diff --git a/browser/base/content/test/Makefile.in b/browser/base/content/test/Makefile.in
index 1dc05eaed6d..cbed37e6921 100644
--- a/browser/base/content/test/Makefile.in
+++ b/browser/base/content/test/Makefile.in
@@ -51,6 +51,7 @@ endif
 # The following tests are disabled because they are unreliable:
 #   browser_bug423833.js is bug 428712
 #   browser_sanitize-download-history.js is bug 432425
+#   browser_aboutHome.js is bug 890409
 #
 # browser_sanitizeDialog_treeView.js is disabled until the tree view is added
 # back to the clear recent history dialog (sanitize.xul), if it ever is (bug
@@ -71,7 +72,6 @@ MOCHITEST_BROWSER_FILES = \
                  blockPluginVulnerableNoUpdate.xml \
                  blockPluginVulnerableUpdatable.xml \
                  browser_aboutHealthReport.js \
-                 browser_aboutHome.js \
                  browser_aboutSyncProgress.js \
                  browser_addKeywordSearch.js \
                  browser_addon_bar_aomlistener.js \
diff --git a/browser/base/content/test/browser_aboutHome.js b/browser/base/content/test/browser_aboutHome.js
index deb229be479..12c3080a6b3 100644
--- a/browser/base/content/test/browser_aboutHome.js
+++ b/browser/base/content/test/browser_aboutHome.js
@@ -89,27 +89,8 @@ let gTests = [
 },
 
 {
-  desc: "Check that performing a search fires a search event.",
-  setup: function () { },
-  run: function () {
-    let deferred = Promise.defer();
-    let doc = gBrowser.contentDocument;
-
-    doc.addEventListener("AboutHomeSearchEvent", function onSearch(e) {
-      is(e.detail, doc.documentElement.getAttribute("searchEngineName"), "Detail is search engine name");
-
-      gBrowser.stop();
-      deferred.resolve();
-    }, true, true);
-
-    doc.getElementById("searchText").value = "it works";
-    doc.getElementById("searchSubmit").click();
-    return deferred.promise;
-  }
-},
-
-{
-  desc: "Check that performing a search records to Firefox Health Report.",
+  desc: "Check that performing a search fires a search event and records to " +
+        "Firefox Health Report.",
   setup: function () { },
   run: function () {
     try {
@@ -120,46 +101,33 @@ let gTests = [
       return Promise.resolve();
     }
 
+    let numSearchesBefore = 0;
     let deferred = Promise.defer();
     let doc = gBrowser.contentDocument;
 
     // We rely on the listener in browser.js being installed and fired before
     // this one. If this ever changes, we should add an executeSoon() or similar.
     doc.addEventListener("AboutHomeSearchEvent", function onSearch(e) {
-      executeSoon(gBrowser.stop.bind(gBrowser));
-      let reporter = Components.classes["@mozilla.org/datareporting/service;1"]
-                                       .getService()
-                                       .wrappedJSObject
-                                       .healthReporter;
-      ok(reporter, "Health Reporter instance available.");
+      let engineName = doc.documentElement.getAttribute("searchEngineName");
+      is(e.detail, engineName, "Detail is search engine name");
 
-      reporter.onInit().then(function onInit() {
-        let provider = reporter.getProvider("org.mozilla.searches");
-        ok(provider, "Searches provider is available.");
-
-        let engineName = doc.documentElement.getAttribute("searchEngineName");
-        let id = Services.search.getEngineByName(engineName).identifier;
-
-        let m = provider.getMeasurement("counts", 2);
-        m.getValues().then(function onValues(data) {
-          let now = new Date();
-          ok(data.days.hasDay(now), "Have data for today.");
-
-          let day = data.days.getDay(now);
-          let field = id + ".abouthome";
-          ok(day.has(field), "Have data for about home on this engine.");
-
-          // Note the search from the previous test.
-          is(day.get(field), 2, "Have searches recorded.");
-
-          deferred.resolve();
-        });
+      gBrowser.stop();
 
+      getNumberOfSearches().then(num => {
+        is(num, numSearchesBefore + 1, "One more search recorded.");
+        deferred.resolve();
       });
     }, true, true);
 
