+                if s.starts_with(b"HEAD") || s.starts_with(b"GET") {
+                    let result = socket
+                        .write(
+                            format!(
+                                "HTTP/1.1 200 OK\r\nContent-Length: {}\r\n\r\n",
+                                responses.body.len()
+                            )
+                            .as_bytes(),
+                        )
+                        .and_then(|_| socket.flush());
+                    if let Err(e) = result {
+                        eprintln!("error writing header {:?}", e);
+                        continue;
+                    }
+                }
+
+                if s.starts_with(b"GET") {
+                    sleep(&shutdown, responses.delay)?;
+                    let result = socket.write(&responses.body).and_then(|_| socket.flush());
+                    if let Err(e) = result {
+                        eprintln!("error writing content {:?}", e);
+                        continue;
+                    }
+                }
+            }
+        });
+
+    MockHttpServerHandle {
+        port,
+        join: Some(join.unwrap()),
+        shutdown: caller_shutdown,
+    }
+}
+
+// Test wrapper to ensure jobs are canceled, set up name strings
+macro_rules! test {
+    (fn $name:ident($param:ident : &str, $tmpdir:ident : &TempDir) $body:block) => {
+        #[test]
+        fn $name() {
+            let $param = stringify!($name);
+            let $tmpdir = &Builder::new().prefix($param).tempdir().unwrap();
+
+            let result = panic::catch_unwind(|| $body);
+
+            cancel_jobs(&format_job_name($param));
+
+            if let Err(e) = result {
+                panic::resume_unwind(e);
+            }
+        }
+    };
+}
+
+test! {
+    fn start_monitor_and_cancel(name: &str, tmp_dir: &TempDir) {
+        let mut server = mock_http_server(name, HttpServerResponses {
+            body: name.to_owned().into_boxed_str().into_boxed_bytes(),
+            delay: 10_000,
+        });
+
+        let mut client = InProcessClient::new(format_job_name(name), tmp_dir.path().into()).unwrap();
+
+        let no_progress_timeout_secs = 60;
+        let interval = 10_000;
+        let timeout = 10_000;
+
+        let (StartJobSuccess {guid}, mut monitor) =
+            client.start_job(
+                server.format_url(name),
+                name.into(),
+                BitsProxyUsage::Preconfig,
+                no_progress_timeout_secs,
+                interval,
+                ).unwrap();
+
+        // cancel in ~250ms
+        let _join = thread::Builder::new()
+            .spawn(move || {
+                thread::sleep(Duration::from_millis(250));
+                client.cancel_job(guid).unwrap();
+            });
+
+        let start = Instant::now();
+
+        // First immediate report
+        monitor.get_status(timeout).expect("should initially be ok").unwrap();
+
+        // ~250ms the cancel should cause an immediate disconnect (otherwise we wouldn't get
+        // an update until 10s when the transfer completes or the interval expires)
+        match monitor.get_status(timeout) {
+            Err(Error::NotConnected) => {},
+            Ok(r) => panic!("unexpected success from get_status() {:?}", r),
+            Err(e) => panic!("unexpected failure from get_status() {:?}", e),
+        }
+        assert!(start.elapsed() < Duration::from_millis(9_000));
+
+        server.shutdown();
+    }
+}
+
+test! {
+    fn start_monitor_and_complete(name: &str, tmp_dir: &TempDir) {
+        let file_path = tmp_dir.path().join(name);
+
+        let mut server = mock_http_server(name, HttpServerResponses {
+            body: name.to_owned().into_boxed_str().into_boxed_bytes(),
+            delay: 500,
+        });
+
+        let mut client = InProcessClient::new(format_job_name(name), format_dir_prefix(tmp_dir)).unwrap();
+
+        let no_progress_timeout_secs = 60;
+        let interval = 100;
+        let timeout = 10_000;
+
+        let (StartJobSuccess {guid}, mut monitor) =
+            client.start_job(
+                server.format_url(name),
+                name.into(),
+                BitsProxyUsage::Preconfig,
+                no_progress_timeout_secs,
+                interval,
+                ).unwrap();
+
+        let start = Instant::now();
+
+        // Get status reports until transfer finishes (~500ms)
+        let mut completed = false;
+        loop {
+            match monitor.get_status(timeout) {
+                Err(e) => {
+                    if completed {
+                        break;
+                    } else {
+                        panic!("monitor failed before completion {:?}", e);
+                    }
+                }
+                Ok(Ok(status)) => match BitsJobState::from(status.state) {
+                    BitsJobState::Queued | BitsJobState::Connecting
+                        | BitsJobState::Transferring => {
+                            //eprintln!("{:?}", BitsJobState::from(status.state));
+                            //eprintln!("{:?}", status);
+
+                            // As long as there is no error, setting the timeout to 0 will not
+                            // fail an active transfer.
+                            client.set_no_progress_timeout(guid.clone(), 0).unwrap();
+                        }
+                    BitsJobState::Transferred => {
+                        client.complete_job(guid.clone()).unwrap();
+                        completed = true;
+                    }
+                    _ => {
+                        panic!("{:?}", status);
+                    }
+                }
+                Ok(Err(e)) => panic!("{:?}", e),
+            }
+
+            // Timeout to prevent waiting forever
+            assert!(start.elapsed() < Duration::from_millis(60_000));
+        }
+
+
+        // Verify file contents
+        let result = panic::catch_unwind(|| {
+            let mut file = File::open(file_path.clone()).unwrap();
+            let mut v = Vec::new();
+            file.read_to_end(&mut v).unwrap();
+            assert_eq!(v, name.as_bytes());
+        });
+
+        let _ = fs::remove_file(file_path);
+
+        if let Err(e) = result {
+            panic::resume_unwind(e);
+        }
+
+        // Save this for last to ensure the file is removed.
+        server.shutdown();
+    }
+}
+
+test! {
+    fn async_transferred_notification(name: &str, tmp_dir: &TempDir) {
+        let mut server = mock_http_server(name, HttpServerResponses {
+            body: name.to_owned().into_boxed_str().into_boxed_bytes(),
+            delay: 250,
+        });
+
+        let mut client = InProcessClient::new(format_job_name(name), format_dir_prefix(tmp_dir)).unwrap();
+
+        let no_progress_timeout_secs = 60;
+        let interval = 60_000;
+        let timeout = 10_000;
+
+        let (_, mut monitor) =
+            client.start_job(
+                server.format_url(name),
+                name.into(),
+                BitsProxyUsage::Preconfig,
+                no_progress_timeout_secs,
+                interval,
+                ).unwrap();
+
+        // Start the timer now, the initial job creation may be delayed by BITS service startup.
+        let start = Instant::now();
+
+        // First report, immediate
+        let report_1 = monitor.get_status(timeout).expect("should initially be ok").unwrap();
+        let elapsed_to_report_1 = start.elapsed();
+
+        // Transferred notification should come when the job completes in ~250 ms, otherwise we
+        // will be stuck until timeout.
+        let report_2 = monitor.get_status(timeout).expect("should get status update").unwrap();
+        let elapsed_to_report_2 = start.elapsed();
+        assert!(elapsed_to_report_2 < Duration::from_millis(9_000));
+        assert_eq!(report_2.state, BitsJobState::Transferred);
+
+        let short_timeout = 500;
+        let report_3 = monitor.get_status(short_timeout);
+        let elapsed_to_report_3 = start.elapsed();
+
+        if let Ok(report_3) = report_3 {
+          panic!("should be disconnected\n\
+                  report_1 ({}.{:03}): {:?}\n\
+                  report_2 ({}.{:03}): {:?}\n\
+                  report_3 ({}.{:03}): {:?}",
+                  elapsed_to_report_1.as_secs(), elapsed_to_report_1.subsec_millis(), report_1,
+                  elapsed_to_report_2.as_secs(), elapsed_to_report_2.subsec_millis(), report_2,
+                  elapsed_to_report_3.as_secs(), elapsed_to_report_3.subsec_millis(), report_3,
+                  );
+        }
+
+        server.shutdown();
+
+        // job will be cancelled by macro
+    }
+}
+
+test! {
+    fn change_interval(name: &str, tmp_dir: &TempDir) {
+        let mut server = mock_http_server(name, HttpServerResponses {
+            body: name.to_owned().into_boxed_str().into_boxed_bytes(),
+            delay: 1000,
+        });
+
+        let mut client = InProcessClient::new(format_job_name(name), format_dir_prefix(tmp_dir)).unwrap();
+
+        let no_progress_timeout_secs = 0;
+        let interval = 60_000;
+        let timeout = 10_000;
+
+        let (StartJobSuccess { guid }, mut monitor) =
+            client.start_job(
+                server.format_url(name),
+                name.into(),
+                BitsProxyUsage::Preconfig,
+                no_progress_timeout_secs,
+                interval,
+                ).unwrap();
+
+        let start = Instant::now();
+
+        // reduce monitor interval in ~250ms to 500ms
+        let _join = thread::Builder::new()
+            .spawn(move || {
+                thread::sleep(Duration::from_millis(250));
+                client.set_update_interval(guid, 500).unwrap();
+            });
+
+        // First immediate report
+        monitor.get_status(timeout).expect("should initially be ok").unwrap();
+
+        // Next report should be rescheduled to 500ms by the spawned thread, otherwise no status
+        // until the original 10s interval.
+        monitor.get_status(timeout).expect("expected second status").unwrap();
+        assert!(start.elapsed() < Duration::from_millis(9_000));
+        assert!(start.elapsed() > Duration::from_millis(400));
+
+        server.shutdown();
+
+        // job will be cancelled by macro
+    }
+}
+
+test! {
+    fn async_error_notification(name: &str, tmp_dir: &TempDir) {
+        let mut server = mock_http_server(name, HttpServerResponses {
+            body: name.to_owned().into_boxed_str().into_boxed_bytes(),
+            delay: 100,
+        });
+
+        let mut client = InProcessClient::new(format_job_name(name), format_dir_prefix(tmp_dir)).unwrap();
+
+        let no_progress_timeout_secs = 60;
+        let interval = 60_000;
+        let timeout = 10_000;
+
+        let (_, mut monitor) =
+            client.start_job(
+                server.format_url("error_404"),
+                name.into(),
+                BitsProxyUsage::Preconfig,
+                no_progress_timeout_secs,
+                interval,
+                ).unwrap();
+
+        // Start the timer now, the initial job creation may be delayed by BITS service startup.
+        let start = Instant::now();
+
+        // First immediate report
+        monitor.get_status(timeout).expect("should initially be ok").unwrap();
+
+        // Error notification should come with HEAD response in 100ms, otherwise no status until
+        // 10s interval or timeout.
+        let status = monitor.get_status(timeout).expect("should get status update").unwrap();
+        assert!(start.elapsed() < Duration::from_millis(9_000));
+        assert_eq!(status.state, BitsJobState::Error);
+
+        server.shutdown();
+
+        // job will be cancelled by macro
+    }
+}
+
+test! {
+    fn transient_error(name: &str, tmp_dir: &TempDir) {
+        let mut server = mock_http_server(name, HttpServerResponses {
+            body: name.to_owned().into_boxed_str().into_boxed_bytes(),
+            delay: 100,
+        });
+
+        let mut client = InProcessClient::new(format_job_name(name), format_dir_prefix(tmp_dir)).unwrap();
+
+        let no_progress_timeout_secs = 60;
+        let interval = 1_000;
+        let timeout = 10_000;
+
+        let (StartJobSuccess { guid }, mut monitor) =
+            client.start_job(
+                server.format_url("error_500"),
+                name.into(),
+                BitsProxyUsage::Preconfig,
+                no_progress_timeout_secs,
+                interval,
+                ).unwrap();
+
+        // Start the timer now, the initial job creation may be delayed by BITS service startup.
+        let start = Instant::now();
+
+        // First immediate report
+        monitor.get_status(timeout).expect("should initially be ok").unwrap();
+
+        // Transient error notification should come when the interval expires in ~1s.
+        let status = monitor.get_status(timeout).expect("should get status update").unwrap();
+        assert!(start.elapsed() > Duration::from_millis(900));
+        assert!(start.elapsed() < Duration::from_millis(9_000));
+        assert_eq!(status.state, BitsJobState::TransientError);
+
+        // Lower no progress timeout to 0
+        let set_timeout_at = Instant::now();
+        client.set_no_progress_timeout(guid, 0).unwrap();
+
+        // Should convert the transient error to a permanent error immediately.
+        let status = monitor.get_status(timeout).expect("should get status update").unwrap();
+        assert!(set_timeout_at.elapsed() < Duration::from_millis(500));
+        assert_eq!(status.state, BitsJobState::Error);
+
+        server.shutdown();
+
+        // job will be cancelled by macro
+    }
+}
+
+test! {
+    fn transient_to_permanent_error(name: &str, tmp_dir: &TempDir) {
+        let mut server = mock_http_server(name, HttpServerResponses {
+            body: name.to_owned().into_boxed_str().into_boxed_bytes(),
+            delay: 100,
+        });
+
+        let mut client = InProcessClient::new(format_job_name(name), format_dir_prefix(tmp_dir)).unwrap();
+
+        let no_progress_timeout_secs = 0;
+        let interval = 1_000;
+        let timeout = 10_000;
+
+        let (_, mut monitor) =
+            client.start_job(
+                server.format_url("error_500"),
+                name.into(),
+                BitsProxyUsage::Preconfig,
+                no_progress_timeout_secs,
+                interval,
+                ).unwrap();
+
+        // Start the timer now, the initial job creation may be delayed by BITS service startup.
+        let start = Instant::now();
+
+        // First immediate report
+        monitor.get_status(timeout).expect("should initially be ok").unwrap();
+
+        // 500 is a transient error, but with no_progress_timeout_secs = 0 it should immediately
+        // produce an error notification with the HEAD response in 100ms. Otherwise no status
+        // until 10s interval or timeout.
+        let status = monitor.get_status(timeout).expect("should get status update").unwrap();
+        assert!(start.elapsed() < Duration::from_millis(500));
+        assert_eq!(status.state, BitsJobState::Error);
+
+        server.shutdown();
+
+        // job will be cancelled by macro
+    }
+}
diff --git a/toolkit/components/bitsdownload/bits_client/src/lib.rs b/toolkit/components/bitsdownload/bits_client/src/lib.rs
new file mode 100644
index 0000000000..7db887bbc5
--- /dev/null
+++ b/toolkit/components/bitsdownload/bits_client/src/lib.rs
@@ -0,0 +1,258 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! An interface for managing and monitoring BITS jobs.
+//!
+//! BITS is a Windows service for performing downloads in the background, independent from an
+//! application, usually via HTTP/HTTPS.
+//!
+//! [`BitsClient`](enum.BitsClient.html) is the main interface, used to issue commands.
+//!
+//! [`BitsMonitorClient`](enum.BitsMonitorClient.html) delivers periodic status reports about a
+//! job.
+//!
+//! Microsoft's documentation for BITS can be found at
+//! <https://docs.microsoft.com/en-us/windows/desktop/Bits/background-intelligent-transfer-service-portal>
+
+extern crate bits;
+extern crate comedy;
+extern crate guid_win;
+extern crate thiserror;
+
+pub mod bits_protocol;
+
+mod in_process;
+
+use std::ffi;
+
+use bits_protocol::*;
+use thiserror::Error;
+
+pub use bits::status::{BitsErrorContext, BitsJobState, BitsJobTimes};
+pub use bits::{BitsJobProgress, BitsJobStatus, BitsProxyUsage};
+pub use bits_protocol::{JobError, JobStatus};
+pub use comedy::HResult;
+pub use guid_win::Guid;
+
+// These errors would come from a Local Service client but are mostly unused currently.
+// PipeError properly lives in the crate that deals with named pipes, but it isn't in use now.
+#[derive(Clone, Debug, Eq, Error, PartialEq)]
+pub enum PipeError {
+    #[error("Pipe is not connected")]
+    NotConnected,
+    #[error("Operation timed out")]
+    Timeout,
+    #[error("Should have written {0} bytes, wrote {1}")]
+    WriteCount(usize, u32),
+    #[error("Windows API error")]
+    Api(#[from] HResult),
+}
+
+pub use PipeError as Error;
+
+/// A client for interacting with BITS.
+///
+/// Methods on `BitsClient` return a `Result<Result<_, XyzFailure>, Error>`. The outer `Result`
+/// is `Err` if there was a communication error in sending the associated command or receiving
+/// its response. Currently this is always `Ok` as all clients are in-process. The inner
+/// `Result` is `Err` if there was an error executing the command.
+///
+/// A single `BitsClient` can be used with multiple BITS jobs simultaneously; generally a job
+/// is not bound tightly to a client.
+///
+/// A `BitsClient` tracks all [`BitsMonitorClient`s](enum.BitsMonitorClient.html) that it started
+/// with `start_job()` or `monitor_job()`, so that the monitor can be stopped or modified.
+pub enum BitsClient {
+    // The `InProcess` variant does all BITS calls directly.
+    #[doc(hidden)]
+    InProcess(in_process::InProcessClient),
+    // Space is reserved here for the LocalService variant, which will work through an external
+    // process running as Local Service.
+}
+
+use BitsClient::InProcess;
+
+impl BitsClient {
+    /// Create an in-process `BitsClient`.
+    ///
+    /// `job_name` will be used when creating jobs, and this `BitsClient` can only be used to
+    /// manipulate jobs with that name.
+    ///
+    /// `save_path_prefix` will be prepended to the local `save_path` given to `start_job()`, it
+    /// must name an existing directory.
+    pub fn new(
+        job_name: ffi::OsString,
+        save_path_prefix: ffi::OsString,
+    ) -> Result<BitsClient, Error> {
+        Ok(InProcess(in_process::InProcessClient::new(
+            job_name,
+            save_path_prefix,
+        )?))
+    }
+
+    /// Start a job to download a single file at `url` to local path `save_path` (relative to the
+    /// `save_path_prefix` given when constructing the `BitsClient`).
+    ///
+    /// `save_path_prefix` combined with `save_path` must name a file (existing or not) in an
+    /// existing directory, which must be under the directory named by `save_path_prefix`.
+    ///
+    /// `proxy_usage` determines what proxy will be used.
+    ///
+    /// When a successful result `Ok(result)` is returned, `result.0.guid` is the id for the
+    /// new job, and `result.1` is a monitor client that can be polled for periodic updates,
+    /// returning a result approximately once per `monitor_interval_millis` milliseconds.
+    pub fn start_job(
+        &mut self,
+        url: ffi::OsString,
+        save_path: ffi::OsString,
+        proxy_usage: BitsProxyUsage,
+        no_progress_timeout_secs: u32,
+        monitor_interval_millis: u32,
+    ) -> Result<Result<(StartJobSuccess, BitsMonitorClient), StartJobFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client
+                .start_job(
+                    url,
+                    save_path,
+                    proxy_usage,
+                    no_progress_timeout_secs,
+                    monitor_interval_millis,
+                )
+                .map(|(success, monitor)| (success, BitsMonitorClient::InProcess(monitor)))),
+        }
+    }
+
+    /// Start monitoring the job with id `guid` approximately once per `monitor_interval_millis`
+    /// milliseconds.
+    ///
+    /// The returned `Ok(monitor)` is a monitor client to be polled for periodic updates.
+    ///
+    /// There can only be one ongoing `BitsMonitorClient` for each job associated with a given
+    /// `BitsClient`. If a monitor client already exists for the specified job, it will be stopped.
+    pub fn monitor_job(
+        &mut self,
+        guid: Guid,
+        interval_millis: u32,
+    ) -> Result<Result<BitsMonitorClient, MonitorJobFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client
+                .monitor_job(guid, interval_millis)
+                .map(BitsMonitorClient::InProcess)),
+        }
+    }
+
+    /// Suspend job `guid`.
+    pub fn suspend_job(&mut self, guid: Guid) -> Result<Result<(), SuspendJobFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.suspend_job(guid)),
+        }
+    }
+
+    /// Resume job `guid`.
+    pub fn resume_job(&mut self, guid: Guid) -> Result<Result<(), ResumeJobFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.resume_job(guid)),
+        }
+    }
+
+    /// Set the priority of job `guid`.
+    ///
+    /// `foreground == true` will set the priority to `BG_JOB_PRIORITY_FOREGROUND`,
+    /// `false` will use the default `BG_JOB_PRIORITY_NORMAL`.
+    /// See the Microsoft documentation for `BG_JOB_PRIORITY` for details.
+    ///
+    /// A job created by `start_job()` will be foreground priority, by default.
+    pub fn set_job_priority(
+        &mut self,
+        guid: Guid,
+        foreground: bool,
+    ) -> Result<Result<(), SetJobPriorityFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.set_job_priority(guid, foreground)),
+        }
+    }
+
+    /// Set the "no progress timeout" of job `guid`.
+    pub fn set_no_progress_timeout(
+        &mut self,
+        guid: Guid,
+        timeout_secs: u32,
+    ) -> Result<Result<(), SetNoProgressTimeoutFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.set_no_progress_timeout(guid, timeout_secs)),
+        }
+    }
+
+    /// Change the update interval for an ongoing monitor of job `guid`.
+    pub fn set_update_interval(
+        &mut self,
+        guid: Guid,
+        interval_millis: u32,
+    ) -> Result<Result<(), SetUpdateIntervalFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.set_update_interval(guid, interval_millis)),
+        }
+    }
+
+    /// Stop any ongoing monitor for job `guid`.
+    pub fn stop_update(
+        &mut self,
+        guid: Guid,
+    ) -> Result<Result<(), SetUpdateIntervalFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.stop_update(guid)),
+        }
+    }
+
+    /// Complete the job `guid`.
+    ///
+    /// This also stops any ongoing monitor for the job.
+    pub fn complete_job(&mut self, guid: Guid) -> Result<Result<(), CompleteJobFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.complete_job(guid)),
+        }
+    }
+
+    /// Cancel the job `guid`.
+    ///
+    /// This also stops any ongoing monitor for the job.
+    pub fn cancel_job(&mut self, guid: Guid) -> Result<Result<(), CancelJobFailure>, Error> {
+        match self {
+            InProcess(client) => Ok(client.cancel_job(guid)),
+        }
+    }
+}
+
+/// The client side of a monitor for a BITS job.
+///
+/// It is intended to be used by calling `get_status` in a loop to receive notifications about
+/// the status of a job. Because `get_status` blocks, it is recommended to run this loop on its
+/// own thread.
+pub enum BitsMonitorClient {
+    InProcess(in_process::InProcessMonitor),
+}
+
+impl BitsMonitorClient {
+    /// `get_status` will return a result approximately every `monitor_interval_millis`
+    /// milliseconds, but in case a result isn't available within `timeout_millis` milliseconds
+    /// this will return `Err(Error::Timeout)`. Any `Err` returned, including timeout, indicates
+    /// that the monitor has been stopped; the `BitsMonitorClient` should then be discarded.
+    ///
+    /// As with methods on `BitsClient`, `BitsMonitorClient::get_status()` has an inner `Result`
+    /// type which indicates an error returned from the server. Any `Err` here also indicates that
+    /// the monitor has stopped after yielding the result.
+    ///
+    /// The first time `get_status` is called it will return a status without any delay.
+    ///
+    /// If there is an error or the transfer completes, a result may be available sooner than
+    /// the monitor interval.
+    pub fn get_status(
+        &mut self,
+        timeout_millis: u32,
+    ) -> Result<Result<JobStatus, HResultMessage>, Error> {
+        match self {
+            BitsMonitorClient::InProcess(client) => client.get_status(timeout_millis),
+        }
+    }
+}
diff --git a/toolkit/components/bitsdownload/components.conf b/toolkit/components/bitsdownload/components.conf
new file mode 100644
index 0000000000..fe7090d366
--- /dev/null
+++ b/toolkit/components/bitsdownload/components.conf
@@ -0,0 +1,17 @@
+# -*- 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 = []
+if defined('MOZ_BITS_DOWNLOAD'):
+    Classes += [
+        {
+            'cid': '{495d6f3d-9748-4d30-8ce5-0290c0001edf}',
+            'contract_ids': ['@mozilla.org/bits;1'],
+            'singleton': True,
+            'constructor': 'GetBitsService',
+            'headers': ['Bits.h'],
+        },
+    ]
diff --git a/toolkit/components/bitsdownload/moz.build b/toolkit/components/bitsdownload/moz.build
new file mode 100644
index 0000000000..9a2c6e23db
--- /dev/null
+++ b/toolkit/components/bitsdownload/moz.build
@@ -0,0 +1,28 @@
+# -*- 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/.
+
+XPIDL_MODULE = "Bits"
+
+XPIDL_SOURCES += [
+    "nsIBits.idl",
+]
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+if CONFIG["MOZ_BITS_DOWNLOAD"]:
+    EXPORTS += ["Bits.h"]
+    UNIFIED_SOURCES += ["Bits.cpp"]
+
+EXTRA_JS_MODULES += [
+    "Bits.sys.mjs",
+]
+
+FINAL_LIBRARY = "xul"
+
+with Files("**"):
+    BUG_COMPONENT = ("Toolkit", "Application Update")
diff --git a/toolkit/components/bitsdownload/nsIBits.idl b/toolkit/components/bitsdownload/nsIBits.idl
new file mode 100644
index 0000000000..993c10243c
--- /dev/null
+++ b/toolkit/components/bitsdownload/nsIBits.idl
@@ -0,0 +1,427 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIRequest.idl"
+
+interface nsIRequest;
+interface nsIRequestObserver;
+
+interface nsIBitsRequest;
+
+interface nsIBitsNewRequestCallback;
+interface nsIBitsCallback;
+
+typedef long nsProxyUsage;
+typedef long nsBitsErrorType;
+typedef long nsBitsErrorAction;
+typedef long nsBitsErrorStage;
+
+/**
+ * An interface for interacting with Windows Background Intelligent Transfer
+ * Service. This should only be used on Windows.
+ *
+ * It would be preferable for the functions in this interface to return
+ * Promises, but this interface is implemented in Rust, which does not yet have
+ * support for Promises. There is a JS wrapper around this class that should be
+ * preferred over using this interface directly, located in Bits.sys.mjs.
+ *
+ * Methods of this class that take a nsIBitsNewRequestCallback do not return or
+ * throw errors. All errors will be reported through the callback. The only
+ * things that should cause methods to directly throw errors are null arguments.
+ */
+[scriptable, uuid(495d6f3d-9748-4d30-8ce5-0290c0001edf)]
+interface nsIBits : nsISupports
+{
+  /**
+   * nsBitsErrorType values
+   * The BITS interface returns many error codes. These are intended to help
+   * determine appropriate fallback actions and to report to telemetry.
+   */
+  const long ERROR_TYPE_SUCCESS                                           = 0;
+  const long ERROR_TYPE_UNKNOWN                                           = 1;
+  const long ERROR_TYPE_METHOD_THREW                                      = 2;
+  const long ERROR_TYPE_METHOD_TIMEOUT                                    = 3;
+  const long ERROR_TYPE_NULL_ARGUMENT                                     = 4;
+  const long ERROR_TYPE_INVALID_ARGUMENT                                  = 5;
+  const long ERROR_TYPE_NOT_INITIALIZED                                   = 6;
+  const long ERROR_TYPE_NO_UTF8_CONVERSION                                = 7;
+  const long ERROR_TYPE_INVALID_GUID                                      = 8;
+  const long ERROR_TYPE_PIPE_NOT_CONNECTED                                = 9;
+  const long ERROR_TYPE_PIPE_TIMEOUT                                      = 10;
+  const long ERROR_TYPE_PIPE_BAD_WRITE_COUNT                              = 11;
+  const long ERROR_TYPE_PIPE_API_ERROR                                    = 12;
+  const long ERROR_TYPE_FAILED_TO_CREATE_BITS_JOB                         = 13;
+  const long ERROR_TYPE_FAILED_TO_ADD_FILE_TO_JOB                         = 14;
+  const long ERROR_TYPE_FAILED_TO_APPLY_BITS_JOB_SETTINGS                 = 15;
+  const long ERROR_TYPE_FAILED_TO_RESUME_BITS_JOB                         = 16;
+  const long ERROR_TYPE_OTHER_BITS_ERROR                                  = 17;
+  const long ERROR_TYPE_OTHER_BITS_CLIENT_ERROR                           = 18;
+  const long ERROR_TYPE_BITS_JOB_NOT_FOUND                                = 19;
+  const long ERROR_TYPE_FAILED_TO_GET_BITS_JOB                            = 20;
+  const long ERROR_TYPE_FAILED_TO_SUSPEND_BITS_JOB                        = 21;
+  const long ERROR_TYPE_FAILED_TO_COMPLETE_BITS_JOB                       = 22;
+  const long ERROR_TYPE_PARTIALLY_COMPLETED_BITS_JOB                      = 23;
+  const long ERROR_TYPE_FAILED_TO_CANCEL_BITS_JOB                         = 24;
+  const long ERROR_TYPE_MISSING_RESULT_DATA                               = 25;
+  const long ERROR_TYPE_MISSING_CALLBACK                                  = 26;
+  const long ERROR_TYPE_CALLBACK_ON_WRONG_THREAD                          = 27;
+  const long ERROR_TYPE_MISSING_BITS_SERVICE                              = 28;
+  const long ERROR_TYPE_BITS_SERVICE_ON_WRONG_THREAD                      = 29;
+  const long ERROR_TYPE_MISSING_BITS_REQUEST                              = 30;
+  const long ERROR_TYPE_BITS_REQUEST_ON_WRONG_THREAD                      = 31;
+  const long ERROR_TYPE_MISSING_OBSERVER                                  = 32;
+  const long ERROR_TYPE_OBSERVER_ON_WRONG_THREAD                          = 33;
+  const long ERROR_TYPE_MISSING_CONTEXT                                   = 34;
+  const long ERROR_TYPE_CONTEXT_ON_WRONG_THREAD                           = 35;
+  const long ERROR_TYPE_FAILED_TO_START_THREAD                            = 36;
+  const long ERROR_TYPE_FAILED_TO_CONSTRUCT_TASK_RUNNABLE                 = 37;
+  const long ERROR_TYPE_FAILED_TO_DISPATCH_RUNNABLE                       = 38;
+  const long ERROR_TYPE_TRANSFER_ALREADY_COMPLETE                         = 39;
+  const long ERROR_TYPE_OPERATION_ALREADY_IN_PROGRESS                     = 40;
+  const long ERROR_TYPE_MISSING_BITS_CLIENT                               = 41;
+  const long ERROR_TYPE_FAILED_TO_GET_JOB_STATUS                          = 42;
+  const long ERROR_TYPE_BITS_STATE_ERROR                                  = 43;
+  const long ERROR_TYPE_BITS_STATE_TRANSIENT_ERROR                        = 44;
+  const long ERROR_TYPE_BITS_STATE_CANCELLED                              = 45;
+  const long ERROR_TYPE_BITS_STATE_UNEXPECTED                             = 46;
+  const long ERROR_TYPE_VERIFICATION_FAILURE                              = 47;
+  const long ERROR_TYPE_ACCESS_DENIED_EXPECTED                            = 48;
+  const long ERROR_TYPE_FAILED_TO_CONNECT_TO_BCM                          = 49;
+  const long ERROR_TYPE_USE_AFTER_REQUEST_SHUTDOWN                        = 50;
+  const long ERROR_TYPE_BROWSER_SHUTTING_DOWN                             = 51;
+  /**
+   * nsBitsErrorAction values
+   * These values indicate where the error occurred.
+   */
+  const long ERROR_ACTION_UNKNOWN                                         = 1;
+  const long ERROR_ACTION_NONE                                            = 2;
+  const long ERROR_ACTION_START_DOWNLOAD                                  = 3;
+  const long ERROR_ACTION_MONITOR_DOWNLOAD                                = 4;
+  const long ERROR_ACTION_CHANGE_MONITOR_INTERVAL                         = 5;
+  const long ERROR_ACTION_CANCEL                                          = 6;
+  const long ERROR_ACTION_SET_PRIORITY                                    = 7;
+  const long ERROR_ACTION_COMPLETE                                        = 8;
+  const long ERROR_ACTION_SUSPEND                                         = 9;
+  const long ERROR_ACTION_RESUME                                          = 10;
+  const long ERROR_ACTION_SET_NO_PROGRESS_TIMEOUT                         = 11;
+
+  /**
+   * nsBitsErrorStage values
+   * These values allow the caller to determine at what point in the download
+   * mechanism a failure occurred.
+   */
+  const long ERROR_STAGE_UNKNOWN                                          = 1;
+  const long ERROR_STAGE_PRETASK                                          = 2;
+  const long ERROR_STAGE_COMMAND_THREAD                                   = 3;
+  const long ERROR_STAGE_AGENT_COMMUNICATION                              = 4;
+  const long ERROR_STAGE_BITS_CLIENT                                      = 5;
+  const long ERROR_STAGE_MAIN_THREAD                                      = 6;
+  const long ERROR_STAGE_MONITOR                                          = 7;
+  const long ERROR_STAGE_VERIFICATION                                     = 8;
+
+  /**
+   * These values indicate what type of error code was returned. These are used
+   * to allow the different types taken by the different callback failure
+   * functions to be made into one generic error type in Javascript.
+   */
+  const long ERROR_CODE_TYPE_NONE                                         = 1;
+  const long ERROR_CODE_TYPE_NSRESULT                                     = 2;
+  const long ERROR_CODE_TYPE_HRESULT                                      = 3;
+  const long ERROR_CODE_TYPE_STRING                                       = 4;
+  const long ERROR_CODE_TYPE_EXCEPTION                                    = 5;
+
+  /**
+   * Indicates whether init() has been called.
+   */
+  readonly attribute boolean initialized;
+
+  /**
+   * Initializes the BITS interface. Unlike other functions here, this happens
+   * synchronously.
+   * init() should only be called only once.
+   *
+   * @param jobName
+   *        The name of the BITS job. This is used both to set the name during
+   *        job creation and to verify that a job is ours.
+   * @param savePathPrefix
+   *        The directory that downloads will be saved to. Providing a safe
+   *        directory here ensures that the download path cannot be manipulated
+   *        to save files to a malicious location. Downloads are guaranteed to
+   *        be saved to this directory or a subdirectory.
+   * @param monitorTimeoutMs
+   *        The amount of time to wait between download monitor notifications.
+   *        This should be larger than the largest monitorIntervalMs that will
+   *        be passed to startDownload(), monitorDownload(), or
+   *        changeMonitorInterval(). This value may not be 0.
+   */
+  void init(in AUTF8String jobName,
+            in AUTF8String savePathPrefix,
+            in unsigned long monitorTimeoutMs);
+
+  /**
+   * Downloads the specified URL to the specified location within the
+   * savePathPrefix passed to init().
+   *
+   * @param downloadURL
+   *        The URL to be downloaded.
+   * @param saveRelativePath
+   *        The location to download to. The path given should be a path
+   *        relative to the savePathPrefix passed to init(). If this attempts to
+   *        escape the directory specified by savePathPrefix, this call will
+   *        fail (ex: Don't pass "../filename").
+   * @param proxy
+   *        Specifies what proxy to use when downloading. Valid values are
+   *        listed below.
+   * @param noProgressTimeoutSecs
+   *        The number of seconds for the "no progress" timeout. After there has
+   *        been no download progress for this long, BITS will not retry the job
+   *        following a transient error, producing instead a permanent error.
+   * @param monitorIntervalMs
+   *        The number of milliseconds between download status notifications.
+   * @param observer
+   *        An observer to be notified of various events. OnStartRequest is
+   *        called once the BITS job has been created. OnStopRequest is called
+   *        when the file transfer has completed or when an error occurs. If
+   *        this object implements nsIProgressEventSink, then its OnProgress
+   *        method will be called as data is transferred.
+   *        IMPORTANT NOTE: When OnStopRequest is called, the download has
+   *                        completed, but nsIBitsRequest::complete() still
+   *                        needs to be called to save the file to the
+   *                        filesystem.
+   * @param context
+   *        User defined object forwarded to the observer's onProgress method.
+   *        This parameter, unlike others for this interface, can be passed a
+   *        null pointer.
+   * @param callback
+   *        The callback used to relay the response from BITS.
+   */
+  void startDownload(in AUTF8String downloadURL,
+                     in AUTF8String saveRelativePath,
+                     in nsProxyUsage proxy,
+                     in unsigned long noProgressTimeoutSecs,
+                     in unsigned long monitorIntervalMs,
+                     in nsIRequestObserver observer,
+                     in nsISupports context,
+                     in nsIBitsNewRequestCallback callback);
+
+  // nsProxyUsage values
+  const long PROXY_NONE = 1;
+  const long PROXY_PRECONFIG = 2;
+  const long PROXY_AUTODETECT = 3;
+
+  /**
+   * Similar to startDownload, but connects to a BITS transfer that has already
+   * been started.
+   *
+   * @param id
+   *        The GUID of the download to monitor.
+   * @param monitorIntervalMs
+   *        The number of milliseconds between download status notifications.
+   * @param observer
+   *        An observer to be notified of various events. OnStartRequest is
+   *        called once the BITS job has been created. OnStopRequest is called
+   *        when the file transfer has completed or when an error occurs. If
+   *        this object implements nsIProgressEventSink, then its OnProgress
+   *        method will be called as data is transferred.
+   *        IMPORTANT NOTE: When OnStopRequest is called, the download has
+   *                        completed, but nsIBitsRequest::complete() still
+   *                        needs to be called to save the file to the
+   *                        filesystem.
+   * @param context
+   *        User defined object forwarded to the observer's onProgress method.
+   *        This parameter, unlike others for this interface, can be passed a
+   *        null pointer.
+   * @param callback
+   *        The callback used to relay the response from BITS.
+   */
+  void monitorDownload(in AUTF8String id,
+                       in unsigned long monitorIntervalMs,
+                       in nsIRequestObserver observer,
+                       in nsISupports context,
+                       in nsIBitsNewRequestCallback callback);
+};
+
+/**
+ * This callback interface is for use by the nsIBits interface for returning
+ * results asynchronously to the caller.
+ */
+[scriptable, uuid(aa12e433-5b9f-452d-b5c9-840a9541328b)]
+interface nsIBitsNewRequestCallback : nsISupports
+{
+  void success(in nsIBitsRequest request);
+  void failure(in nsBitsErrorType errorType,
+               in nsBitsErrorAction errorAction,
+               in nsBitsErrorStage errorStage);
+  void failureNsresult(in nsBitsErrorType errorType,
+                       in nsBitsErrorAction errorAction,
+                       in nsBitsErrorStage errorStage,
+                       in nsresult errorCode);
+  void failureHresult(in nsBitsErrorType errorType,
+                      in nsBitsErrorAction errorAction,
+                      in nsBitsErrorStage errorStage,
+                      in long errorCode);
+  void failureString(in nsBitsErrorType errorType,
+                     in nsBitsErrorAction errorAction,
+                     in nsBitsErrorStage errorStage,
+                     in AUTF8String errorMessage);
+};
+
+/*
+ * An interface for managing a running BITS download.
+ *
+ * It would be preferable for the functions in this interface to return
+ * Promises, but this interface is implemented in Rust, which does not yet have
+ * support for Promises. There is a JS wrapper around this class that should be
+ * preferred over using this interface directly, located in Bits.sys.mjs.
+ *
+ * Methods of this class that take a nsIBitsCallback do not return or throw
+ * errors. All errors will be reported through the callback. The only
+ * things that should cause methods to directly throw errors are null arguments.
+ *
+ * Note: Although the nsIBits interface derives from nsIRequest, implementations
+ *       may not implement the loadGroup or loadFlags attributes.
+ *
+ * Note: Once the file transfer has stopped (due to completion or error),
+ *       calling any method besides complete() or cancel() will result in an
+ *       error with type nsIBits::ERROR_TYPE_TRANSFER_ALREADY_COMPLETE.
+ *       Calling complete() or cancel() again after either has already been
+ *       called will also result in an ERROR_TYPE_TRANSFER_ALREADY_COMPLETE
+ *       error.
+ *       Attributes and nsIRequest::isPending() can still be accessed at any
+ *       time.
+ */
+[scriptable, uuid(ab9da0e9-06bf-4e73-bb1b-c0f2ea9ecc3e)]
+interface nsIBitsRequest : nsIRequest
+{
+  /**
+   * The BITS id of the download. This will be a string representing a UUID.
+   */
+  readonly attribute AUTF8String bitsId;
+
+  /**
+   * The transfer result of the download, meant to be accessed after the
+   * transfer has stopped (i.e. after the observer's onStopRequest method has
+   * been called). Will be nsIBits::ERROR_TYPE_SUCCESS if the transfer is
+   * successful (and before transfer completion). If the transfer failed, this
+   * will be a different nsBitsErrorType value indicating the cause of the
+   * failure.