-    doc.getElementById("searchText").value = "a search";
-    doc.getElementById("searchSubmit").click();
+    // Get the current number of recorded searches.
+    getNumberOfSearches().then(num => {
+      numSearchesBefore = num;
+
+      info("Perform a search.");
+      doc.getElementById("searchText").value = "a search";
+      doc.getElementById("searchSubmit").click();
+    });
+
     return deferred.promise;
   }
 },
@@ -422,3 +390,54 @@ function promiseBrowserAttributes(aTab)
 
   return deferred.promise;
 }
+
+/**
+ * Retrieves the number of about:home searches recorded for the current day.
+ *
+ * @return {Promise} Returns a promise resolving to the number of searches.
+ */
+function getNumberOfSearches() {
+  let reporter = Components.classes["@mozilla.org/datareporting/service;1"]
+                                   .getService()
+                                   .wrappedJSObject
+                                   .healthReporter;
+  ok(reporter, "Health Reporter instance available.");
+
+  return reporter.onInit().then(function onInit() {
+    let provider = reporter.getProvider("org.mozilla.searches");
+    ok(provider, "Searches provider is available.");
+
+    let m = provider.getMeasurement("counts", 2);
+    return m.getValues().then(data => {
+      let now = new Date();
+      let yday = new Date(now);
+      yday.setDate(yday.getDate() - 1);
+
+      // Add the number of searches recorded yesterday to the number of searches
+      // recorded today. This makes the test not fail intermittently when it is
+      // run at midnight and we accidentally compare the number of searches from
+      // different days. Tests are always run with an empty profile so there
+      // are no searches from yesterday, normally. Should the test happen to run
+      // past midnight we make sure to count them in as well.
+      return getNumberOfSearchesByDate(data, now) +
+             getNumberOfSearchesByDate(data, yday);
+    });
+  });
+}
+
+function getNumberOfSearchesByDate(aData, aDate) {
+  if (aData.days.hasDay(aDate)) {
+    let doc = gBrowser.contentDocument;
+    let engineName = doc.documentElement.getAttribute("searchEngineName");
+    let id = Services.search.getEngineByName(engineName).identifier;
+
+    let day = aData.days.getDay(aDate);
+    let field = id + ".abouthome";
+
+    if (day.has(field)) {
+      return day.get(field) || 0;
+    }
+  }
+
+  return 0; // No records found.
+}
diff --git a/browser/components/sessionstore/src/SessionStore.jsm b/browser/components/sessionstore/src/SessionStore.jsm
index 11932edd82a..8e92478278b 100644
--- a/browser/components/sessionstore/src/SessionStore.jsm
+++ b/browser/components/sessionstore/src/SessionStore.jsm
@@ -460,7 +460,11 @@ let SessionStoreInternal = {
 
     // A Lazy getter for the sessionstore.js backup promise.
     XPCOMUtils.defineLazyGetter(this, "_backupSessionFileOnce", function () {
-      return _SessionFile.createBackupCopy();
+      // We're creating a backup of sessionstore.js by moving it to .bak
+      // because that's a lot faster than creating a copy. sessionstore.js
+      // would be overwritten shortly afterwards anyway so we can save time
+      // and just move instead of copy.
+      return _SessionFile.moveToBackupPath();
     });
 
     // at this point, we've as good as resumed the session, so we can
@@ -504,12 +508,12 @@ let SessionStoreInternal = {
     return Task.spawn(function task() {
       try {
         // Perform background backup
-        yield _SessionFile.createUpgradeBackupCopy("-" + buildID);
+        yield _SessionFile.createBackupCopy("-" + buildID);
 
         this._prefBranch.setCharPref(PREF_UPGRADE, buildID);
 