+   */
+  readonly attribute nsBitsErrorType transferError;
+
+  /**
+   * Requests a change to the frequency that Firefox is receiving download
+   * status notifications.
+   *
+   * @param monitorIntervalMs
+   *        The new number of milliseconds between download status
+   *        notifications.
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void changeMonitorInterval(in unsigned long monitorIntervalMs,
+                             in nsIBitsCallback callback);
+
+  /**
+   * Cancels the download. This function is named this way to avoid conflict
+   * with nsIRequest::cancel.
+   *
+   * @param status
+   *        The reason for cancelling the request. This must be a failure code
+   *        rather than a success code like NS_OK.
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void cancelAsync(in nsresult status,
+                   in nsIBitsCallback callback);
+
+  /**
+   * Sets the priority of the BITS job to high (i.e. foreground download).
+   *
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void setPriorityHigh(in nsIBitsCallback callback);
+
+  /**
+   * Sets the priority of the BITS job to low (i.e. background download).
+   *
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void setPriorityLow(in nsIBitsCallback callback);
+
+  /**
+   * Sets the BITS "no progress" timeout for the job.
+   *
+   * @param timeoutSecs
+   *        The new number of seconds for the timeout. After there has been
+   *        no progress for this long, BITS will not retry the job following
+   *        a transient error, producing instead a permanent error.
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void setNoProgressTimeout(in unsigned long timeoutSecs,
+                             in nsIBitsCallback callback);
+
+  /*
+   * Completes the download, moving it out of the BITS system and onto the
+   * disk location specified when startDownload was called.
+   *
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void complete(in nsIBitsCallback callback);
+
+  /*
+   * Suspends the download, preventing more data from being transferred until
+   * the download is resumed. This function is named this way to avoid conflict
+   * with nsIRequest::suspend.
+   *
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void suspendAsync(in nsIBitsCallback callback);
+
+  /*
+   * Resumes a previously suspended download. This function is named this way
+   * to avoid conflict with nsIRequest::resume.
+   *
+   * @param callback
+   *        The callback function used to relay success or failure.
+   */
+  void resumeAsync(in nsIBitsCallback callback);
+};
+
+/**
+ * This callback interface is for use by the nsIBitsRequest interface for
+ * returning results asynchronously to the caller.
+ */
+[scriptable, uuid(ea657e66-6bad-4e41-84d9-c6d107e9799d)]
+interface nsIBitsCallback : nsISupports
+{
+  void success();
+  void failure(in nsBitsErrorType errorType,
+               in nsBitsErrorAction errorAction,
+               in nsBitsErrorStage errorStage);
+  void failureNsresult(in nsBitsErrorType errorType,
+                       in nsBitsErrorAction errorAction,
+                       in nsBitsErrorStage errorStage,
+                       in nsresult errorCode);
+  void failureHresult(in nsBitsErrorType errorType,
+                      in nsBitsErrorAction errorAction,
+                      in nsBitsErrorStage errorStage,
+                      in long errorCode);
+  void failureString(in nsBitsErrorType errorType,
+                     in nsBitsErrorAction errorAction,
+                     in nsBitsErrorStage errorStage,
+                     in AUTF8String errorMessage);
+};
+
+%{C++
+#define NS_BITS_CID \
+  { 0xa334de05, 0xb9de, 0x46a1, \
+    { 0x98, 0xa9, 0x3f, 0x5c, 0xed, 0x82, 0x1e, 0x68 } }
+#define NS_BITS_CONTRACTID "@mozilla.org/bits;1"
+%}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/action.rs b/toolkit/components/bitsdownload/src/bits_interface/action.rs
new file mode 100644
index 0000000000..1019e29827
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/action.rs
@@ -0,0 +1,118 @@
+/* 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/. */
+
+//! `Action` is an enum describing what BITS action is being processed. This is
+//! used mostly for logging and error reporting reasons.
+//! The values of `Action` describe actions that could be in progress for
+//! BitsService or BitsRequest. When specifying a type, `ServiceAction` or
+//! `RequestAction`, can be used to restrict the action type to one of the two
+//! categories.
+//! A value of type `ServiceAction` or `RequestAction` can easily be converted
+//! to an `Action` using the `into()` method.
+
+use std::convert::From;
+use xpcom::interfaces::nsIBits;
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum Action {
+    StartDownload,
+    MonitorDownload,
+    Complete,
+    Cancel,
+    SetMonitorInterval,
+    SetPriority,
+    SetNoProgressTimeout,
+    Resume,
+    Suspend,
+}
+
+impl Action {
+    pub fn description(&self) -> &'static str {
+        match self {
+            Action::StartDownload => "starting download",
+            Action::MonitorDownload => "monitoring download",
+            Action::Complete => "completing download",
+            Action::Cancel => "cancelling download",
+            Action::SetMonitorInterval => "changing monitor interval",
+            Action::SetPriority => "setting download priority",
+            Action::SetNoProgressTimeout => "setting no progress timeout",
+            Action::Resume => "resuming download",
+            Action::Suspend => "suspending download",
+        }
+    }
+
+    pub fn as_error_code(&self) -> i32 {
+        match self {
+            Action::StartDownload => nsIBits::ERROR_ACTION_START_DOWNLOAD,
+            Action::MonitorDownload => nsIBits::ERROR_ACTION_MONITOR_DOWNLOAD,
+            Action::Complete => nsIBits::ERROR_ACTION_COMPLETE,
+            Action::Cancel => nsIBits::ERROR_ACTION_CANCEL,
+            Action::SetMonitorInterval => nsIBits::ERROR_ACTION_CHANGE_MONITOR_INTERVAL,
+            Action::SetPriority => nsIBits::ERROR_ACTION_SET_PRIORITY,
+            Action::SetNoProgressTimeout => nsIBits::ERROR_ACTION_SET_NO_PROGRESS_TIMEOUT,
+            Action::Resume => nsIBits::ERROR_ACTION_RESUME,
+            Action::Suspend => nsIBits::ERROR_ACTION_SUSPEND,
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum ServiceAction {
+    StartDownload,
+    MonitorDownload,
+}
+
+impl From<ServiceAction> for Action {
+    fn from(action: ServiceAction) -> Action {
+        match action {
+            ServiceAction::StartDownload => Action::StartDownload,
+            ServiceAction::MonitorDownload => Action::MonitorDownload,
+        }
+    }
+}
+
+impl ServiceAction {
+    pub fn as_error_code(&self) -> i32 {
+        Action::as_error_code(&(self.clone()).into())
+    }
+
+    pub fn description(&self) -> &'static str {
+        Action::description(&(self.clone()).into())
+    }
+}
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum RequestAction {
+    Complete,
+    Cancel,
+    SetMonitorInterval,
+    SetPriority,
+    SetNoProgressTimeout,
+    Resume,
+    Suspend,
+}
+
+impl From<RequestAction> for Action {
+    fn from(action: RequestAction) -> Action {
+        match action {
+            RequestAction::Complete => Action::Complete,
+            RequestAction::Cancel => Action::Cancel,
+            RequestAction::SetMonitorInterval => Action::SetMonitorInterval,
+            RequestAction::SetPriority => Action::SetPriority,
+            RequestAction::SetNoProgressTimeout => Action::SetNoProgressTimeout,
+            RequestAction::Resume => Action::Resume,
+            RequestAction::Suspend => Action::Suspend,
+        }
+    }
+}
+
+impl RequestAction {
+    pub fn as_error_code(&self) -> i32 {
+        Action::as_error_code(&(self.clone()).into())
+    }
+
+    pub fn description(&self) -> &'static str {
+        Action::description(&(self.clone()).into())
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/dispatch_callback.rs b/toolkit/components/bitsdownload/src/bits_interface/dispatch_callback.rs
new file mode 100644
index 0000000000..b42637a993
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/dispatch_callback.rs
@@ -0,0 +1,199 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use super::{
+    error::{BitsTaskError, ErrorCode, ErrorType},
+    BitsRequest,
+};
+use log::{error, info, warn};
+use nserror::{nsresult, NS_ERROR_FAILURE, NS_OK};
+use xpcom::{
+    interfaces::{nsIBitsCallback, nsIBitsNewRequestCallback},
+    RefPtr,
+};
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum IsCallbackExpected {
+    CallbackExpected,
+    CallbackOptional,
+}
+pub use self::IsCallbackExpected::{CallbackExpected, CallbackOptional};
+
+// This is meant to be called at the end of a nsIBits Task. It attempts to
+// return the result via the callback given. If the callback is unavailable, a
+// log message will be printed indicating the results and (possibly) warning
+// than an expected callback was missing.
+pub fn maybe_dispatch_request_via_callback(
+    result: Result<RefPtr<BitsRequest>, BitsTaskError>,
+    maybe_callback: Result<&nsIBitsNewRequestCallback, BitsTaskError>,
+    expected: IsCallbackExpected,
+) -> Result<(), nsresult> {
+    if let Err(error) = maybe_callback.as_ref() {
+        if expected == CallbackExpected || error.error_type == ErrorType::CallbackOnWrongThread {
+            error!(
+                "Unexpected error when {} - No callback: {:?}",
+                error.error_action.description(),
+                error,
+            );
+        }
+    }
+    match result {
+        Ok(request) => match (maybe_callback, expected) {
+            (Ok(callback), _) => unsafe { callback.Success(request.coerce()) },
+            (Err(error), CallbackExpected) => {
+                error!(
+                    "Success {} but there is no callback to return the result with",
+                    error.error_action.description(),
+                );
+                NS_ERROR_FAILURE
+            }
+            (Err(error), CallbackOptional) => {
+                info!("Success {}", error.error_action.description());
+                NS_OK
+            }
+        },
+        Err(error) => match (maybe_callback, expected) {
+            (Ok(callback), _) => match error.error_code {
+                ErrorCode::None => unsafe {
+                    callback.Failure(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                    )
+                },
+                ErrorCode::Hresult(error_code) => unsafe {
+                    callback.FailureHresult(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                        error_code,
+                    )
+                },
+                ErrorCode::Nsresult(error_code) => unsafe {
+                    callback.FailureNsresult(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                        error_code,
+                    )
+                },
+                ErrorCode::Message(message) => unsafe {
+                    callback.FailureString(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                        &*message,
+                    )
+                },
+            },
+            (Err(_), CallbackExpected) => {
+                error!("Error {}: {:?}", error.error_action.description(), error);
+                NS_ERROR_FAILURE
+            }
+            (Err(_), CallbackOptional) => {
+                warn!("Error {}: {:?}", error.error_action.description(), error);
+                NS_ERROR_FAILURE
+            }
+        },
+    }
+    .to_result()
+}
+
+// Intended to be used by an nsIBits XPCOM wrapper to return errors that occur
+// before dispatching a task off-thread. No return value is returned because it
+// will represent the return value of the callback function, which should not be
+// propagated.
+pub fn dispatch_pretask_interface_error(
+    error: BitsTaskError,
+    callback: &nsIBitsNewRequestCallback,
+) {
+    let _ = maybe_dispatch_request_via_callback(Err(error), Ok(callback), CallbackExpected);
+}
+
+// This is meant to be called at the end of a nsIBitsRequest Task. It attempts
+// to return the result via the callback given. If the callback is unavailable,
+// a log message will be printed indicating the results and (possibly) warning
+// than an expected callback was missing.
+pub fn maybe_dispatch_via_callback(
+    result: Result<(), BitsTaskError>,
+    maybe_callback: Result<&nsIBitsCallback, BitsTaskError>,
+    expected: IsCallbackExpected,
+) -> Result<(), nsresult> {
+    if let Err(error) = maybe_callback.as_ref() {
+        if expected == CallbackExpected || error.error_type == ErrorType::CallbackOnWrongThread {
+            error!(
+                "Unexpected error when {} - No callback: {:?}",
+                error.error_action.description(),
+                error,
+            );
+        }
+    }
+    match result {
+        Ok(()) => match (maybe_callback, expected) {
+            (Ok(callback), _) => unsafe { callback.Success() },
+            (Err(error), CallbackExpected) => {
+                error!(
+                    "Success {} but there is no callback to return the result with",
+                    error.error_action.description(),
+                );
+                NS_ERROR_FAILURE
+            }
+            (Err(error), CallbackOptional) => {
+                info!("Success {}", error.error_action.description());
+                NS_OK
+            }
+        },
+        Err(error) => match (maybe_callback, expected) {
+            (Ok(callback), _) => match error.error_code {
+                ErrorCode::None => unsafe {
+                    callback.Failure(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                    )
+                },
+                ErrorCode::Hresult(error_code) => unsafe {
+                    callback.FailureHresult(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                        error_code,
+                    )
+                },
+                ErrorCode::Nsresult(error_code) => unsafe {
+                    callback.FailureNsresult(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                        error_code,
+                    )
+                },
+                ErrorCode::Message(message) => unsafe {
+                    callback.FailureString(
+                        error.error_type.bits_code(),
+                        error.error_action.as_error_code(),
+                        error.error_stage.bits_code(),
+                        &*message,
+                    )
+                },
+            },
+            (Err(_), CallbackExpected) => {
+                error!("Error {}: {:?}", error.error_action.description(), error);
+                NS_ERROR_FAILURE
+            }
+            (Err(_), CallbackOptional) => {
+                warn!("Error {}: {:?}", error.error_action.description(), error);
+                NS_ERROR_FAILURE
+            }
+        },
+    }
+    .to_result()
+}
+
+// Intended to be used by an nsIBitsRequest XPCOM wrapper to return errors that
+// occur before dispatching a task off-thread. No return value is returned
+// because it will represent the return value of the callback function, which
+// should not be propagated.
+pub fn dispatch_pretask_request_error(error: BitsTaskError, callback: &nsIBitsCallback) {
+    let _ = maybe_dispatch_via_callback(Err(error), Ok(callback), CallbackExpected);
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/error.rs b/toolkit/components/bitsdownload/src/bits_interface/error.rs
new file mode 100644
index 0000000000..c046ae5e70
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/error.rs
@@ -0,0 +1,686 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use super::action::Action;
+use bits_client::{
+    bits_protocol::{
+        CancelJobFailure, CompleteJobFailure, HResultMessage, MonitorJobFailure, ResumeJobFailure,
+        SetJobPriorityFailure, SetNoProgressTimeoutFailure, SetUpdateIntervalFailure,
+        StartJobFailure, SuspendJobFailure,
+    },
+    PipeError,
+};
+use comedy::error::HResult as ComedyError;
+use nserror::{nsresult, NS_ERROR_FAILURE};
+use nsstring::nsCString;
+use std::convert::From;
+use xpcom::interfaces::nsIBits;
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum ErrorType {
+    NullArgument,
+    InvalidArgument,
+    NotInitialized,
+    NoUtf8Conversion,
+    InvalidGuid,
+    PipeNotConnected,
+    PipeTimeout,
+    PipeBadWriteCount,
+    PipeApiError,
+    FailedToCreateBitsJob,
+    FailedToAddFileToJob,
+    FailedToApplyBitsJobSettings,
+    FailedToResumeBitsJob,
+    OtherBitsError,
+    OtherBitsClientError,
+    BitsJobNotFound,
+    FailedToGetBitsJob,
+    FailedToSuspendBitsJob,
+    FailedToCompleteBitsJob,
+    PartiallyCompletedBitsJob,
+    FailedToCancelBitsJob,
+    MissingResultData,
+    MissingCallback,
+    CallbackOnWrongThread,
+    MissingBitsService,
+    BitsServiceOnWrongThread,
+    MissingBitsRequest,
+    BitsRequestOnWrongThread,
+    MissingObserver,
+    ObserverOnWrongThread,
+    MissingContext,
+    ContextOnWrongThread,
+    FailedToStartThread,
+    FailedToConstructTaskRunnable,
+    FailedToDispatchRunnable,
+    TransferAlreadyComplete,
+    OperationAlreadyInProgress,
+    MissingBitsClient,
+    FailedToGetJobStatus,
+    BitsStateError,
+    BitsStateTransientError,
+    BitsStateCancelled,
+    BitsStateUnexpected,
+    FailedToConnectToBcm,
+}
+
+impl ErrorType {
+    pub fn bits_code(&self) -> i32 {
+        match self {
+            ErrorType::NullArgument => nsIBits::ERROR_TYPE_NULL_ARGUMENT,
+            ErrorType::InvalidArgument => nsIBits::ERROR_TYPE_INVALID_ARGUMENT,
+            ErrorType::NotInitialized => nsIBits::ERROR_TYPE_NOT_INITIALIZED,
+            ErrorType::NoUtf8Conversion => nsIBits::ERROR_TYPE_NO_UTF8_CONVERSION,
+            ErrorType::InvalidGuid => nsIBits::ERROR_TYPE_INVALID_GUID,
+            ErrorType::PipeNotConnected => nsIBits::ERROR_TYPE_PIPE_NOT_CONNECTED,
+            ErrorType::PipeTimeout => nsIBits::ERROR_TYPE_PIPE_TIMEOUT,
+            ErrorType::PipeBadWriteCount => nsIBits::ERROR_TYPE_PIPE_BAD_WRITE_COUNT,
+            ErrorType::PipeApiError => nsIBits::ERROR_TYPE_PIPE_API_ERROR,
+            ErrorType::FailedToCreateBitsJob => nsIBits::ERROR_TYPE_FAILED_TO_CREATE_BITS_JOB,
+            ErrorType::FailedToAddFileToJob => nsIBits::ERROR_TYPE_FAILED_TO_ADD_FILE_TO_JOB,
+            ErrorType::FailedToApplyBitsJobSettings => {
+                nsIBits::ERROR_TYPE_FAILED_TO_APPLY_BITS_JOB_SETTINGS
+            }
+            ErrorType::FailedToResumeBitsJob => nsIBits::ERROR_TYPE_FAILED_TO_RESUME_BITS_JOB,
+            ErrorType::OtherBitsError => nsIBits::ERROR_TYPE_OTHER_BITS_ERROR,
+            ErrorType::OtherBitsClientError => nsIBits::ERROR_TYPE_OTHER_BITS_CLIENT_ERROR,
+            ErrorType::BitsJobNotFound => nsIBits::ERROR_TYPE_BITS_JOB_NOT_FOUND,
+            ErrorType::FailedToGetBitsJob => nsIBits::ERROR_TYPE_FAILED_TO_GET_BITS_JOB,
+            ErrorType::FailedToSuspendBitsJob => nsIBits::ERROR_TYPE_FAILED_TO_SUSPEND_BITS_JOB,
+            ErrorType::FailedToCompleteBitsJob => nsIBits::ERROR_TYPE_FAILED_TO_COMPLETE_BITS_JOB,
+            ErrorType::PartiallyCompletedBitsJob => {
+                nsIBits::ERROR_TYPE_PARTIALLY_COMPLETED_BITS_JOB
+            }
+            ErrorType::FailedToCancelBitsJob => nsIBits::ERROR_TYPE_FAILED_TO_CANCEL_BITS_JOB,
+            ErrorType::MissingResultData => nsIBits::ERROR_TYPE_MISSING_RESULT_DATA,
+            ErrorType::MissingCallback => nsIBits::ERROR_TYPE_MISSING_CALLBACK,
+            ErrorType::CallbackOnWrongThread => nsIBits::ERROR_TYPE_CALLBACK_ON_WRONG_THREAD,
+            ErrorType::MissingBitsService => nsIBits::ERROR_TYPE_MISSING_BITS_SERVICE,
+            ErrorType::BitsServiceOnWrongThread => nsIBits::ERROR_TYPE_BITS_SERVICE_ON_WRONG_THREAD,
+            ErrorType::MissingBitsRequest => nsIBits::ERROR_TYPE_MISSING_BITS_REQUEST,
+            ErrorType::BitsRequestOnWrongThread => nsIBits::ERROR_TYPE_BITS_REQUEST_ON_WRONG_THREAD,
+            ErrorType::MissingObserver => nsIBits::ERROR_TYPE_MISSING_OBSERVER,
+            ErrorType::ObserverOnWrongThread => nsIBits::ERROR_TYPE_OBSERVER_ON_WRONG_THREAD,
+            ErrorType::MissingContext => nsIBits::ERROR_TYPE_MISSING_CONTEXT,
+            ErrorType::ContextOnWrongThread => nsIBits::ERROR_TYPE_CONTEXT_ON_WRONG_THREAD,
+            ErrorType::FailedToStartThread => nsIBits::ERROR_TYPE_FAILED_TO_START_THREAD,
+            ErrorType::FailedToConstructTaskRunnable => {
+                nsIBits::ERROR_TYPE_FAILED_TO_CONSTRUCT_TASK_RUNNABLE
+            }
+            ErrorType::FailedToDispatchRunnable => nsIBits::ERROR_TYPE_FAILED_TO_DISPATCH_RUNNABLE,
+            ErrorType::TransferAlreadyComplete => nsIBits::ERROR_TYPE_TRANSFER_ALREADY_COMPLETE,
+            ErrorType::OperationAlreadyInProgress => {
+                nsIBits::ERROR_TYPE_OPERATION_ALREADY_IN_PROGRESS
+            }
+            ErrorType::MissingBitsClient => nsIBits::ERROR_TYPE_MISSING_BITS_CLIENT,
+            ErrorType::FailedToGetJobStatus => nsIBits::ERROR_TYPE_FAILED_TO_GET_JOB_STATUS,
+            ErrorType::BitsStateError => nsIBits::ERROR_TYPE_BITS_STATE_ERROR,
+            ErrorType::BitsStateTransientError => nsIBits::ERROR_TYPE_BITS_STATE_TRANSIENT_ERROR,
+            ErrorType::BitsStateCancelled => nsIBits::ERROR_TYPE_BITS_STATE_CANCELLED,
+            ErrorType::BitsStateUnexpected => nsIBits::ERROR_TYPE_BITS_STATE_UNEXPECTED,
+            ErrorType::FailedToConnectToBcm => nsIBits::ERROR_TYPE_FAILED_TO_CONNECT_TO_BCM,
+        }
+    }
+}
+
+impl From<&PipeError> for ErrorType {
+    fn from(error: &PipeError) -> Self {
+        match error {
+            PipeError::NotConnected => ErrorType::PipeNotConnected,
+            PipeError::Timeout => ErrorType::PipeTimeout,
+            PipeError::WriteCount(_, _) => ErrorType::PipeBadWriteCount,
+            PipeError::Api(_) => ErrorType::PipeApiError,
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum ErrorStage {
+    Pretask,
+    CommandThread,
+    AgentCommunication,
+    BitsClient,
+    MainThread,
+}
+
+impl ErrorStage {
+    pub fn bits_code(&self) -> i32 {
+        let val = match self {
+            ErrorStage::Pretask => nsIBits::ERROR_STAGE_PRETASK,
+            ErrorStage::CommandThread => nsIBits::ERROR_STAGE_COMMAND_THREAD,
+            ErrorStage::AgentCommunication => nsIBits::ERROR_STAGE_AGENT_COMMUNICATION,
+            ErrorStage::BitsClient => nsIBits::ERROR_STAGE_BITS_CLIENT,
+            ErrorStage::MainThread => nsIBits::ERROR_STAGE_MAIN_THREAD,
+        };
+        val as i32
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum ErrorCode {
+    None,
+    Hresult(i32),
+    Nsresult(nsresult),
+    Message(nsCString),
+}
+
+impl From<ComedyError> for ErrorCode {
+    fn from(error: ComedyError) -> Self {
+        ErrorCode::Hresult(error.code())
+    }
+}
+
+impl From<HResultMessage> for ErrorCode {
+    fn from(result: HResultMessage) -> Self {
+        ErrorCode::Hresult(result.hr)
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct BitsTaskError {
+    pub error_type: ErrorType,
+    pub error_action: Action,
+    pub error_stage: ErrorStage,
+    pub error_code: ErrorCode,
+}
+
+impl BitsTaskError {
+    pub fn new(
+        error_type: ErrorType,
+        error_action: Action,
+        error_stage: ErrorStage,
+    ) -> BitsTaskError {
+        BitsTaskError {
+            error_type,
+            error_action,
+            error_stage,
+            error_code: ErrorCode::None,
+        }
+    }
+
+    pub fn missing_result(error_action: Action) -> BitsTaskError {
+        BitsTaskError {
+            error_type: ErrorType::MissingResultData,
+            error_action,
+            error_stage: ErrorStage::MainThread,
+            error_code: ErrorCode::None,
+        }
+    }
+
+    pub fn from_nsresult(
+        error_type: ErrorType,
+        error_action: Action,
+        error_stage: ErrorStage,
+        error_code: nsresult,
+    ) -> BitsTaskError {
+        BitsTaskError {
+            error_type,
+            error_action,
+            error_stage,
+            error_code: ErrorCode::Nsresult(error_code),
+        }
+    }
+
+    pub fn from_hresult(
+        error_type: ErrorType,
+        error_action: Action,
+        error_stage: ErrorStage,
+        error_code: i32,
+    ) -> BitsTaskError {
+        BitsTaskError {
+            error_type,
+            error_action,
+            error_stage,
+            error_code: ErrorCode::Hresult(error_code),
+        }
+    }
+
+    pub fn from_comedy(
+        error_type: ErrorType,
+        error_action: Action,
+        error_stage: ErrorStage,
+        comedy_error: ComedyError,
+    ) -> BitsTaskError {
+        BitsTaskError {
+            error_type,
+            error_action,
+            error_stage,
+            error_code: comedy_error.into(),
+        }
+    }
+
+    pub fn from_pipe(error_action: Action, pipe_error: PipeError) -> BitsTaskError {
+        let error_type = (&pipe_error).into();
+        match pipe_error {
+            PipeError::Api(comedy_error) => BitsTaskError {
+                error_type,
+                error_action,
+                error_stage: ErrorStage::AgentCommunication,
+                error_code: comedy_error.into(),
+            },
+            _ => BitsTaskError {
+                error_type,
+                error_action,
+                error_stage: ErrorStage::AgentCommunication,
+                error_code: ErrorCode::None,
+            },
+        }
+    }
+}
+
+impl From<BitsTaskError> for nsresult {
+    fn from(error: BitsTaskError) -> Self {
+        if let ErrorCode::Nsresult(rv) = error.error_code {
+            rv
+        } else {
+            NS_ERROR_FAILURE
+        }
+    }
+}
+
+impl From<StartJobFailure> for BitsTaskError {
+    fn from(error: StartJobFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::StartDownload;
+        match error {
+            StartJobFailure::ArgumentValidation(message) => BitsTaskError {
+                error_type: ErrorType::InvalidArgument,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+            StartJobFailure::Create(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToCreateBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            StartJobFailure::AddFile(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToAddFileToJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            StartJobFailure::ApplySettings(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToApplyBitsJobSettings,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            StartJobFailure::Resume(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToResumeBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            StartJobFailure::ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            StartJobFailure::OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            StartJobFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<MonitorJobFailure> for BitsTaskError {
+    fn from(error: MonitorJobFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::MonitorDownload;
+        match error {
+            MonitorJobFailure::ArgumentValidation(message) => BitsTaskError {
+                error_type: ErrorType::InvalidArgument,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+            MonitorJobFailure::NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            MonitorJobFailure::GetJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToGetBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            MonitorJobFailure::ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            MonitorJobFailure::OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            MonitorJobFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<SuspendJobFailure> for BitsTaskError {
+    fn from(error: SuspendJobFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::Suspend;
+        match error {
+            SuspendJobFailure::NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            SuspendJobFailure::GetJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToGetBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SuspendJobFailure::SuspendJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToSuspendBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SuspendJobFailure::ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SuspendJobFailure::OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SuspendJobFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<ResumeJobFailure> for BitsTaskError {
+    fn from(error: ResumeJobFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::Resume;
+        match error {
+            ResumeJobFailure::NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            ResumeJobFailure::GetJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToGetBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            ResumeJobFailure::ResumeJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToResumeBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            ResumeJobFailure::ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            ResumeJobFailure::OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            ResumeJobFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<SetJobPriorityFailure> for BitsTaskError {
+    fn from(error: SetJobPriorityFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::SetPriority;
+        match error {
+            SetJobPriorityFailure::NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            SetJobPriorityFailure::GetJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToGetBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SetJobPriorityFailure::ApplySettings(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToApplyBitsJobSettings,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SetJobPriorityFailure::ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SetJobPriorityFailure::OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            SetJobPriorityFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<SetNoProgressTimeoutFailure> for BitsTaskError {
+    fn from(error: SetNoProgressTimeoutFailure) -> Self {
+        use self::SetNoProgressTimeoutFailure::*;
+
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::SetNoProgressTimeout;
+        match error {
+            NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            GetJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToGetBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            ApplySettings(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToApplyBitsJobSettings,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<SetUpdateIntervalFailure> for BitsTaskError {
+    fn from(error: SetUpdateIntervalFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::SetMonitorInterval;
+        match error {
+            SetUpdateIntervalFailure::ArgumentValidation(message) => BitsTaskError {
+                error_type: ErrorType::InvalidArgument,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+            SetUpdateIntervalFailure::NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            SetUpdateIntervalFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<CompleteJobFailure> for BitsTaskError {
+    fn from(error: CompleteJobFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::Complete;
+        match error {
+            CompleteJobFailure::NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            CompleteJobFailure::GetJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToGetBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CompleteJobFailure::CompleteJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToCompleteBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CompleteJobFailure::PartialComplete => BitsTaskError {
+                error_type: ErrorType::PartiallyCompletedBitsJob,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            CompleteJobFailure::ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CompleteJobFailure::OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CompleteJobFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
+
+impl From<CancelJobFailure> for BitsTaskError {
+    fn from(error: CancelJobFailure) -> Self {
+        let error_stage = ErrorStage::BitsClient;
+        let error_action = Action::Cancel;
+        match error {
+            CancelJobFailure::NotFound => BitsTaskError {
+                error_type: ErrorType::BitsJobNotFound,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::None,
+            },
+            CancelJobFailure::GetJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToGetBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CancelJobFailure::CancelJob(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToCancelBitsJob,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CancelJobFailure::ConnectBcm(error_code) => BitsTaskError {
+                error_type: ErrorType::FailedToConnectToBcm,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CancelJobFailure::OtherBITS(error_code) => BitsTaskError {
+                error_type: ErrorType::OtherBitsError,
+                error_action,
+                error_stage,
+                error_code: error_code.into(),
+            },
+            CancelJobFailure::Other(message) => BitsTaskError {
+                error_type: ErrorType::OtherBitsClientError,
+                error_action,
+                error_stage,
+                error_code: ErrorCode::Message(nsCString::from(message)),
+            },
+        }
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/mod.rs b/toolkit/components/bitsdownload/src/bits_interface/mod.rs
new file mode 100644
index 0000000000..93d2ddeadd
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/mod.rs
@@ -0,0 +1,329 @@
+/* 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/. */
+mod action;
+mod dispatch_callback;
+mod error;
+mod monitor;
+mod request;
+mod string;
+mod task;
+mod xpcom_methods;
+
+pub use self::error::BitsTaskError;
+use self::{
+    action::Action,
+    error::{
+        ErrorStage::Pretask,
+        ErrorType::{
+            FailedToConstructTaskRunnable, FailedToDispatchRunnable, FailedToStartThread,
+            InvalidArgument, NotInitialized,
+        },
+    },
+    request::BitsRequest,
+    string::Guid_from_nsCString,
+    task::{ClientInitData, MonitorDownloadTask, StartDownloadTask},
+};
+use nsIBits_method; // From xpcom_method.rs
+
+use bits_client::BitsProxyUsage;
+use log::{info, warn};
+use moz_task::{create_thread, Task, TaskRunnable};
+use nserror::{nsresult, NS_ERROR_ALREADY_INITIALIZED, NS_OK};
+use nsstring::{nsACString, nsCString};
+use std::cell::Cell;
+use xpcom::{
+    interfaces::{
+        nsIBits, nsIBitsNewRequestCallback, nsIRequestObserver, nsISupports, nsIThread,
+        nsProxyUsage,
+    },
+    xpcom, xpcom_method, RefPtr,
+};
+
+#[no_mangle]
+pub unsafe extern "C" fn new_bits_service(result: *mut *const nsIBits) {
+    let service: RefPtr<BitsService> = BitsService::new();
+    RefPtr::new(service.coerce::<nsIBits>()).forget(&mut *result);
+}
+
+#[xpcom(implement(nsIBits), nonatomic)]
+pub struct BitsService {
+    // This command thread will be used to send commands (ex: Suspend, Resume)
+    // to a running job. It will be started up when the first job is created and
+    // shutdown when all jobs have been completed or cancelled.
+    command_thread: Cell<Option<RefPtr<nsIThread>>>,
+    client_init_data: Cell<Option<ClientInitData>>,
+    // This count will track the number of in-progress requests so that the
+    // service knows when the command_thread is no longer being used and can be
+    // shut down.
+    // `BitsRequest::new()` will increment this and it will be decremented
+    // either when cancel/complete is called, or when the request is dropped
+    // (if it didn't decrement it already).
+    // The count will also be incremented when an action to create a request
+    // starts and decremented when the action ends and returns the result via
+    // the callback. This prevents the command thread from being shut down while
+    // a job is being created.
+    request_count: Cell<u32>,
+}
+
+/// This implements the nsIBits interface, documented in nsIBits.idl, to enable
+/// BITS job management. Specifically, this interface can start a download or
+/// connect to an existing download. Doing so will create a BitsRequest through
+/// which the transfer can be further manipulated.
+///
+/// This is a primarily asynchronous interface, which is accomplished via
+/// callbacks of type nsIBitsNewRequestCallback. The callback is passed in as
+/// an argument and is then passed off-thread via a Task. The Task interacts
+/// with BITS and is dispatched back to the main thread with the BITS result.
+/// Back on the main thread, it returns that result via the callback including,
+/// if successful, a BitsRequest.
+impl BitsService {
+    pub fn new() -> RefPtr<BitsService> {
+        BitsService::allocate(InitBitsService {
+            command_thread: Cell::new(None),
+            client_init_data: Cell::new(None),
+            request_count: Cell::new(0),
+        })
+    }
+
+    fn get_client_init(&self) -> Option<ClientInitData> {
+        let maybe_init_data = self.client_init_data.take();
+        self.client_init_data.set(maybe_init_data.clone());
+        maybe_init_data
+    }
+
+    // Returns the handle to the command thread. If it has not been started yet,
+    // the thread will be started.
+    fn get_command_thread(&self) -> Result<RefPtr<nsIThread>, nsresult> {
+        let mut command_thread = self.command_thread.take();
+        if command_thread.is_none() {
+            command_thread.replace(create_thread("BitsCommander")?);
+        }
+        self.command_thread.set(command_thread.clone());
+        Ok(command_thread.unwrap())
+    }
+
+    // Asynchronously shuts down the command thread. The thread is not shutdown
+    // until the event queue is empty, so any tasks that were dispatched before
+    // this is called will still run.
+    // Leaves None in self.command_thread
+    fn shutdown_command_thread(&self) {
+        if let Some(command_thread) = self.command_thread.take() {
+            if let Err(rv) = unsafe { command_thread.AsyncShutdown() }.to_result() {
+                warn!("Failed to shut down command thread: {}", rv);
+                warn!("Releasing reference to thread that failed to shut down!");
+            }
+        }
+    }
+
+    fn dispatch_runnable_to_command_thread(
+        &self,
+        task: Box<dyn Task + Send + Sync>,
+        task_runnable_name: &'static str,
+        action: Action,
+    ) -> Result<(), BitsTaskError> {
+        let command_thread = self
+            .get_command_thread()
+            .map_err(|rv| BitsTaskError::from_nsresult(FailedToStartThread, action, Pretask, rv))?;
+        let runnable = TaskRunnable::new(task_runnable_name, task).map_err(|rv| {
+            BitsTaskError::from_nsresult(FailedToConstructTaskRunnable, action, Pretask, rv)
+        })?;
+        TaskRunnable::dispatch(runnable, &command_thread).map_err(|rv| {
+            BitsTaskError::from_nsresult(FailedToDispatchRunnable, action, Pretask, rv)
+        })
+    }
+
+    fn inc_request_count(&self) {
+        self.request_count.set(self.request_count.get() + 1);
+    }
+
+    fn dec_request_count(&self) {
+        let mut count = self.request_count.get();
+        if count == 0 {
+            warn!("Attempted to decrement request count, but it is 0");
+            return;
+        }
+        count -= 1;
+        self.request_count.set(count);
+
+        if count == 0 {
+            self.shutdown_command_thread();
+        }
+    }
+
+    xpcom_method!(
+        get_initialized => GetInitialized() -> bool
+    );
+    fn get_initialized(&self) -> Result<bool, nsresult> {
+        Ok(self.get_client_init().is_some())
+    }
+
+    xpcom_method!(
+        init => Init(
+            job_name: *const nsACString,
+            save_path_prefix: *const nsACString,
+            monitor_timeout_ms: u32
+        )
+    );
+    fn init(
+        &self,
+        job_name: &nsACString,
+        save_path_prefix: &nsACString,
+        monitor_timeout_ms: u32,
+    ) -> Result<(), nsresult> {
+        let previous_data = self.client_init_data.take();
+        if previous_data.is_some() {
+            self.client_init_data.set(previous_data);
+            return Err(NS_ERROR_ALREADY_INITIALIZED);
+        }
+
+        info!(
+            "BitsService initialized with job_name: {}, save_path_prefix: {}, timeout: {}",
+            job_name, save_path_prefix, monitor_timeout_ms,
+        );
+
+        self.client_init_data.set(Some(ClientInitData::new(
+            nsCString::from(job_name),
+            nsCString::from(save_path_prefix),
+            monitor_timeout_ms,
+        )));
+
+        Ok(())
+    }
+
+    nsIBits_method!(
+        [Action::StartDownload]
+        start_download => StartDownload(
+            download_url: *const nsACString,
+            save_rel_path: *const nsACString,
+            proxy: nsProxyUsage,
+            no_progress_timeout_secs: u32,
+            update_interval_ms: u32,
+            observer: *const nsIRequestObserver,
+            [optional] context: *const nsISupports,
+        )
+    );
+    fn start_download(
+        &self,
+        download_url: &nsACString,
+        save_rel_path: &nsACString,
+        proxy: nsProxyUsage,
+        no_progress_timeout_secs: u32,
+        update_interval_ms: u32,
+        observer: &nsIRequestObserver,
+        context: Option<&nsISupports>,
+        callback: &nsIBitsNewRequestCallback,
+    ) -> Result<(), BitsTaskError> {
+        let client_init_data = self
+            .get_client_init()
+            .ok_or_else(|| BitsTaskError::new(NotInitialized, Action::StartDownload, Pretask))?;
+        if update_interval_ms >= client_init_data.monitor_timeout_ms {
+            return Err(BitsTaskError::new(
+                InvalidArgument,
+                Action::StartDownload,
+                Pretask,
+            ));
+        }
+        let proxy = match proxy {
+            nsIBits::PROXY_NONE => BitsProxyUsage::NoProxy,
+            nsIBits::PROXY_PRECONFIG => BitsProxyUsage::Preconfig,
+            nsIBits::PROXY_AUTODETECT => BitsProxyUsage::AutoDetect,
+            _ => {
+                return Err(BitsTaskError::new(
+                    InvalidArgument,
+                    Action::StartDownload,
+                    Pretask,
+                ));
+            }
+        };
+
+        let task: Box<StartDownloadTask> = Box::new(StartDownloadTask::new(
+            client_init_data,
+            nsCString::from(download_url),
+            nsCString::from(save_rel_path),
+            proxy,
+            no_progress_timeout_secs,
+            update_interval_ms,
+            RefPtr::new(self),
+            RefPtr::new(observer),
+            context.map(RefPtr::new),
+            RefPtr::new(callback),
+        ));
+
+        let dispatch_result = self.dispatch_runnable_to_command_thread(
+            task,
+            "BitsService::start_download",
+            Action::StartDownload,
+        );
+
+        if dispatch_result.is_ok() {
+            // Increment the request count when we dispatch an action to start
+            // a job, decrement it when the action completes. See the
+            // declaration of InitBitsService::request_count for details.