         // In case of success, remove previous backup.
-        yield _SessionFile.removeUpgradeBackup("-" + latestBackup);
+        yield _SessionFile.removeBackupCopy("-" + latestBackup);
       } catch (ex) {
         debug("Could not perform upgrade backup " + ex);
         debug(ex.stack);
@@ -772,12 +776,12 @@ let SessionStoreInternal = {
           this._restoreCount = this._initialState.windows ? this._initialState.windows.length : 0;
           this.restoreWindow(aWindow, this._initialState,
                              this._isCmdLineEmpty(aWindow, this._initialState));
-
-          // _loadState changed from "stopped" to "running"
-          // force a save operation so that crashes happening during startup are correctly counted
-          this._initialState.session.state = STATE_RUNNING_STR;
-          this._saveStateObject(this._initialState);
           this._initialState = null;
+
+          // _loadState changed from "stopped" to "running". Save the session's
+          // load state immediately so that crashes happening during startup
+          // are correctly counted.
+          _SessionFile.writeLoadStateOnceAfterStartup(STATE_RUNNING_STR);
         }
       }
       else {
diff --git a/browser/components/sessionstore/src/SessionWorker.js b/browser/components/sessionstore/src/SessionWorker.js
new file mode 100644
index 00000000000..a4d17813ce9
--- /dev/null
+++ b/browser/components/sessionstore/src/SessionWorker.js
@@ -0,0 +1,221 @@
+/* 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/. */
+
+/**
+ * A worker dedicated to handle I/O for Session Store.
+ */
+
+"use strict";
+
+importScripts("resource://gre/modules/osfile.jsm");
+
+let File = OS.File;
+let Encoder = new TextEncoder();
+let Decoder = new TextDecoder();
+
+/**
+ * Communications with the controller.
+ *
+ * Accepts messages:
+ * {fun:function_name, args:array_of_arguments_or_null, id: custom_id}
+ *
+ * Sends messages:
+ * {ok: result, id: custom_id} / {fail: serialized_form_of_OS.File.Error,
+ *                                id: custom_id}
+ */
+self.onmessage = function (msg) {
+  let data = msg.data;
+  if (!(data.fun in Agent)) {
+    throw new Error("Cannot find method " + data.fun);
+  }
+
+  let result;
+  let id = data.id;
+
+  try {
+    result = Agent[data.fun].apply(Agent, data.args);
+  } catch (ex if ex instanceof OS.File.Error) {
+    // Instances of OS.File.Error know how to serialize themselves
+    // (deserialization ensures that we end up with OS-specific
+    // instances of |OS.File.Error|)
+    self.postMessage({fail: OS.File.Error.toMsg(ex), id: id});
+    return;
+  }
+
+  // Other exceptions do not, and should be propagated through DOM's
+  // built-in mechanism for uncaught errors, although this mechanism
+  // may lose interesting information.
+  self.postMessage({ok: result, id: id});
+};
+
+let Agent = {
+  // The initial session string as read from disk.
+  initialState: null,
+
+  // Boolean that tells whether we already wrote
+  // the loadState to disk once after startup.
+  hasWrittenLoadStateOnce: false,
+
+  // The path to sessionstore.js
+  path: OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.js"),
+
+  // The path to sessionstore.bak
+  backupPath: OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.bak"),
+
+  /**
+   * This method is only intended to be called by _SessionFile.syncRead() and
+   * can be removed when we're not supporting synchronous SessionStore
+   * initialization anymore. When sessionstore.js is read from disk
+   * synchronously the state string must be supplied to the worker manually by
+   * calling this method.
+   */
+  setInitialState: function (aState) {
+    // _SessionFile.syncRead() should not be called after startup has finished.
+    // Thus we also don't support any setInitialState() calls after we already
+    // wrote the loadState to disk.
+    if (this.hasWrittenLoadStateOnce) {
+      throw new Error("writeLoadStateOnceAfterStartup() must only be called once.");
+    }
+
+    // Initial state might have been filled by read() already but yet we might
+    // be called by _SessionFile.syncRead() before SessionStore.jsm had a chance
+    // to call writeLoadStateOnceAfterStartup(). It's safe to ignore
+    // setInitialState() calls if this happens.
+    if (!this.initialState) {
+      this.initialState = aState;
+    }
+  },
+
+  /**
+   * Read the session from disk.
+   * In case sessionstore.js does not exist, attempt to read sessionstore.bak.
+   */
+  read: function () {
+    for (let path of [this.path, this.backupPath]) {
+      try {
+        return this.initialState = Decoder.decode(File.read(path));
+      } catch (ex if isNoSuchFileEx(ex)) {
+        // Ignore exceptions about non-existent files.
+      }
+    }
+
+    // No sessionstore data files found. Return an empty string.
+    return "";
+  },
+
+  /**
+   * Write the session to disk.
+   */
+  write: function (stateString) {
+    let bytes = Encoder.encode(stateString);
+    return File.writeAtomic(this.path, bytes, {tmpPath: this.path + ".tmp"});
+  },
+
+  /**
+   * Writes the session state to disk again but changes session.state to
+   * 'running' before doing so. This is intended to be called only once, shortly
+   * after startup so that we detect crashes on startup correctly.
+   */
+  writeLoadStateOnceAfterStartup: function (loadState) {
+    if (this.hasWrittenLoadStateOnce) {
+      throw new Error("writeLoadStateOnceAfterStartup() must only be called once.");
+    }
+
+    if (!this.initialState) {
+      throw new Error("writeLoadStateOnceAfterStartup() must not be called " +
+                      "without a valid session state or before it has been " +
+                      "read from disk.");
+    }
+
+    // Make sure we can't call this function twice.
+    this.hasWrittenLoadStateOnce = true;
+
+    let state;
+    try {
+      state = JSON.parse(this.initialState);
+    } finally {
+      this.initialState = null;
+    }
+
+    state.session = state.session || {};
+    state.session.state = loadState;
+    return this.write(JSON.stringify(state));
+  },
+
+  /**
+   * Moves sessionstore.js to sessionstore.bak.
+   */
+  moveToBackupPath: function () {
+    try {
+      return File.move(this.path, this.backupPath);
+    } catch (ex if isNoSuchFileEx(ex)) {
+      // Ignore exceptions about non-existent files.
+      return true;
+    }
+  },
+
+  /**
+   * Creates a copy of sessionstore.js.
+   */
+  createBackupCopy: function (ext) {
+    try {
+      return File.copy(this.path, this.backupPath + ext);
+    } catch (ex if isNoSuchFileEx(ex)) {
+      // Ignore exceptions about non-existent files.
+      return true;
+    }
+  },
+
+  /**
+   * Removes a backup copy.
+   */
+  removeBackupCopy: function (ext) {
+    try {
+      return File.remove(this.backupPath + ext);
+    } catch (ex if isNoSuchFileEx(ex)) {
+      // Ignore exceptions about non-existent files.
+      return true;
+    }
+  },
+
+  /**
+   * Wipes all files holding session data from disk.
+   */
+  wipe: function () {
+    let exn;
+
+    // Erase session state file
+    try {
+      File.remove(this.path);
+    } catch (ex if isNoSuchFileEx(ex)) {
+      // Ignore exceptions about non-existent files.
+    } catch (ex) {
+      // Don't stop immediately.
+      exn = ex;
+    }
+
+    // Erase any backup, any file named "sessionstore.bak[-buildID]".
+    let iter = new File.DirectoryIterator(OS.Constants.Path.profileDir);
+    for (let entry in iter) {
+      if (!entry.isDir && entry.path.startsWith(this.backupPath)) {
+        try {
+          File.remove(entry.path);
+        } catch (ex) {
+          // Don't stop immediately.
+          exn = exn || ex;
+        }
+      }
+    }
+
+    if (exn) {
+      throw exn;
+    }
+
+    return true;
+  }
+};
+
+function isNoSuchFileEx(aReason) {
+  return aReason instanceof OS.File.Error && aReason.becauseNoSuchFile;
+}
diff --git a/browser/components/sessionstore/src/_SessionFile.jsm b/browser/components/sessionstore/src/_SessionFile.jsm
index 83afb92d4f4..4a69b36be47 100644
--- a/browser/components/sessionstore/src/_SessionFile.jsm
+++ b/browser/components/sessionstore/src/_SessionFile.jsm
@@ -32,6 +32,7 @@ const Ci = Components.interfaces;
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 Cu.import("resource://gre/modules/osfile.jsm");
+Cu.import("resource://gre/modules/osfile/_PromiseWorker.jsm", this);
 Cu.import("resource://gre/modules/commonjs/sdk/core/promise.js");
 