+            self.inc_request_count();
+        }
+
+        dispatch_result
+    }
+
+    nsIBits_method!(
+        [Action::MonitorDownload]
+        monitor_download => MonitorDownload(
+            id: *const nsACString,
+            update_interval_ms: u32,
+            observer: *const nsIRequestObserver,
+            [optional] context: *const nsISupports,
+        )
+    );
+    fn monitor_download(
+        &self,
+        id: &nsACString,
+        update_interval_ms: u32,
+        observer: &nsIRequestObserver,
+        context: Option<&nsISupports>,
+        callback: &nsIBitsNewRequestCallback,
+    ) -> Result<(), BitsTaskError> {
+        let client_init_data = self
+            .get_client_init()
+            .ok_or_else(|| BitsTaskError::new(NotInitialized, Action::MonitorDownload, Pretask))?;
+        if update_interval_ms >= client_init_data.monitor_timeout_ms {
+            return Err(BitsTaskError::new(
+                InvalidArgument,
+                Action::MonitorDownload,
+                Pretask,
+            ));
+        }
+        let guid = Guid_from_nsCString(&nsCString::from(id), Action::MonitorDownload, Pretask)?;
+
+        let task: Box<MonitorDownloadTask> = Box::new(MonitorDownloadTask::new(
+            client_init_data,
+            guid,
+            update_interval_ms,
+            RefPtr::new(self),
+            RefPtr::new(observer),
+            context.map(RefPtr::new),
+            RefPtr::new(callback),
+        ));
+
+        let dispatch_result = self.dispatch_runnable_to_command_thread(
+            task,
+            "BitsService::monitor_download",
+            Action::MonitorDownload,
+        );
+
+        if dispatch_result.is_ok() {
+            // Increment the request count when we dispatch an action to start
+            // a job, decrement it when the action completes. See the
+            // declaration of InitBitsService::request_count for details.
+            self.inc_request_count();
+        }
+
+        dispatch_result
+    }
+}
+
+impl Drop for BitsService {
+    fn drop(&mut self) {
+        self.shutdown_command_thread();
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/monitor.rs b/toolkit/components/bitsdownload/src/bits_interface/monitor.rs
new file mode 100644
index 0000000000..4332efe04b
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/monitor.rs
@@ -0,0 +1,247 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use bits_interface::{error::ErrorType, BitsRequest};
+
+use bits_client::{
+    bits_protocol::HResultMessage, BitsJobState, BitsMonitorClient, Guid, JobStatus, PipeError,
+};
+use crossbeam_utils::atomic::AtomicCell;
+use log::error;
+use moz_task::{get_main_thread, is_main_thread};
+use nserror::{nsresult, NS_ERROR_ABORT, NS_ERROR_FAILURE, NS_OK};
+use nsstring::{nsACString, nsCString};
+use xpcom::{
+    interfaces::{nsIEventTarget, nsIThread},
+    xpcom, xpcom_method, RefPtr, ThreadBoundRefPtr,
+};
+
+/// This function takes the output of BitsMonitorClient::get_status() and uses
+/// it to determine whether the the transfer has started. If the argument
+/// contains an error, the transfer is considered started because we also
+/// consider a transfer stopped on error.
+/// This function is used to determine whether the OnStartRequest and OnProgress
+/// observer functions should be called.
+fn transfer_started(status: &Result<Result<JobStatus, HResultMessage>, PipeError>) -> bool {
+    match status.as_ref() {
+        Ok(Ok(job_status)) => match job_status.state {
+            BitsJobState::Queued | BitsJobState::Connecting => false,
+            _ => true,
+        },
+        Ok(Err(_)) => true,
+        Err(_) => true,
+    }
+}
+
+/// This function takes the output of BitsMonitorClient::get_status() and uses
+/// it to determine whether the the transfer has stopped. If the argument
+/// contains an error, the transfer is considered stopped.
+/// A number of things will be done when a transfer is completed, such as
+/// calling the observer's OnStopRequest method.
+fn transfer_completed(status: &Result<Result<JobStatus, HResultMessage>, PipeError>) -> bool {
+    match status.as_ref() {
+        Ok(Ok(job_status)) => match job_status.state {
+            BitsJobState::Error
+            | BitsJobState::Transferred
+            | BitsJobState::Acknowledged
+            | BitsJobState::Cancelled => true,
+            _ => false,
+        },
+        Ok(Err(_)) => true,
+        Err(_) => true,
+    }
+}
+
+/// BitsRequest implements nsIRequest, which means that it must be able to
+/// provide an nsresult status code. This function provides such a status code
+/// based on the output of BitsMonitorClient::get_status().
+fn status_to_nsresult(status: &Result<Result<JobStatus, HResultMessage>, PipeError>) -> nsresult {
+    match status.as_ref() {
+        Ok(Ok(job_status)) => match job_status.state {
+            BitsJobState::Cancelled => NS_ERROR_ABORT,
+            BitsJobState::Transferred | BitsJobState::Acknowledged => NS_OK,
+            _ => NS_ERROR_FAILURE,
+        },
+        Ok(Err(_)) => NS_ERROR_FAILURE,
+        Err(_) => NS_ERROR_FAILURE,
+    }
+}
+
+/// This function takes the output of BitsMonitorClient::get_status() and uses
+/// it to determine the result value of the request. This will take the form of
+/// an Optional ErrorType value with a None value indicating success.
+fn status_to_request_result(
+    status: &Result<Result<JobStatus, HResultMessage>, PipeError>,
+) -> Option<ErrorType> {
+    match status.as_ref() {
+        Ok(Ok(job_status)) => match job_status.state {
+            BitsJobState::Transferred | BitsJobState::Acknowledged => None,
+            BitsJobState::Cancelled => Some(ErrorType::BitsStateCancelled),
+            BitsJobState::Error => Some(ErrorType::BitsStateError),
+            BitsJobState::TransientError => Some(ErrorType::BitsStateTransientError),
+            _ => Some(ErrorType::BitsStateUnexpected),
+        },
+        Ok(Err(_)) => Some(ErrorType::FailedToGetJobStatus),
+        Err(pipe_error) => Some(pipe_error.into()),
+    }
+}
+
+/// MonitorRunnable is an nsIRunnable meant to be dispatched off thread. It will
+/// perform the following actions:
+///   1. Call BitsMonitorClient::get_status and store the result.
+///   2. Dispatch itself back to the main thread.
+///   3. Report the status to the observer.
+///   4. If the transfer has finished, free its data and return, otherwise:
+///   5. Dispatch itself back to its original thread and repeat from step 1.
+#[xpcom(implement(nsIRunnable, nsINamed), atomic)]
+pub struct MonitorRunnable {
+    request: AtomicCell<Option<ThreadBoundRefPtr<BitsRequest>>>,
+    id: Guid,
+    timeout: u32,
+    monitor_client: AtomicCell<Option<BitsMonitorClient>>,
+    // This cell contains an Option, possibly containing the return value of
+    // BitsMonitorClient::get_status.
+    status: AtomicCell<Option<Result<Result<JobStatus, HResultMessage>, PipeError>>>,
+    request_started: AtomicCell<bool>,
+    in_error_state: AtomicCell<bool>,
+}
+
+impl MonitorRunnable {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        timeout: u32,
+        monitor_client: BitsMonitorClient,
+    ) -> RefPtr<MonitorRunnable> {
+        MonitorRunnable::allocate(InitMonitorRunnable {
+            request: AtomicCell::new(Some(ThreadBoundRefPtr::new(request))),
+            id,
+            timeout,
+            monitor_client: AtomicCell::new(Some(monitor_client)),
+            status: AtomicCell::new(None),
+            request_started: AtomicCell::new(false),
+            in_error_state: AtomicCell::new(false),
+        })
+    }
+
+    pub fn dispatch(&self, thread: RefPtr<nsIThread>) -> Result<(), nsresult> {
+        unsafe { thread.DispatchFromScript(self.coerce(), nsIEventTarget::DISPATCH_NORMAL) }
+            .to_result()
+    }
+
+    fn free_mainthread_data(&self) {
+        if is_main_thread() {
+            // This is not safe to free unless on the main thread
+            self.request.swap(None);
+        } else {
+            error!("Attempting to free data on the main thread, but not on the main thread");
+        }
+    }
+
+    xpcom_method!(run => Run());
+
+    /// This method is essentially a error-handling wrapper around try_run.
+    /// This is done to make it easier to ensure that main-thread data is freed
+    /// on the main thread.
+    pub fn run(&self) -> Result<(), nsresult> {
+        if self.in_error_state.load() {
+            self.free_mainthread_data();
+            return Err(NS_ERROR_FAILURE);
+        }
+
+        self.try_run().or_else(|error_message| {
+            error!("{}", error_message);
+
+            // Once an error has been encountered, we need to free all of our
+            // data, but it all needs to be freed on the main thread.
+            self.in_error_state.store(true);
+            if is_main_thread() {
+                self.free_mainthread_data();
+                Err(NS_ERROR_FAILURE)
+            } else {
+                self.dispatch(get_main_thread()?)
+            }
+        })
+    }
+
+    /// This function performs all the primary functionality of MonitorRunnable.
+    /// See the documentation for InitMonitorRunnable/MonitorRunnable for
+    /// details.
+    pub fn try_run(&self) -> Result<(), String> {
+        if !is_main_thread() {
+            let mut monitor_client = self
+                .monitor_client
+                .swap(None)
+                .ok_or("Missing monitor client")?;
+            self.status
+                .store(Some(monitor_client.get_status(self.timeout)));
+            self.monitor_client.store(Some(monitor_client));
+
+            let main_thread =
+                get_main_thread().map_err(|rv| format!("Unable to get main thread: {}", rv))?;
+
+            self.dispatch(main_thread)
+                .map_err(|rv| format!("Unable to dispatch to main thread: {}", rv))
+        } else {
+            let status = self.status.swap(None).ok_or("Missing status object")?;
+            let tb_request = self.request.swap(None).ok_or("Missing request")?;
+
+            // This block bounds the scope for request to ensure that it ends
+            // before re-storing tb_request.
+            let maybe_next_thread: Option<RefPtr<nsIThread>> = {
+                let request = tb_request
+                    .get_ref()
+                    .ok_or("BitsRequest is on the wrong thread")?;
+
+                if !self.request_started.load() && transfer_started(&status) {
+                    self.request_started.store(true);
+                    request.on_start();
+                }
+
+                if self.request_started.load() {
+                    if let Ok(Ok(job_status)) = status.as_ref() {
+                        let transferred_bytes = job_status.progress.transferred_bytes as i64;
+                        let total_bytes = match job_status.progress.total_bytes {
+                            Some(total) => total as i64,
+                            None => -1i64,
+                        };
+                        request.on_progress(transferred_bytes, total_bytes);
+                    }
+                }
+
+                if transfer_completed(&status) {
+                    request.on_stop(Some((
+                        status_to_nsresult(&status),
+                        status_to_request_result(&status),
+                    )));
+
+                    // Transfer completed. No need to dispatch back to the monitor thread.
+                    None
+                } else {
+                    Some(
+                        request
+                            .get_monitor_thread()
+                            .ok_or("Missing monitor thread")?,
+                    )
+                }
+            };
+
+            self.request.store(Some(tb_request));
+
+            match maybe_next_thread {
+                Some(next_thread) => self
+                    .dispatch(next_thread)
+                    .map_err(|rv| format!("Unable to dispatch to thread: {}", rv)),
+                None => {
+                    self.free_mainthread_data();
+                    Ok(())
+                }
+            }
+        }
+    }
+
+    xpcom_method!(get_name => GetName() -> nsACString);
+    fn get_name(&self) -> Result<nsCString, nsresult> {
+        Ok(nsCString::from("BitsRequest::Monitor"))
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/request.rs b/toolkit/components/bitsdownload/src/bits_interface/request.rs
new file mode 100644
index 0000000000..460e3b4950
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/request.rs
@@ -0,0 +1,763 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use super::{
+    action::{Action, ServiceAction},
+    error::{
+        ErrorStage::{MainThread, Pretask},
+        ErrorType,
+        ErrorType::{
+            BitsStateCancelled, FailedToDispatchRunnable, FailedToStartThread, InvalidArgument,
+            OperationAlreadyInProgress, TransferAlreadyComplete,
+        },
+    },
+    monitor::MonitorRunnable,
+    task::{
+        CancelTask, ChangeMonitorIntervalTask, CompleteTask, Priority, ResumeTask,
+        SetNoProgressTimeoutTask, SetPriorityTask, SuspendTask,
+    },
+    BitsService, BitsTaskError,
+};
+use nsIBitsRequest_method; // From xpcom_method.rs
+
+use bits_client::{BitsMonitorClient, Guid};
+use log::{error, info, warn};
+use moz_task::create_thread;
+use nserror::{nsresult, NS_ERROR_ABORT, NS_ERROR_NOT_IMPLEMENTED, NS_OK};
+use nsstring::{nsACString, nsCString};
+use std::{cell::Cell, fmt};
+use xpcom::{
+    interfaces::{
+        nsIBits, nsIBitsCallback, nsILoadGroup, nsIProgressEventSink, nsIRequestObserver,
+        nsISupports, nsIThread, nsLoadFlags,
+    },
+    xpcom, xpcom_method, RefPtr, XpCom,
+};
+
+/// This structure exists to resolve a race condition. If cancel is called, we
+/// don't want to immediately set the request state to cancelled, because the
+/// cancel action could fail. But it's possible that on_stop() could be called
+/// before the cancel action resolves, and the correct status should be sent to
+/// OnStopRequest.
+/// This is how this race condition will be resolved:
+///   1.  cancel() is called, which sets the CancelAction to InProgress and
+///       stores in it the status that should be set if it succeeds.
+///   2.  cancel() dispatches the cancel task off thread.
+/// At this point, things unfold in one of two ways, depending on the race
+/// condition. Either:
+///   3.  The cancel task returns to the main thread and calls
+///       BitsRequest::finish_cancel_action.
+///   4.  If the cancel action succeeded, the appropriate status codes are set
+///       and the CancelAction is set to RequestEndPending.
+///       If the cancel action failed, the CancelAction is set to NotInProgress.
+///   5.  The MonitorRunnable detects that the transfer has ended and calls
+///       BitsRequest::on_stop, passing different status codes.
+///   6.  BitsRequest::on_stop checks the CancelAction and
+///       If the cancel action succeeded and RequestEndPending is set, the
+///       status codes that were set by BitsRequest::finish_cancel_action are
+///       left untouched.
+///       If the cancel action failed and NotInProgress is set, the status codes
+///       passed to BitsRequest::on_stop are set.
+///   7.  onStopRequest is called with the correct status code.
+/// Or, if MonitorRunnable calls on_stop before the cancel task can finish:
+///   3.  The MonitorRunnable detects that the transfer has ended and calls
+///       BitsRequest::on_stop, passing status codes to it.
+///   4.  BitsRequest::on_stop checks the CancelAction, sees it is set to
+///       InProgress, and sets it to RequestEndedWhileInProgress, carrying over
+///       the status code from InProgress.
+///   5.  BitsRequest::on_stop sets the status to the value passed to it, which
+///       will be overwritten if the cancel action succeeds, but kept if it
+///       fails.
+///   6.  BitsRequest::on_stop returns early, without calling OnStopRequest.
+///   7.  The cancel task returns to the main thread and calls
+///       BitsRequest::finish_cancel_action.
+///   8.  If the cancel action succeeded, the status codes are set from the
+///       value stored in RequestEndedWhileInProgress.
+///       If the cancel action failed, the status codes are not changed.
+///   9.  The CancelAction is set to NotInProgress.
+///   10. BitsRequest::finish_cancel_action calls BitsRequest::on_stop without
+///       passing it any status codes.
+///   11. onStopRequest is called with the correct status code.
+#[derive(Clone, Copy, PartialEq)]
+enum CancelAction {
+    NotInProgress,
+    InProgress(Option<nsresult>),
+    RequestEndedWhileInProgress(Option<nsresult>),
+    RequestEndPending,
+}
+
+#[xpcom(implement(nsIBitsRequest), nonatomic)]
+pub struct BitsRequest {
+    bits_id: Guid,
+    bits_service: RefPtr<BitsService>,
+    // Stores the value to be returned by nsIRequest::IsPending.
+    download_pending: Cell<bool>,
+    // Stores the value to be returned by nsIRequest::GetStatus.
+    download_status_nsresult: Cell<nsresult>,
+    // Stores an ErrorType if the request has failed, or None to represent the
+    // success state.
+    download_status_error_type: Cell<Option<ErrorType>>,
+    // This option will be None only after OnStopRequest has been fired.
+    monitor_thread: Cell<Option<RefPtr<nsIThread>>>,
+    monitor_timeout_ms: u32,
+    observer: RefPtr<nsIRequestObserver>,
+    // started indicates whether or not OnStartRequest has been fired.
+    started: Cell<bool>,
+    // finished indicates whether or not we have called
+    // BitsService::dec_request_count() to (assuming that there are no other
+    // requests) shutdown the command thread.
+    finished: Cell<bool>,
+    cancel_action: Cell<CancelAction>,
+}
+
+impl fmt::Debug for BitsRequest {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "BitsRequest {{ id: {} }}", self.bits_id)
+    }
+}
+
+/// This implements the nsIBitsRequest interface, documented in nsIBits.idl, to
+/// enable BITS job management. This interface deals only with BITS jobs that
+/// already exist. Jobs can be created via BitsService, which will create a
+/// BitsRequest for that job.
+///
+/// This is a primarily asynchronous interface, which is accomplished via
+/// callbacks of type nsIBitsCallback. The callback is passed in as an argument
+/// and is then passed off-thread via a Task. The Task interacts with BITS and
+/// is dispatched back to the main thread with the BITS result. Back on the main
+/// thread, it returns that result via the callback.
+impl BitsRequest {
+    pub fn new(
+        id: Guid,
+        bits_service: RefPtr<BitsService>,
+        monitor_timeout_ms: u32,
+        observer: RefPtr<nsIRequestObserver>,
+        context: Option<RefPtr<nsISupports>>,
+        monitor_client: BitsMonitorClient,
+        action: ServiceAction,
+    ) -> Result<RefPtr<BitsRequest>, BitsTaskError> {
+        let _ = context;
+        let action: Action = action.into();
+        let monitor_thread = create_thread("BitsMonitor").map_err(|rv| {
+            BitsTaskError::from_nsresult(FailedToStartThread, action, MainThread, rv)
+        })?;
+
+        // BitsRequest.drop() will call dec_request_count
+        bits_service.inc_request_count();
+        let request: RefPtr<BitsRequest> = BitsRequest::allocate(InitBitsRequest {
+            bits_id: id.clone(),
+            bits_service,
+            download_pending: Cell::new(true),
+            download_status_nsresult: Cell::new(NS_OK),
+            download_status_error_type: Cell::new(None),
+            monitor_thread: Cell::new(Some(monitor_thread.clone())),
+            monitor_timeout_ms,
+            observer,
+            started: Cell::new(false),
+            finished: Cell::new(false),
+            cancel_action: Cell::new(CancelAction::NotInProgress),
+        });
+
+        let monitor_runnable =
+            MonitorRunnable::new(request.clone(), id, monitor_timeout_ms, monitor_client);
+
+        if let Err(rv) = monitor_runnable.dispatch(monitor_thread.clone()) {
+            request.shutdown_monitor_thread();
+            return Err(BitsTaskError::from_nsresult(
+                FailedToDispatchRunnable,
+                action,
+                MainThread,
+                rv,
+            ));
+        }
+
+        Ok(request)
+    }
+
+    pub fn get_monitor_thread(&self) -> Option<RefPtr<nsIThread>> {
+        let monitor_thread = self.monitor_thread.take();
+        self.monitor_thread.set(monitor_thread.clone());
+        monitor_thread
+    }
+
+    fn has_monitor_thread(&self) -> bool {
+        let maybe_monitor_thread = self.monitor_thread.take();
+        let transferred = maybe_monitor_thread.is_some();
+        self.monitor_thread.set(maybe_monitor_thread);
+        transferred
+    }
+
+    /// If this returns an true, it means that:
+    ///   - The monitor thread and monitor runnable may have been shut down
+    ///   - The BITS job is not in the TRANSFERRING state
+    ///   - The download either completed, failed, or was cancelled
+    ///   - The BITS job may or may not still need complete() or cancel() to be
+    ///     called on it
+    fn request_has_transferred(&self) -> bool {
+        self.request_has_completed() || !self.has_monitor_thread()
+    }
+
+    /// If this returns an error, it means that:
+    ///   - complete() or cancel() has been called on the BITS job.
+    ///   - BitsService::dec_request_count has already been called.
+    ///   - The BitsClient object that this request was using may have been
+    ///     dropped.