 XPCOMUtils.defineLazyModuleGetter(this, "TelemetryStopwatch",
@@ -44,66 +45,62 @@ XPCOMUtils.defineLazyModuleGetter(this, "Task",
   "resource://gre/modules/Task.jsm");
 XPCOMUtils.defineLazyServiceGetter(this, "Telemetry",
   "@mozilla.org/base/telemetry;1", "nsITelemetry");
-
-// An encoder to UTF-8.
-XPCOMUtils.defineLazyGetter(this, "gEncoder", function () {
-  return new TextEncoder();
-});
-// A decoder.
-XPCOMUtils.defineLazyGetter(this, "gDecoder", function () {
-  return new TextDecoder();
-});
+XPCOMUtils.defineLazyModuleGetter(this, "Deprecated",
+  "resource://gre/modules/Deprecated.jsm");
 
 this._SessionFile = {
-  /**
-   * A promise fulfilled once initialization (either synchronous or
-   * asynchronous) is complete.
-   */
-  promiseInitialized: function SessionFile_initialized() {
-    return SessionFileInternal.promiseInitialized;
-  },
   /**
    * Read the contents of the session file, asynchronously.
    */
-  read: function SessionFile_read() {
+  read: function () {
     return SessionFileInternal.read();
   },
   /**
    * Read the contents of the session file, synchronously.
    */
-  syncRead: function SessionFile_syncRead() {
+  syncRead: function () {
+    Deprecated.warning(
+      "syncRead is deprecated and will be removed in a future version",
+      "https://bugzilla.mozilla.org/show_bug.cgi?id=532150")
     return SessionFileInternal.syncRead();
   },
   /**
    * Write the contents of the session file, asynchronously.
    */
-  write: function SessionFile_write(aData) {
+  write: function (aData) {
     return SessionFileInternal.write(aData);
   },
+  /**
+   * Writes the initial state to disk again only to change the session's load
+   * state. This must only be called once, it will throw an error otherwise.
+   */
+  writeLoadStateOnceAfterStartup: function (aLoadState) {
+    return SessionFileInternal.writeLoadStateOnceAfterStartup(aLoadState);
+  },
   /**
    * Create a backup copy, asynchronously.
    */
-  createBackupCopy: function SessionFile_createBackupCopy() {
-    return SessionFileInternal.createBackupCopy();
+  moveToBackupPath: function () {
+    return SessionFileInternal.moveToBackupPath();
   },
   /**
    * Create a backup copy, asynchronously.
    * This is designed to perform backup on upgrade.
    */
-  createUpgradeBackupCopy: function(ext) {
-    return SessionFileInternal.createUpgradeBackupCopy(ext);
+  createBackupCopy: function (ext) {
+    return SessionFileInternal.createBackupCopy(ext);
   },
   /**
    * Remove a backup copy, asynchronously.
    * This is designed to clean up a backup on upgrade.
    */
-  removeUpgradeBackup: function(ext) {
-    return SessionFileInternal.removeUpgradeBackup(ext);
+  removeBackupCopy: function (ext) {
+    return SessionFileInternal.removeBackupCopy(ext);
   },
   /**
    * Wipe the contents of the session file, asynchronously.
    */
-  wipe: function SessionFile_wipe() {
+  wipe: function () {
     return SessionFileInternal.wipe();
   }
 };
@@ -147,11 +144,6 @@ const TaskUtils = {
 };
 
 let SessionFileInternal = {
-  /**
-   * A promise fulfilled once initialization is complete
-   */
-  promiseInitialized: Promise.defer(),
-
   /**
    * The path to sessionstore.js
    */
@@ -168,7 +160,7 @@ let SessionFileInternal = {
    *        A path to read the file from.
    * @returns string if successful, undefined otherwise.
    */
-  readAuxSync: function ssfi_readAuxSync(aPath) {
+  readAuxSync: function (aPath) {
     let text;
     try {
       let file = new FileUtils.File(aPath);
@@ -197,7 +189,7 @@ let SessionFileInternal = {
    * happened between backup and write), attempt to read the sessionstore.bak
    * instead.
    */
-  syncRead: function ssfi_syncRead() {
+  syncRead: function () {
     // Start measuring the duration of the synchronous read.
     TelemetryStopwatch.start("FX_SESSION_RESTORE_SYNC_READ_FILE_MS");
     // First read the sessionstore.js.
@@ -208,83 +200,26 @@ let SessionFileInternal = {
     }
     // Finish the telemetry probe and return an empty string.
     TelemetryStopwatch.finish("FX_SESSION_RESTORE_SYNC_READ_FILE_MS");
-    return text || "";
+    text = text || "";
+
+    // The worker needs to know the initial state read from
+    // disk so that writeLoadStateOnceAfterStartup() works.
+    SessionWorker.post("setInitialState", [text]);
+    return text;
   },
 