+    fn request_has_completed(&self) -> bool {
+        self.finished.get()
+    }
+
+    fn shutdown_monitor_thread(&self) {
+        if let Some(monitor_thread) = self.monitor_thread.take() {
+            if let Err(rv) = unsafe { monitor_thread.AsyncShutdown() }.to_result() {
+                warn!("Failed to shut down monitor thread: {:?}", rv);
+                warn!("Releasing reference to thread that failed to shut down!");
+            }
+        }
+    }
+
+    /**
+     * To be called when the transfer starts. Fires observer.OnStartRequest exactly once.
+     */
+    pub fn on_start(&self) {
+        if self.started.get() {
+            return;
+        }
+        self.started.set(true);
+        if let Err(rv) = unsafe { self.observer.OnStartRequest(self.coerce()) }.to_result() {
+            // This behavior is specified by nsIRequestObserver.
+            // See nsIRequestObserver.idl
+            info!(
+                "Cancelling download because OnStartRequest rejected with: {:?}",
+                rv
+            );
+            if let Err(rv) = self.cancel(NS_ERROR_ABORT, None) {
+                warn!("Failed to cancel download: {:?}", rv);
+            }
+        }
+    }
+
+    pub fn on_progress(&self, transferred_bytes: i64, total_bytes: i64) {
+        if let Some(progress_event_sink) = self.observer.query_interface::<nsIProgressEventSink>() {
+            unsafe {
+                progress_event_sink.OnProgress(self.coerce(), transferred_bytes, total_bytes);
+            }
+        }
+    }
+
+    /// To be called when the transfer stops (fails or completes). Fires
+    /// observer.OnStopRequest exactly once, though the call may be delayed to
+    /// resolve a race condition.
+    ///
+    /// The status values, if passed, will be stored in download_status_nsresult
+    /// and download_status_error_type, unless they have been overridden by a
+    /// cancel action.
+    ///
+    /// See the documentation for CancelAction for details.
+    pub fn on_stop(&self, maybe_status: Option<(nsresult, Option<ErrorType>)>) {
+        if !self.has_monitor_thread() {
+            // If the request has already stopped, don't stop it again
+            return;
+        }
+
+        match self.cancel_action.get() {
+            CancelAction::InProgress(saved_status)
+            | CancelAction::RequestEndedWhileInProgress(saved_status) => {
+                if let Some((status, result)) = maybe_status {
+                    self.download_status_nsresult.set(status);
+                    self.download_status_error_type.set(result);
+                }
+
+                info!("Deferring OnStopRequest until Cancel Task completes");
+                self.cancel_action
+                    .set(CancelAction::RequestEndedWhileInProgress(saved_status));
+                return;
+            }
+            CancelAction::NotInProgress => {
+                if let Some((status, result)) = maybe_status {
+                    self.download_status_nsresult.set(status);
+                    self.download_status_error_type.set(result);
+                }
+            }
+            CancelAction::RequestEndPending => {
+                // Don't set the status variables if the end of this request was
+                // the result of a cancel action. The cancel action already set
+                // those values and they should not be changed.
+                // See the CancelAction documentation for details.
+            }
+        }
+
+        self.download_pending.set(false);
+        self.shutdown_monitor_thread();
+        unsafe {
+            self.observer
+                .OnStopRequest(self.coerce(), self.download_status_nsresult.get());
+        }
+    }
+
+    /// To be called after a cancel or complete task has run successfully. If
+    /// this is the only BitsRequest running, this will shut down
+    /// BitsService's command thread, destroying the BitsClient.
+    pub fn on_finished(&self) {
+        if self.finished.get() {
+            return;
+        }
+        self.finished.set(true);
+        self.bits_service.dec_request_count();
+    }
+
+    // Return the same thing for GetBitsId() and GetName().
+    xpcom_method!(
+        maybe_get_bits_id => GetBitsId() -> nsACString
+    );
+    xpcom_method!(
+        maybe_get_bits_id => GetName() -> nsACString
+    );
+    fn maybe_get_bits_id(&self) -> Result<nsCString, nsresult> {
+        Ok(self.get_bits_id())
+    }
+    pub fn get_bits_id(&self) -> nsCString {
+        nsCString::from(self.bits_id.to_string())
+    }
+
+    xpcom_method!(
+        get_bits_transfer_error_nsIBitsRequest => GetTransferError() -> i32
+    );
+    #[allow(non_snake_case)]
+    fn get_bits_transfer_error_nsIBitsRequest(&self) -> Result<i32, nsresult> {
+        let error_type = match self.download_status_error_type.get() {
+            None => nsIBits::ERROR_TYPE_SUCCESS,
+            Some(error_type) => error_type.bits_code(),
+        };
+        Ok(error_type)
+    }
+
+    xpcom_method!(
+        is_pending => IsPending() -> bool
+    );
+    fn is_pending(&self) -> Result<bool, nsresult> {
+        Ok(self.download_pending.get())
+    }
+
+    xpcom_method!(
+        get_status_nsIRequest => GetStatus() -> nsresult
+    );
+    #[allow(non_snake_case)]
+    fn get_status_nsIRequest(&self) -> Result<nsresult, nsresult> {
+        Ok(self.get_status())
+    }
+    pub fn get_status(&self) -> nsresult {
+        self.download_status_nsresult.get()
+    }
+
+    nsIBitsRequest_method!(
+        [Action::SetMonitorInterval]
+        change_monitor_interval => ChangeMonitorInterval(update_interval_ms: u32)
+    );
+    fn change_monitor_interval(
+        &self,
+        update_interval_ms: u32,
+        callback: &nsIBitsCallback,
+    ) -> Result<(), BitsTaskError> {
+        if update_interval_ms == 0 || update_interval_ms >= self.monitor_timeout_ms {
+            return Err(BitsTaskError::new(
+                InvalidArgument,
+                Action::SetMonitorInterval,
+                Pretask,
+            ));
+        }
+        if self.request_has_transferred() {
+            return Err(BitsTaskError::new(
+                TransferAlreadyComplete,
+                Action::SetMonitorInterval,
+                Pretask,
+            ));
+        }
+
+        let task: Box<ChangeMonitorIntervalTask> = Box::new(ChangeMonitorIntervalTask::new(
+            RefPtr::new(self),
+            self.bits_id.clone(),
+            update_interval_ms,
+            RefPtr::new(callback),
+        ));
+
+        self.bits_service.dispatch_runnable_to_command_thread(
+            task,
+            "BitsRequest::change_monitor_interval",
+            Action::SetMonitorInterval,
+        )
+    }
+
+    nsIBitsRequest_method!(
+        [Action::Cancel]
+        cancel_nsIBitsRequest => CancelAsync(status: nsresult)
+    );
+    #[allow(non_snake_case)]
+    fn cancel_nsIBitsRequest(
+        &self,
+        status: nsresult,
+        callback: &nsIBitsCallback,
+    ) -> Result<(), BitsTaskError> {
+        self.cancel(status, Some(RefPtr::new(callback)))
+    }
+    xpcom_method!(
+        cancel_nsIRequest => Cancel(status: nsresult)
+    );
+    #[allow(non_snake_case)]
+    fn cancel_nsIRequest(&self, status: nsresult) -> Result<(), BitsTaskError> {
+        self.cancel(status, None)
+    }
+
+    fn cancel(
+        &self,
+        status: nsresult,
+        callback: Option<RefPtr<nsIBitsCallback>>,
+    ) -> Result<(), BitsTaskError> {
+        if status.clone().succeeded() {
+            return Err(BitsTaskError::new(InvalidArgument, Action::Cancel, Pretask));
+        }
+        if self.request_has_completed() {
+            return Err(BitsTaskError::new(
+                TransferAlreadyComplete,
+                Action::Cancel,
+                Pretask,
+            ));
+        }
+
+        // If the transfer is still in a success state, cancelling it should move it to the failure
+        // state that was passed. But if the transfer already failed, the only reason to call cancel
+        // is to remove the job from BITS. So in that case, we should keep the failure status that
+        // we already have.
+        let maybe_status: Option<nsresult> = if self.download_status_nsresult.get().failed() {
+            None
+        } else {
+            Some(status)
+        };
+
+        if self.cancel_action.get() != CancelAction::NotInProgress {
+            return Err(BitsTaskError::new(
+                OperationAlreadyInProgress,
+                Action::Cancel,
+                Pretask,
+            ));
+        }
+        self.cancel_action
+            .set(CancelAction::InProgress(maybe_status));
+
+        let task: Box<CancelTask> = Box::new(CancelTask::new(
+            RefPtr::new(self),
+            self.bits_id.clone(),
+            callback,
+        ));
+
+        self.bits_service.dispatch_runnable_to_command_thread(
+            task,
+            "BitsRequest::cancel",
+            Action::Cancel,
+        )
+    }
+
+    /// This function must be called when a cancel action completes.
+    ///
+    /// See the documentation for CancelAction for details.
+    pub fn finish_cancel_action(&self, cancelled_successfully: bool) {
+        let (maybe_status, transfer_ended) = match self.cancel_action.get() {
+            CancelAction::InProgress(maybe_status) => (maybe_status, false),
+            CancelAction::RequestEndedWhileInProgress(maybe_status) => (maybe_status, true),
+            _ => {
+                error!("End of cancel action, but cancel action is not in progress!");
+                return;
+            }
+        };
+        info!(
+            "Finishing cancel action. cancel success = {}",
+            cancelled_successfully
+        );
+        if cancelled_successfully {
+            // If no status was provided, it is because this cancel action removed the BITS job
+            // after the job had already failed. Keep the original error codes.
+            if let Some(status) = maybe_status {
+                self.download_status_nsresult.set(status);
+                self.download_status_error_type
+                    .set(Some(BitsStateCancelled));
+            }
+        }
+
+        let next_stage = if cancelled_successfully && !transfer_ended {
+            // This signals on_stop not to allow the status codes set above to
+            // be overridden by the ones passed to it.
+            CancelAction::RequestEndPending
+        } else {
+            CancelAction::NotInProgress
+        };
+        self.cancel_action.set(next_stage);
+
+        if cancelled_successfully {
+            self.on_finished();
+        }
+
+        if transfer_ended {
+            info!("Running deferred OnStopRequest");
+            self.on_stop(None);
+        }
+    }
+
+    nsIBitsRequest_method!(
+        [Action::SetPriority]
+        set_priority_high => SetPriorityHigh()
+    );
+    fn set_priority_high(&self, callback: &nsIBitsCallback) -> Result<(), BitsTaskError> {
+        self.set_priority(Priority::High, callback)
+    }
+
+    nsIBitsRequest_method!(
+        [Action::SetPriority]
+        set_priority_low => SetPriorityLow()
+    );
+    fn set_priority_low(&self, callback: &nsIBitsCallback) -> Result<(), BitsTaskError> {
+        self.set_priority(Priority::Low, callback)
+    }
+
+    fn set_priority(
+        &self,
+        priority: Priority,
+        callback: &nsIBitsCallback,
+    ) -> Result<(), BitsTaskError> {
+        if self.request_has_transferred() {
+            return Err(BitsTaskError::new(
+                TransferAlreadyComplete,
+                Action::SetPriority,
+                Pretask,
+            ));
+        }
+
+        let task: Box<SetPriorityTask> = Box::new(SetPriorityTask::new(
+            RefPtr::new(self),
+            self.bits_id.clone(),
+            priority,
+            RefPtr::new(callback),
+        ));
+
+        self.bits_service.dispatch_runnable_to_command_thread(
+            task,
+            "BitsRequest::set_priority",
+            Action::SetPriority,
+        )
+    }
+
+    nsIBitsRequest_method!(
+        [Action::SetNoProgressTimeout]
+        set_no_progress_timeout => SetNoProgressTimeout(timeout_secs: u32)
+    );
+    fn set_no_progress_timeout(
+        &self,
+        timeout_secs: u32,
+        callback: &nsIBitsCallback,
+    ) -> Result<(), BitsTaskError> {
+        if self.request_has_transferred() {
+            return Err(BitsTaskError::new(
+                TransferAlreadyComplete,
+                Action::SetNoProgressTimeout,
+                Pretask,
+            ));
+        }
+
+        let task: Box<SetNoProgressTimeoutTask> = Box::new(SetNoProgressTimeoutTask::new(
+            RefPtr::new(self),
+            self.bits_id.clone(),
+            timeout_secs,
+            RefPtr::new(callback),
+        ));
+
+        self.bits_service.dispatch_runnable_to_command_thread(
+            task,
+            "BitsRequest::set_no_progress_timeout",
+            Action::SetNoProgressTimeout,
+        )
+    }
+
+    nsIBitsRequest_method!(
+        [Action::Complete]
+        complete => Complete()
+    );
+    fn complete(&self, callback: &nsIBitsCallback) -> Result<(), BitsTaskError> {
+        if self.request_has_completed() {
+            return Err(BitsTaskError::new(
+                TransferAlreadyComplete,
+                Action::Complete,
+                Pretask,
+            ));
+        }
+
+        let task: Box<CompleteTask> = Box::new(CompleteTask::new(
+            RefPtr::new(self),
+            self.bits_id.clone(),
+            RefPtr::new(callback),
+        ));
+
+        self.bits_service.dispatch_runnable_to_command_thread(
+            task,
+            "BitsRequest::complete",
+            Action::Complete,
+        )
+    }
+
+    nsIBitsRequest_method!(
+        [Action::Suspend]
+        suspend_nsIBitsRequest => SuspendAsync()
+    );
+    #[allow(non_snake_case)]
+    fn suspend_nsIBitsRequest(&self, callback: &nsIBitsCallback) -> Result<(), BitsTaskError> {
+        self.suspend(Some(RefPtr::new(callback)))
+    }
+    xpcom_method!(
+        suspend_nsIRequest => Suspend()
+    );
+    #[allow(non_snake_case)]
+    fn suspend_nsIRequest(&self) -> Result<(), BitsTaskError> {
+        self.suspend(None)
+    }
+
+    fn suspend(&self, callback: Option<RefPtr<nsIBitsCallback>>) -> Result<(), BitsTaskError> {
+        if self.request_has_transferred() {
+            return Err(BitsTaskError::new(
+                TransferAlreadyComplete,
+                Action::Suspend,
+                Pretask,
+            ));
+        }
+
+        let task: Box<SuspendTask> = Box::new(SuspendTask::new(
+            RefPtr::new(self),
+            self.bits_id.clone(),
+            callback,
+        ));
+
+        self.bits_service.dispatch_runnable_to_command_thread(
+            task,
+            "BitsRequest::suspend",
+            Action::Suspend,
+        )
+    }
+
+    nsIBitsRequest_method!(
+        [Action::Resume]
+        resume_nsIBitsRequest => ResumeAsync()
+    );
+    #[allow(non_snake_case)]
+    fn resume_nsIBitsRequest(&self, callback: &nsIBitsCallback) -> Result<(), BitsTaskError> {
+        self.resume(Some(RefPtr::new(callback)))
+    }
+    xpcom_method!(
+        resume_nsIRequest => Resume()
+    );
+    #[allow(non_snake_case)]
+    fn resume_nsIRequest(&self) -> Result<(), BitsTaskError> {
+        self.resume(None)
+    }
+
+    fn resume(&self, callback: Option<RefPtr<nsIBitsCallback>>) -> Result<(), BitsTaskError> {
+        if self.request_has_transferred() {
+            return Err(BitsTaskError::new(
+                TransferAlreadyComplete,
+                Action::Resume,
+                Pretask,
+            ));
+        }
+
+        let task: Box<ResumeTask> = Box::new(ResumeTask::new(
+            RefPtr::new(self),
+            self.bits_id.clone(),
+            callback,
+        ));
+
+        self.bits_service.dispatch_runnable_to_command_thread(
+            task,
+            "BitsRequest::resume",
+            Action::Resume,
+        )
+    }
+
+    xpcom_method!(
+        get_load_group => GetLoadGroup() -> *const nsILoadGroup
+    );
+
+    /**
+     * As stated in nsIBits.idl, nsIBits interfaces are not expected to
+     * implement the loadGroup or loadFlags attributes. This implementation
+     * provides only null implementations only for these methods.
+     */
+    fn get_load_group(&self) -> Result<RefPtr<nsILoadGroup>, nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        set_load_group => SetLoadGroup(_load_group: *const nsILoadGroup)
+    );
+    fn set_load_group(&self, _load_group: &nsILoadGroup) -> Result<(), nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        get_load_flags => GetLoadFlags() -> nsLoadFlags
+    );
+    fn get_load_flags(&self) -> Result<nsLoadFlags, nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        set_load_flags => SetLoadFlags(_load_flags: nsLoadFlags)
+    );
+    fn set_load_flags(&self, _load_flags: nsLoadFlags) -> Result<(), nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        get_trr_mode => GetTRRMode() -> u32
+    );
+    fn get_trr_mode(&self) -> Result<u32, nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        set_trr_mode => SetTRRMode(_trr_mode: u32)
+    );
+    fn set_trr_mode(&self, _trr_mode: u32) -> Result<(), nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        get_canceled_reason => GetCanceledReason() -> nsACString
+    );
+    fn get_canceled_reason(&self) -> Result<nsCString, nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        set_canceled_reason => SetCanceledReason(_reason: *const nsACString)
+    );
+    fn set_canceled_reason(&self, _reason: *const nsACString) -> Result<(), nsresult> {
+        Err(NS_ERROR_NOT_IMPLEMENTED)
+    }
+
+    xpcom_method!(
+        cancel_with_reason_nsIRequest => CancelWithReason(status: nsresult, _reason: *const nsACString)
+    );
+    #[allow(non_snake_case)]
+    fn cancel_with_reason_nsIRequest(
+        &self,
+        status: nsresult,
+        _reason: *const nsACString,
+    ) -> Result<(), BitsTaskError> {
+        self.cancel(status, None)
+    }
+}
+
+impl Drop for BitsRequest {
+    fn drop(&mut self) {
+        // Make sure that the monitor thread gets cleaned up.
+        self.shutdown_monitor_thread();
+        // Make sure we tell BitsService that we are done with the command thread.
+        self.on_finished();
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/string.rs b/toolkit/components/bitsdownload/src/bits_interface/string.rs
new file mode 100644
index 0000000000..c3111b7cba
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/string.rs
@@ -0,0 +1,80 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use super::{
+    action::Action,
+    error::{BitsTaskError, ErrorStage, ErrorType},
+};
+
+use bits_client::Guid;
+use nsstring::nsCString;
+use std::ffi::OsString;
+use std::{str, str::FromStr};
+
+/// This function is fallible, and the consumers would prefer a BitsTaskError
+/// in the failure case. To facilitate that, this function takes some params
+/// that give it the data necessary to construct the BitsTaskError if it fails.
+/// If it succeeds, those values will be unused.
+#[allow(non_snake_case)]
+pub fn nsCString_to_String(
+    value: &nsCString,
+    error_action: Action,
+    error_stage: ErrorStage,
+) -> Result<String, BitsTaskError> {
+    match String::from_utf8(value[..].to_vec()) {
+        Ok(s) => Ok(s),
+        Err(_) => Err(BitsTaskError::new(
+            ErrorType::NoUtf8Conversion,
+            error_action,
+            error_stage,
+        )),
+    }
+}
+
+/// This function is fallible, and the consumers would prefer a BitsTaskError
+/// in the failure case. To facilitate that, this function takes some params
+/// that give it the data necessary to construct the BitsTaskError if it fails.
+/// If it succeeds, those values will be unused.
+#[allow(non_snake_case)]
+pub fn nsCString_to_OsString(
+    value: &nsCString,
+    error_action: Action,
+    error_stage: ErrorStage,
+) -> Result<OsString, BitsTaskError> {
+    Ok(OsString::from(nsCString_to_String(
+        value,
+        error_action,
+        error_stage,
+    )?))
+}
+
+/// This function is fallible, and the consumers would prefer a BitsTaskError
+/// in the failure case. To facilitate that, this function takes some params
+/// that give it the data necessary to construct the BitsTaskError if it fails.
+/// If it succeeds, those values will be unused.
+#[allow(non_snake_case)]
+pub fn Guid_from_nsCString(
+    value: &nsCString,
+    error_action: Action,
+    error_stage: ErrorStage,
+) -> Result<Guid, BitsTaskError> {
+    let vector = &value[..].to_vec();
+    let string = match str::from_utf8(vector) {
+        Ok(s) => s,
+        Err(_) => {
+            return Err(BitsTaskError::new(
+                ErrorType::NoUtf8Conversion,
+                error_action,
+                error_stage,
+            ));
+        }
+    };
+    Guid::from_str(string).map_err(|comedy_error| {
+        BitsTaskError::from_comedy(
+            ErrorType::InvalidGuid,
+            error_action,
+            error_stage,
+            comedy_error,
+        )
+    })
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/task/client.rs b/toolkit/components/bitsdownload/src/bits_interface/task/client.rs
new file mode 100644
index 0000000000..edbf4d0698
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/task/client.rs
@@ -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/. */
+use super::{
+    action::Action,
+    error::{BitsTaskError, ErrorStage::CommandThread, ErrorType::MissingBitsClient},
+    string::nsCString_to_OsString,
+};
+
+use bits_client::BitsClient;
+use nsstring::nsCString;
+use std::cell::Cell;
+
+thread_local! {
+    // This is used to store the `BitsClient` on the Command thread.
+    // Keeping it here solves the problem of how to allow multiple runnables to
+    // be simultaneously queued on the Command thread while giving them all
+    // access to the same `BitsClient`.
+    static BITS_CLIENT: Cell<Option<BitsClient>> = Cell::new(None);
+}
+
+/// This structure holds the data needed to initialize `BitsClient` and
+/// `BitsMonitorClient`.
+#[derive(Debug, Clone)]
+pub struct ClientInitData {
+    pub job_name: nsCString,
+    pub save_path_prefix: nsCString,
+    pub monitor_timeout_ms: u32,
+}
+
+impl ClientInitData {
+    pub fn new(
+        job_name: nsCString,
+        save_path_prefix: nsCString,
+        monitor_timeout_ms: u32,
+    ) -> ClientInitData {
+        ClientInitData {
+            job_name,
+            save_path_prefix,
+            monitor_timeout_ms,
+        }
+    }
+}
+
+/// This function constructs a `BitsClient`, if one does not already exist. If
+/// the `BitsClient` cannot be constructed, a `BitsTaskError` will be returned.
+/// If the `BitsClient` could be obtained, then the function then calls the
+/// closure passed to it, passing a mutable reference to the `BitsClient`.
+/// This function will then return whatever the closure returned, which must be
+/// a `Result<_, BitsTaskError>`.
+pub fn with_maybe_new_bits_client<F, R>(
+    init_data: &ClientInitData,
+    action: Action,
+    closure: F,
+) -> Result<R, BitsTaskError>
+where
+    F: FnOnce(&mut BitsClient) -> Result<R, BitsTaskError>,
+{
+    _with_bits_client(Some(init_data), action, closure)
+}
+
+/// This function assumes that a `BitsClient` has already been constructed. If
+/// there is not one available, a `BitsTaskError` will be returned. Otherwise,
+/// the function calls the closure passed to it, passing a mutable reference to
+/// the `BitsClient`. This function will then return whatever the closure
+/// returned, which must be a `Result<_, BitsTaskError>`.
+pub fn with_bits_client<F, R>(action: Action, closure: F) -> Result<R, BitsTaskError>
+where
+    F: FnOnce(&mut BitsClient) -> Result<R, BitsTaskError>,
+{
+    _with_bits_client(None, action, closure)
+}
+
+fn _with_bits_client<F, R>(
+    maybe_init_data: Option<&ClientInitData>,
+    action: Action,
+    closure: F,
+) -> Result<R, BitsTaskError>
+where
+    F: FnOnce(&mut BitsClient) -> Result<R, BitsTaskError>,
+{
+    BITS_CLIENT.with(|cell| {
+        let maybe_client = cell.take();
+        let mut client = match (maybe_client, maybe_init_data) {
+            (Some(r), _) => r,
+            (None, Some(init_data)) => {
+                // Immediately invoked function to allow for the ? operator
+                BitsClient::new(
+                    nsCString_to_OsString(&init_data.job_name, action, CommandThread)?,
+                    nsCString_to_OsString(&init_data.save_path_prefix, action, CommandThread)?,
+                )
+                .map_err(|pipe_error| BitsTaskError::from_pipe(action, pipe_error))?
+            }
+            (None, None) => {
+                return Err(BitsTaskError::new(MissingBitsClient, action, CommandThread));
+            }
+        };
+        let result = closure(&mut client);
+        cell.set(Some(client));
+        result
+    })
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/task/from_threadbound.rs b/toolkit/components/bitsdownload/src/bits_interface/task/from_threadbound.rs
new file mode 100644
index 0000000000..2cf2e9189a
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/task/from_threadbound.rs
@@ -0,0 +1,125 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use super::{
+    action::Action,
+    error::{BitsTaskError, ErrorStage, ErrorType},
+};
+use log::warn;
+use xpcom::{RefCounted, ThreadBoundRefPtr};
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum DataType {
+    Callback,
+    BitsService,
+    BitsRequest,
+    Observer,
+    Context,
+}
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+enum GetThreadboundError {
+    Missing,
+    WrongThread,
+}
+
+impl DataType {
+    fn error_type(&self, error: GetThreadboundError) -> ErrorType {
+        match self {
+            DataType::Callback => match error {
+                GetThreadboundError::Missing => ErrorType::MissingCallback,
+                GetThreadboundError::WrongThread => ErrorType::CallbackOnWrongThread,
+            },
+            DataType::BitsService => match error {
+                GetThreadboundError::Missing => ErrorType::MissingBitsService,
+                GetThreadboundError::WrongThread => ErrorType::BitsServiceOnWrongThread,
+            },
+            DataType::BitsRequest => match error {
+                GetThreadboundError::Missing => ErrorType::MissingBitsRequest,
+                GetThreadboundError::WrongThread => ErrorType::BitsRequestOnWrongThread,
+            },
+            DataType::Observer => match error {
+                GetThreadboundError::Missing => ErrorType::MissingObserver,
+                GetThreadboundError::WrongThread => ErrorType::ObserverOnWrongThread,
+            },
+            DataType::Context => match error {
+                GetThreadboundError::Missing => ErrorType::MissingContext,
+                GetThreadboundError::WrongThread => ErrorType::ContextOnWrongThread,
+            },
+        }
+    }
+
+    fn name(&self) -> &'static str {
+        match self {
+            DataType::Callback => "Callback",
+            DataType::BitsService => "BITS Service",
+            DataType::BitsRequest => "BITS Request",
+            DataType::Observer => "Observer",
+            DataType::Context => "Context",
+        }
+    }
+}
+
+/// Given a reference to a threadbound option
+/// (i.e. `&Option<ThreadBoundRefPtr<_>>`), this function will attempt to
+/// retrieve a reference to the value stored within. If it is not available
+/// (option is `None` or value is on the wrong thread), `None` is returned
+/// instead.
+pub fn get_from_threadbound_option<T>(
+    maybe_threadbound: &Option<ThreadBoundRefPtr<T>>,
+    data_type: DataType,
+    action: Action,
+) -> Option<&T>
+where
+    T: RefCounted + 'static,
+{
+    maybe_threadbound.as_ref().and_then(|threadbound| {
+        let maybe_reference = threadbound.get_ref();
+        if maybe_reference.is_none() {
+            warn!(
+                "Unexpected error {}: {} is on the wrong thread",
+                action.description(),
+                data_type.name(),
+            );
+        }
+        maybe_reference
+    })
+}
+
+/// Given a reference to a threadbound option
+/// (i.e. `&Option<ThreadBoundRefPtr<_>>`), this function will attempt to
+/// retrieve a reference to the value stored within. If it is not available
+/// (option is `None` or value is on the wrong thread), a `BitsTaskError` is
+/// returned instead.
+pub fn expect_from_threadbound_option<T>(
+    maybe_threadbound: &Option<ThreadBoundRefPtr<T>>,
+    data_type: DataType,
+    action: Action,
+) -> Result<&T, BitsTaskError>
+where
+    T: RefCounted + 'static,
+{
+    match maybe_threadbound.as_ref() {
+        Some(threadbound) => {
+            match threadbound.get_ref() {
+                Some(reference) => Ok(reference),
+                None => Err(BitsTaskError::new(
+                    data_type.error_type(GetThreadboundError::WrongThread),
+                    action,
+                    // Retrieving data from threadbounds all happens on the main thread.
+                    // No data is ever bound to other threads so there would be no
+                    // reason to retrieve it there.
+                    ErrorStage::MainThread,
+                )),
+            }
+        }
+        None => Err(BitsTaskError::new(
+            data_type.error_type(GetThreadboundError::Missing),
+            action,
+            // Retrieving data from threadbounds all happens on the main thread.
+            // No data is ever bound to other threads so there would be no
+            // reason to retrieve it there.
+            ErrorStage::MainThread,
+        )),
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/task/mod.rs b/toolkit/components/bitsdownload/src/bits_interface/task/mod.rs
new file mode 100644
index 0000000000..b6b96d887b
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/task/mod.rs
@@ -0,0 +1,18 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+mod from_threadbound;
+
+use super::{action, dispatch_callback, error, request::BitsRequest, string, BitsService};
+
+mod client;
+pub use self::client::ClientInitData;
+
+mod service_task;
+pub use self::service_task::{MonitorDownloadTask, StartDownloadTask};
+
+mod request_task;
+pub use self::request_task::{
+    CancelTask, ChangeMonitorIntervalTask, CompleteTask, Priority, ResumeTask,
+    SetNoProgressTimeoutTask, SetPriorityTask, SuspendTask,
+};
diff --git a/toolkit/components/bitsdownload/src/bits_interface/task/request_task.rs b/toolkit/components/bitsdownload/src/bits_interface/task/request_task.rs
new file mode 100644
index 0000000000..f0fd331ed0
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/task/request_task.rs
@@ -0,0 +1,425 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use super::{
+    action::{Action, RequestAction},
+    client::with_bits_client,
+    dispatch_callback::{
+        maybe_dispatch_via_callback, CallbackExpected, CallbackOptional, IsCallbackExpected,
+    },
+    error::BitsTaskError,
+    from_threadbound::{expect_from_threadbound_option, DataType},
+    BitsRequest,
+};
+
+use bits_client::{BitsClient, Guid};
+use crossbeam_utils::atomic::AtomicCell;
+use log::info;
+use moz_task::Task;
+use nserror::nsresult;
+use xpcom::{interfaces::nsIBitsCallback, RefPtr, ThreadBoundRefPtr};
+
+type RunFn<D> = fn(Guid, &D, &mut BitsClient) -> Result<(), BitsTaskError>;
+type DoneFn = fn(&BitsRequest, bool) -> Result<(), BitsTaskError>;
+
+pub struct RequestTask<D> {
+    request: AtomicCell<Option<ThreadBoundRefPtr<BitsRequest>>>,
+    guid: Guid,
+    action: RequestAction,
+    task_data: D,
+    run_fn: RunFn<D>,
+    maybe_done_fn: Option<DoneFn>,
+    callback: AtomicCell<Option<ThreadBoundRefPtr<nsIBitsCallback>>>,
+    callback_presence: IsCallbackExpected,
+    result: AtomicCell<Option<Result<(), BitsTaskError>>>,
+}
+
+impl<D> RequestTask<D>
+where
+    D: Sync + Send,
+{
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        guid: Guid,
+        action: RequestAction,
+        task_data: D,
+        run_fn: RunFn<D>,
+        maybe_done_fn: Option<DoneFn>,
+        callback: Option<RefPtr<nsIBitsCallback>>,
+        callback_presence: IsCallbackExpected,
+    ) -> RequestTask<D> {
+        RequestTask {
+            request: AtomicCell::new(Some(ThreadBoundRefPtr::new(request))),
+            guid,
+            action,
+            task_data,
+            run_fn,
+            maybe_done_fn,
+            callback: AtomicCell::new(callback.map(ThreadBoundRefPtr::new)),
+            result: AtomicCell::new(None),
+            callback_presence,
+        }
+    }
+}
+
+impl<D> Task for RequestTask<D> {
+    fn run(&self) {
+        let result = with_bits_client(self.action.into(), |client| {
+            (self.run_fn)(self.guid.clone(), &self.task_data, client)
+        });
+        self.result.store(Some(result));
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        // If TaskRunnable.run() calls Task.done() to return a result
+        // on the main thread before TaskRunnable.run() returns on the worker
+        // thread, then the Task will get dropped on the worker thread.
+        //
+        // But the callback is an nsXPCWrappedJS that isn't safe to release
+        // on the worker thread.  So we move it out of the Task here to ensure
+        // it gets released on the main thread.
+        let maybe_tb_callback = self.callback.swap(None);
+        // It also isn't safe to drop the BitsRequest RefPtr off-thread,
+        // because BitsRequest refcounting is non-atomic
+        let maybe_tb_request = self.request.swap(None);
+
+        let action: Action = self.action.into();
+        let maybe_callback =
+            expect_from_threadbound_option(&maybe_tb_callback, DataType::Callback, action);
+
+        // Immediately invoked function expression to allow for the ? operator
+        let result: Result<(), BitsTaskError> = (|| {
+            let request =
+                expect_from_threadbound_option(&maybe_tb_request, DataType::BitsRequest, action)?;
+
+            let maybe_result = self.result.swap(None);
+
+            let success = if let Some(result) = maybe_result.as_ref() {
+                result.is_ok()
+            } else {
+                false
+            };
+
+            if let Some(done_fn) = self.maybe_done_fn {
+                done_fn(request, success)?;
+            }
+
+            maybe_result.ok_or_else(|| BitsTaskError::missing_result(action))?
+        })();
+        info!("BITS Request Task completed: {:?}", result);
+        maybe_dispatch_via_callback(result, maybe_callback, self.callback_presence)
+    }
+}
+
+pub struct CompleteTask(RequestTask<()>);
+
+impl Task for CompleteTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl CompleteTask {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        callback: RefPtr<nsIBitsCallback>,
+    ) -> CompleteTask {
+        CompleteTask(RequestTask::new(
+            request,
+            id,
+            RequestAction::Complete,
+            (),
+            CompleteTask::run_fn,
+            Some(CompleteTask::done_fn),
+            Some(callback),
+            CallbackExpected,
+        ))
+    }
+
+    fn run_fn(id: Guid, _data: &(), client: &mut BitsClient) -> Result<(), BitsTaskError> {
+        client
+            .complete_job(id)
+            .map_err(|pipe_error| BitsTaskError::from_pipe(Action::Complete, pipe_error))??;
+        Ok(())
+    }
+
+    fn done_fn(request: &BitsRequest, success: bool) -> Result<(), BitsTaskError> {
+        if success {
+            request.on_finished();
+        }
+        Ok(())
+    }
+}
+
+pub struct CancelTask(RequestTask<()>);
+
+impl Task for CancelTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl CancelTask {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        callback: Option<RefPtr<nsIBitsCallback>>,
+    ) -> CancelTask {
+        let callback_presence = if callback.is_some() {
+            CallbackExpected
+        } else {
+            CallbackOptional
+        };
+
+        CancelTask(RequestTask::new(
+            request,
+            id,
+            RequestAction::Cancel,
+            (),
+            CancelTask::run_fn,
+            Some(CancelTask::done_fn),
+            callback,
+            callback_presence,
+        ))
+    }
+
+    fn run_fn(id: Guid, _data: &(), client: &mut BitsClient) -> Result<(), BitsTaskError> {
+        client
+            .cancel_job(id)
+            .map_err(|pipe_error| BitsTaskError::from_pipe(Action::Cancel, pipe_error))??;
+        Ok(())
+    }
+
+    fn done_fn(request: &BitsRequest, success: bool) -> Result<(), BitsTaskError> {
+        request.finish_cancel_action(success);
+        Ok(())
+    }
+}
+
+pub struct SuspendTask(RequestTask<()>);
+
+impl Task for SuspendTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl SuspendTask {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        callback: Option<RefPtr<nsIBitsCallback>>,
+    ) -> SuspendTask {
+        let callback_presence = if callback.is_some() {
+            CallbackExpected
+        } else {
+            CallbackOptional
+        };
+
+        SuspendTask(RequestTask::new(
+            request,
+            id,
+            RequestAction::Suspend,
+            (),
+            SuspendTask::run_fn,
+            None,
+            callback,
+            callback_presence,
+        ))
+    }
+
+    fn run_fn(id: Guid, _data: &(), client: &mut BitsClient) -> Result<(), BitsTaskError> {
+        client
+            .suspend_job(id)
+            .map_err(|pipe_error| BitsTaskError::from_pipe(Action::Suspend, pipe_error))??;
+        Ok(())
+    }
+}
+
+pub struct ResumeTask(RequestTask<()>);
+
+impl Task for ResumeTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl ResumeTask {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        callback: Option<RefPtr<nsIBitsCallback>>,
+    ) -> ResumeTask {
+        let callback_presence = if callback.is_some() {
+            CallbackExpected
+        } else {
+            CallbackOptional
+        };
+
+        ResumeTask(RequestTask::new(
+            request,
+            id,
+            RequestAction::Resume,
+            (),
+            ResumeTask::run_fn,
+            None,
+            callback,
+            callback_presence,
+        ))
+    }
+
+    fn run_fn(id: Guid, _data: &(), client: &mut BitsClient) -> Result<(), BitsTaskError> {
+        client
+            .resume_job(id)
+            .map_err(|pipe_error| BitsTaskError::from_pipe(Action::Resume, pipe_error))??;
+        Ok(())
+    }
+}
+
+pub struct ChangeMonitorIntervalTask(RequestTask<u32>);
+
+impl Task for ChangeMonitorIntervalTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl ChangeMonitorIntervalTask {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        update_interval_ms: u32,
+        callback: RefPtr<nsIBitsCallback>,
+    ) -> ChangeMonitorIntervalTask {
+        ChangeMonitorIntervalTask(RequestTask::new(
+            request,
+            id,
+            RequestAction::SetMonitorInterval,
+            update_interval_ms,
+            ChangeMonitorIntervalTask::run_fn,
+            None,
+            Some(callback),
+            CallbackExpected,
+        ))
+    }
+
+    fn run_fn(
+        id: Guid,
+        update_interval_ms: &u32,
+        client: &mut BitsClient,
+    ) -> Result<(), BitsTaskError> {
+        client
+            .set_update_interval(id, *update_interval_ms)
+            .map_err(|pipe_error| {
+                BitsTaskError::from_pipe(Action::SetMonitorInterval, pipe_error)
+            })??;
+        Ok(())
+    }
+}
+
+#[derive(Debug, PartialEq, Clone, Copy)]
+pub enum Priority {
+    High,
+    Low,
+}
+
+pub struct SetPriorityTask(RequestTask<Priority>);
+
+impl Task for SetPriorityTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl SetPriorityTask {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        priority: Priority,
+        callback: RefPtr<nsIBitsCallback>,
+    ) -> SetPriorityTask {
+        SetPriorityTask(RequestTask::new(
+            request,
+            id,
+            RequestAction::SetPriority,
+            priority,
+            SetPriorityTask::run_fn,
+            None,
+            Some(callback),
+            CallbackExpected,
+        ))
+    }
+
+    fn run_fn(id: Guid, priority: &Priority, client: &mut BitsClient) -> Result<(), BitsTaskError> {
+        client
+            .set_job_priority(id, *priority == Priority::High)
+            .map_err(|pipe_error| BitsTaskError::from_pipe(Action::SetPriority, pipe_error))??;
+        Ok(())
+    }
+}
+
+pub struct SetNoProgressTimeoutTask(RequestTask<u32>);
+
+impl Task for SetNoProgressTimeoutTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl SetNoProgressTimeoutTask {
+    pub fn new(
+        request: RefPtr<BitsRequest>,
+        id: Guid,
+        timeout_secs: u32,
+        callback: RefPtr<nsIBitsCallback>,
+    ) -> SetNoProgressTimeoutTask {
+        SetNoProgressTimeoutTask(RequestTask::new(
+            request,
+            id,
+            RequestAction::SetNoProgressTimeout,
+            timeout_secs,
+            SetNoProgressTimeoutTask::run_fn,
+            None,
+            Some(callback),
+            CallbackExpected,
+        ))
+    }
+
+    fn run_fn(id: Guid, timeout_secs: &u32, client: &mut BitsClient) -> Result<(), BitsTaskError> {
+        client
+            .set_no_progress_timeout(id, *timeout_secs)
+            .map_err(|pipe_error| {
+                BitsTaskError::from_pipe(Action::SetNoProgressTimeout, pipe_error)
+            })??;
+        Ok(())
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/task/service_task.rs b/toolkit/components/bitsdownload/src/bits_interface/task/service_task.rs
new file mode 100644
index 0000000000..c66f127f7a
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/task/service_task.rs
@@ -0,0 +1,332 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+use super::{
+    action::{
+        Action,
+        Action::{MonitorDownload, StartDownload},
+        ServiceAction,
+    },
+    client::{with_maybe_new_bits_client, ClientInitData},
+    dispatch_callback::{maybe_dispatch_request_via_callback, CallbackExpected},
+    error::{BitsTaskError, ErrorStage::CommandThread},
+    from_threadbound::{expect_from_threadbound_option, get_from_threadbound_option, DataType},
+    string::nsCString_to_OsString,
+    BitsRequest, BitsService,
+};
+
+use bits_client::{BitsClient, BitsMonitorClient, BitsProxyUsage, Guid};
+use crossbeam_utils::atomic::AtomicCell;
+use log::{info, warn};
+use moz_task::Task;
+use nserror::nsresult;
+use nsstring::nsCString;
+use xpcom::{
+    interfaces::{nsIBitsNewRequestCallback, nsIRequestObserver, nsISupports},
+    RefPtr, ThreadBoundRefPtr,
+};
+
+// D is the Data Type that the RunFn function needs to make S.
+// S is the Success Type that the RunFn returns on success and that the
+//   DoneFn needs to make the BitsRequest.
+type RunFn<D, S> = fn(&D, &mut BitsClient) -> Result<S, BitsTaskError>;
+type DoneFn<D, S> = fn(
+    &D,
+    S,
+    &ClientInitData,
+    &BitsService,
+    &nsIRequestObserver,
+    Option<&nsISupports>,
+) -> Result<RefPtr<BitsRequest>, BitsTaskError>;
+
+pub struct ServiceTask<D, S> {
+    client_init_data: ClientInitData,
+    action: ServiceAction,
+    task_data: D,
+    run_fn: RunFn<D, S>,
+    done_fn: DoneFn<D, S>,
+    bits_service: AtomicCell<Option<ThreadBoundRefPtr<BitsService>>>,
+    observer: AtomicCell<Option<ThreadBoundRefPtr<nsIRequestObserver>>>,
+    context: AtomicCell<Option<ThreadBoundRefPtr<nsISupports>>>,
+    callback: AtomicCell<Option<ThreadBoundRefPtr<nsIBitsNewRequestCallback>>>,
+    result: AtomicCell<Option<Result<S, BitsTaskError>>>,
+}
+
+impl<D, S> ServiceTask<D, S>
+where
+    D: Sync + Send,
+    S: Sync + Send,
+{
+    pub fn new(
+        client_init_data: ClientInitData,
+        action: ServiceAction,
+        task_data: D,
+        run_fn: RunFn<D, S>,
+        done_fn: DoneFn<D, S>,
+        bits_service: RefPtr<BitsService>,
+        observer: RefPtr<nsIRequestObserver>,
+        context: Option<RefPtr<nsISupports>>,
+        callback: RefPtr<nsIBitsNewRequestCallback>,
+    ) -> ServiceTask<D, S> {
+        ServiceTask {
+            client_init_data,
+            action,
+            task_data,
+            run_fn,
+            done_fn,
+            bits_service: AtomicCell::new(Some(ThreadBoundRefPtr::new(bits_service))),
+            observer: AtomicCell::new(Some(ThreadBoundRefPtr::new(observer))),
+            context: AtomicCell::new(context.map(ThreadBoundRefPtr::new)),
+            callback: AtomicCell::new(Some(ThreadBoundRefPtr::new(callback))),
+            result: AtomicCell::new(None),
+        }
+    }
+}
+
+impl<D, S> Task for ServiceTask<D, S> {
+    fn run(&self) {
+        let result =
+            with_maybe_new_bits_client(&self.client_init_data, self.action.into(), |client| {
+                (self.run_fn)(&self.task_data, client)
+            });
+        self.result.store(Some(result));
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        // If TaskRunnable.run() calls Task.done() to return a result
+        // on the main thread before TaskRunnable.run() returns on the worker
+        // thread, then the Task will get dropped on the worker thread.
+        //
+        // But the callback is an nsXPCWrappedJS that isn't safe to release
+        // on the worker thread.  So we move it out of the Task here to ensure
+        // it gets released on the main thread.
+        let maybe_tb_callback = self.callback.swap(None);
+        // It also isn't safe to drop the BitsService RefPtr off-thread,
+        // because BitsService refcounting is non-atomic
+        let maybe_tb_service = self.bits_service.swap(None);
+        // The observer and context are also an nsXPCWrappedJS that aren't safe
+        // to release on the worker thread.
+        let maybe_tb_observer = self.observer.swap(None);
+        let maybe_tb_context = self.context.swap(None);
+
+        let action: Action = self.action.into();
+        let maybe_callback =
+            expect_from_threadbound_option(&maybe_tb_callback, DataType::Callback, action);
+
+        // Immediately invoked function expression to allow for the ? operator
+        let result: Result<RefPtr<BitsRequest>, BitsTaskError> = (|| {
+            let bits_service =
+                expect_from_threadbound_option(&maybe_tb_service, DataType::BitsService, action)?;
+            let observer =
+                expect_from_threadbound_option(&maybe_tb_observer, DataType::Observer, action)?;
+            let maybe_context =
+                get_from_threadbound_option(&maybe_tb_context, DataType::Context, action);
+            let success = self
+                .result
+                .swap(None)
+                .ok_or_else(|| BitsTaskError::missing_result(action))??;
+
+            (self.done_fn)(
+                &self.task_data,
+                success,
+                &self.client_init_data,
+                bits_service,
+                observer,
+                maybe_context,
+            )
+        })();
+        info!("BITS Interface Task completed: {:?}", result);
+        // We incremented the request count when we dispatched an action to
+        // start the job. Now we will decrement since the action completed.
+        // See the declaration of InitBitsService::request_count for details.
+        let bits_service_result =
+            expect_from_threadbound_option(&maybe_tb_service, DataType::BitsService, action);
+        match bits_service_result {
+            Ok(bits_service) => {
+                bits_service.dec_request_count();
+            }
+            Err(error) => {
+                warn!(
+                    concat!(
+                        "Unable to decrement the request count when finishing ServiceTask. ",
+                        "The command thread may not be shut down. Error: {:?}"
+                    ),
+                    error
+                );
+            }
+        }
+
+        maybe_dispatch_request_via_callback(result, maybe_callback, CallbackExpected)
+    }
+}
+
+struct StartDownloadData {
+    download_url: nsCString,
+    save_rel_path: nsCString,
+    proxy: BitsProxyUsage,
+    no_progress_timeout_secs: u32,
+    update_interval_ms: u32,
+}
+
+struct StartDownloadSuccess {
+    guid: Guid,
+    monitor_client: BitsMonitorClient,
+}
+
+pub struct StartDownloadTask(ServiceTask<StartDownloadData, StartDownloadSuccess>);
+
+impl Task for StartDownloadTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl StartDownloadTask {
+    pub fn new(
+        client_init_data: ClientInitData,
+        download_url: nsCString,
+        save_rel_path: nsCString,
+        proxy: BitsProxyUsage,
+        no_progress_timeout_secs: u32,
+        update_interval_ms: u32,
+        bits_service: RefPtr<BitsService>,
+        observer: RefPtr<nsIRequestObserver>,
+        context: Option<RefPtr<nsISupports>>,
+        callback: RefPtr<nsIBitsNewRequestCallback>,
+    ) -> StartDownloadTask {
+        StartDownloadTask(ServiceTask::new(
+            client_init_data,
+            ServiceAction::StartDownload,
+            StartDownloadData {
+                download_url,
+                save_rel_path,
+                proxy,
+                no_progress_timeout_secs,
+                update_interval_ms,
+            },
+            StartDownloadTask::run_fn,
+            StartDownloadTask::done_fn,
+            bits_service,
+            observer,
+            context,
+            callback,
+        ))
+    }
+
+    fn run_fn(
+        data: &StartDownloadData,
+        client: &mut BitsClient,
+    ) -> Result<StartDownloadSuccess, BitsTaskError> {
+        let url = nsCString_to_OsString(&data.download_url, StartDownload, CommandThread)?;
+        let path = nsCString_to_OsString(&data.save_rel_path, StartDownload, CommandThread)?;
+        let (success, monitor_client) = client
+            .start_job(
+                url,
+                path,
+                data.proxy,
+                data.no_progress_timeout_secs,
+                data.update_interval_ms,
+            )
+            .map_err(|pipe_error| BitsTaskError::from_pipe(StartDownload, pipe_error))??;
+        Ok(StartDownloadSuccess {
+            guid: success.guid,
+            monitor_client,
+        })
+    }
+
+    fn done_fn(
+        _data: &StartDownloadData,
+        success: StartDownloadSuccess,
+        client_init_data: &ClientInitData,
+        bits_service: &BitsService,
+        observer: &nsIRequestObserver,
+        maybe_context: Option<&nsISupports>,
+    ) -> Result<RefPtr<BitsRequest>, BitsTaskError> {
+        BitsRequest::new(
+            success.guid.clone(),
+            RefPtr::new(bits_service),
+            client_init_data.monitor_timeout_ms,
+            RefPtr::new(&observer),
+            maybe_context.map(RefPtr::new),
+            success.monitor_client,
+            ServiceAction::StartDownload,
+        )
+    }
+}
+
+struct MonitorDownloadData {
+    guid: Guid,
+    update_interval_ms: u32,
+}
+
+pub struct MonitorDownloadTask(ServiceTask<MonitorDownloadData, BitsMonitorClient>);
+
+impl Task for MonitorDownloadTask {
+    fn run(&self) {
+        self.0.run();
+    }
+
+    fn done(&self) -> Result<(), nsresult> {
+        self.0.done()
+    }
+}
+
+impl MonitorDownloadTask {
+    pub fn new(
+        client_init_data: ClientInitData,
+        guid: Guid,
+        update_interval_ms: u32,
+        bits_service: RefPtr<BitsService>,
+        observer: RefPtr<nsIRequestObserver>,
+        context: Option<RefPtr<nsISupports>>,
+        callback: RefPtr<nsIBitsNewRequestCallback>,
+    ) -> MonitorDownloadTask {
+        MonitorDownloadTask(ServiceTask::new(
+            client_init_data,
+            ServiceAction::MonitorDownload,
+            MonitorDownloadData {
+                guid,
+                update_interval_ms,
+            },
+            MonitorDownloadTask::run_fn,
+            MonitorDownloadTask::done_fn,
+            bits_service,
+            observer,
+            context,
+            callback,
+        ))
+    }
+
+    fn run_fn(
+        data: &MonitorDownloadData,
+        client: &mut BitsClient,
+    ) -> Result<BitsMonitorClient, BitsTaskError> {
+        let result = client
+            .monitor_job(data.guid.clone(), data.update_interval_ms)
+            .map_err(|pipe_error| BitsTaskError::from_pipe(MonitorDownload, pipe_error));
+        Ok(result??)
+    }
+
+    fn done_fn(
+        data: &MonitorDownloadData,
+        monitor_client: BitsMonitorClient,
+        client_init_data: &ClientInitData,
+        bits_service: &BitsService,
+        observer: &nsIRequestObserver,
+        maybe_context: Option<&nsISupports>,
+    ) -> Result<RefPtr<BitsRequest>, BitsTaskError> {
+        BitsRequest::new(
+            data.guid.clone(),
+            RefPtr::new(bits_service),
+            client_init_data.monitor_timeout_ms,
+            RefPtr::new(&observer),
+            maybe_context.map(RefPtr::new),
+            monitor_client,
+            ServiceAction::MonitorDownload,
+        )
+    }
+}
diff --git a/toolkit/components/bitsdownload/src/bits_interface/xpcom_methods.rs b/toolkit/components/bitsdownload/src/bits_interface/xpcom_methods.rs
new file mode 100644
index 0000000000..cb267b9634
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/bits_interface/xpcom_methods.rs
@@ -0,0 +1,195 @@
+/* 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 macro is very similar to xpcom_macro, but works a bit differently:
+///
+/// When possible, it returns errors via the callback rather than via the return
+/// value.
+///
+/// It implicitly adds the callback argument of type: nsIBitsNewRequestCallback
+///
+/// It needs an action type, to be specified before the rust name, in square
+/// brackets.
+///
+/// The rustic implementation that the xpcom method calls is expected to return
+/// the type: Result<_, BitsTaskError>. If this value is Ok, it will be ignored.
+/// If the value is Err, it will be returned via the callback passed.
+///
+/// Usage like this:
+///
+/// ```ignore
+///     nsIBits_method!(
+///         [ActionType]
+///         rust_method => XpcomMethod(
+///             foo: *const nsACString,
+///             bar: *const nsIBar,
+///             baz: bool,
+///             [optional] qux: *const nsIQux,
+///         )
+///     );
+/// ```
+///
+/// Results in the macro generating a method like:
+///
+/// ```ignore
+///     unsafe fn XpcomMethod(
+///         &self,
+///         foo: *const nsACString,
+///         bar: *const nsIBar,
+///         baz: bool,
+///         qux: *const nsIQux,
+///         callback: *const nsIBitsNewRequestCallback,
+///     ) -> nsresult {
+///         let callback: &nsIBitsNewRequestCallback = match xpcom::Ensure::ensure(callback) {
+///             Ok(val) => val,
+///             Err(result) => return result,
+///         };
+///         let foo = match xpcom::Ensure::ensure(foo) {
+///             Ok(val) => val,
+///             Err(_) => {
+///                 dispatch_pretask_interface_error(BitsTaskError::new(ErrorType::NullArgument, ActionType.into(), ErrorStage::Pretask), callback);
+///                 return NS_OK;
+///             }
+///         };
+///         let bar = match xpcom::Ensure::ensure(bar) {
+///             Ok(val) => val,
+///             Err(_) => {
+///                 dispatch_pretask_interface_error(BitsTaskError::new(ErrorType::NullArgument, ActionType.into(), ErrorStage::Pretask), callback);
+///                 return NS_OK;
+///             }
+///         };
+///         let baz = match xpcom::Ensure::ensure(baz) {
+///             Ok(val) => val,
+///             Err(_) => {
+///                 dispatch_pretask_interface_error(BitsTaskError::new(ErrorType::NullArgument, ActionType.into(), ErrorStage::Pretask), callback);
+///                 return NS_OK;
+///             }
+///         };
+///         let qux = match xpcom::Ensure::ensure(qux) {
+///             Ok(val) => Some(val),
+///             Err(_) => None,
+///         };
+///
+///         if let Err(error) = self.rust_method(foo, bar, baz, qux, callback) {
+///             dispatch_pretask_interface_error(error, callback);
+///         }
+///
+///         NS_OK
+///     }
+/// ```
+///
+/// Which expects a Rustic implementation method like:
+///
+/// ```ignore
+///     fn rust_method(
+///         &self,
+///         foo: &nsACString,
+///         bar: &nsIBar,
+///         baz: bool,
+///         qux: Option<&nsIQux>,
+///         callback: &nsIBitsNewRequestCallback,
+///     ) -> Result<(), BitsTaskError> {
+///         do_something()
+///     }
+/// ```
+#[macro_export]
+macro_rules! nsIBits_method {
+    // The internal rule @ensure_param converts raw pointer arguments to
+    // references, calling dispatch_pretask_interface_error and returning if the
+    // argument is null.
+    // If, however, the type is optional, the reference will also be wrapped
+    // in an option and null pointers will be converted to None.
+    (@ensure_param [optional] $name:ident, $action:expr, $callback:ident) => {
+        let $name = match Ensure::ensure($name) {
+            Ok(val) => Some(val),
+            Err(_) => None,
+        };
+    };
+    (@ensure_param $name:ident, $action:expr, $callback:ident) => {
+        let $name = match Ensure::ensure($name) {
+            Ok(val) => val,
+            Err(_) => {
+                dispatch_pretask_interface_error(BitsTaskError::new(NullArgument, $action.into(), Pretask), $callback);
+                return NS_OK;
+            }
+        };
+    };
+
+    ([$action:expr] $rust_name:ident => $xpcom_name:ident($($([$param_required:ident])* $param_name:ident: $param_type:ty $(,)*)*)) => {
+        #[allow(non_snake_case)]
+        unsafe fn $xpcom_name(&self, $($param_name: $param_type, )* callback: *const nsIBitsNewRequestCallback) -> nsresult {
+            use xpcom::Ensure;
+            use nserror::NS_OK;
+            // When no params are passed, the imports below will not be used, so silence the
+            // warning
+            #[allow(unused_imports)]
+            use bits_interface::{
+                dispatch_callback::dispatch_pretask_interface_error,
+                error::{BitsTaskError, ErrorStage::Pretask, ErrorType::NullArgument},
+            };
+
+            let callback: &nsIBitsNewRequestCallback = match Ensure::ensure(callback) {
+                Ok(val) => val,
+                Err(result) => return result,
+            };
+            $(nsIBits_method!(@ensure_param $([$param_required])* $param_name, $action, callback);)*
+            if let Err(error) = self.$rust_name($($param_name, )* callback) {
+                dispatch_pretask_interface_error(error, callback);
+            }
+            NS_OK
+        }
+    };
+}
+
+/*
+ * Same as above, but expects a nsIBitsCallback as its callback.
+ */
+#[macro_export]
+macro_rules! nsIBitsRequest_method {
+    // The internal rule @ensure_param converts raw pointer arguments to
+    // references, calling dispatch_pretask_interface_error and returning if the
+    // argument is null.
+    // If, however, the type is optional, the reference will also be wrapped
+    // in an option and null pointers will be converted to None.
+    (@ensure_param [optional] $name:ident, $action:expr, $callback:ident) => {
+        let $name = match Ensure::ensure($name) {
+            Ok(val) => Some(val),
+            Err(_) => None,
+        };
+    };
+    (@ensure_param $name:ident, $action:expr, $callback:ident) => {
+        let $name = match Ensure::ensure($name) {
+            Ok(val) => val,
+            Err(_) => {
+                dispatch_pretask_request_error(BitsTaskError::new(NullArgument, $action.into(), Pretask), $callback);
+                return NS_OK;
+            }
+        };
+    };
+
+    ([$action:expr] $rust_name:ident => $xpcom_name:ident($($([$param_required:ident])* $param_name:ident: $param_type:ty $(,)*)*)) => {
+        #[allow(non_snake_case)]
+        unsafe fn $xpcom_name(&self, $($param_name: $param_type, )* callback: *const nsIBitsCallback) -> nsresult {
+            use xpcom::Ensure;
+            use nserror::NS_OK;
+            // When no params are passed, the imports below will not be used, so silence the
+            // warning
+            #[allow(unused_imports)]
+            use bits_interface::{
+                dispatch_callback::dispatch_pretask_request_error,
+                error::{BitsTaskError, ErrorStage::Pretask, ErrorType::NullArgument},
+            };
+
+            let callback: &nsIBitsCallback = match Ensure::ensure(callback) {
+                Ok(val) => val,
+                Err(result) => return result,
+            };
+            $(nsIBitsRequest_method!(@ensure_param $([$param_required])* $param_name, $action, callback);)*
+            if let Err(error) = self.$rust_name($($param_name, )* callback) {
+                dispatch_pretask_request_error(error, callback);
+            }
+            NS_OK
+        }
+    };
+}
diff --git a/toolkit/components/bitsdownload/src/lib.rs b/toolkit/components/bitsdownload/src/lib.rs
new file mode 100644
index 0000000000..3b8ef021da
--- /dev/null
+++ b/toolkit/components/bitsdownload/src/lib.rs
@@ -0,0 +1,23 @@
+/* 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 crate is meant to be used in Windows only. It provides the
+//! bits_interface module, which implements the nsIBits an nsIBitsRequest
+//! XPCOM interfaces. These interfaces allow usage of the Windows component:
+//! BITS (Background Intelligent Transfer Service). Further documentation can
+//! be found in the XPCOM interface definition, located in nsIBits.idl
+
+#![cfg(target_os = "windows")]
+
+extern crate bits_client;
+extern crate comedy;
+extern crate crossbeam_utils;
+extern crate libc;
+extern crate log;
+extern crate moz_task;
+extern crate nserror;
+extern crate nsstring;
+extern crate xpcom;
+
+pub mod bits_interface;