-  /**
-   * Utility function to safely read a file asynchronously.
-   * @param aPath
-   *        A path to read the file from.
-   * @param aReadOptions
-   *        Read operation options.
-   *        |outExecutionDuration| option will be reused and can be
-   *        incrementally updated by the worker process.
-   * @returns string if successful, undefined otherwise.
-   */
-  readAux: function ssfi_readAux(aPath, aReadOptions) {
-    let self = this;
-    return TaskUtils.spawn(function () {
-      let text;
-      try {
-        let bytes = yield OS.File.read(aPath, undefined, aReadOptions);
-        text = gDecoder.decode(bytes);
-        // If the file is read successfully, add a telemetry probe based on
-        // the updated duration value of the |outExecutionDuration| option.
-        let histogram = Telemetry.getHistogramById(
-          "FX_SESSION_RESTORE_READ_FILE_MS");
-        histogram.add(aReadOptions.outExecutionDuration);
-      } catch (ex if self._isNoSuchFile(ex)) {
-        // Ignore exceptions about non-existent files.
-      } catch (ex) {
-        Cu.reportError(ex);
-      }
-      throw new Task.Result(text);
-    });
+  read: function () {
+    return SessionWorker.post("read").then(msg => msg.ok);
   },
 
-  /**
-   * Read the sessionstore file asynchronously.
-   *
-   * In case sessionstore.js file does not exist or is corrupted (something
-   * happened between backup and write), attempt to read the sessionstore.bak
-   * instead.
-   */
-  read: function ssfi_read() {
-    let self = this;
-    return TaskUtils.spawn(function task() {
-      // Specify |outExecutionDuration| option to hold the combined duration of
-      // the asynchronous reads off the main thread (of both sessionstore.js and
-      // sessionstore.bak, if necessary). If sessionstore.js does not exist or
-      // is corrupted, |outExecutionDuration| will register the time it took to
-      // attempt to read the file. It will then be subsequently incremented by
-      // the read time of sessionsore.bak.
-      let readOptions = {
-        outExecutionDuration: null
-      };
-      // First read the sessionstore.js.
-      let text = yield self.readAux(self.path, readOptions);
-      if (typeof text === "undefined") {
-        // If sessionstore.js does not exist or is corrupted, read the
-        // sessionstore.bak.
-        text = yield self.readAux(self.backupPath, readOptions);
-      }
-      // Return either the content of the sessionstore.bak if it was read
-      // successfully or an empty string otherwise.
-      throw new Task.Result(text || "");
-    });
-  },
-
-  write: function ssfi_write(aData) {
+  write: function (aData) {
     let refObj = {};
-    let self = this;
     return TaskUtils.spawn(function task() {
       TelemetryStopwatch.start("FX_SESSION_RESTORE_WRITE_FILE_MS", refObj);
       TelemetryStopwatch.start("FX_SESSION_RESTORE_WRITE_FILE_LONGEST_OP_MS", refObj);
 
-      let bytes = gEncoder.encode(aData);
-
       try {
-        let promise = OS.File.writeAtomic(self.path, bytes, {tmpPath: self.path + ".tmp"});
+        let promise = SessionWorker.post("write", [aData]);
         // At this point, we measure how long we stop the main thread
         TelemetryStopwatch.finish("FX_SESSION_RESTORE_WRITE_FILE_LONGEST_OP_MS", refObj);
 
@@ -294,93 +229,51 @@ let SessionFileInternal = {
       } catch (ex) {
         TelemetryStopwatch.cancel("FX_SESSION_RESTORE_WRITE_FILE_LONGEST_OP_MS", refObj);
         TelemetryStopwatch.cancel("FX_SESSION_RESTORE_WRITE_FILE_MS", refObj);
-        Cu.reportError("Could not write session state file " + self.path
-                       + ": " + aReason);
-      }
-    });
-  },
-
-  createBackupCopy: function ssfi_createBackupCopy() {
-    let backupCopyOptions = {
-      outExecutionDuration: null
-    };
-    let self = this;
-    return TaskUtils.spawn(function task() {
-      try {
-        yield OS.File.move(self.path, self.backupPath, backupCopyOptions);
-        Telemetry.getHistogramById("FX_SESSION_RESTORE_BACKUP_FILE_MS").add(
-          backupCopyOptions.outExecutionDuration);
-      } catch (ex if self._isNoSuchFile(ex)) {
-        // Ignore exceptions about non-existent files.
-      } catch (ex) {
-        Cu.reportError("Could not backup session state file: " + ex);
-        throw ex;
-      }
-    });
-  },
-
-  createUpgradeBackupCopy: function(ext) {
-    return TaskUtils.spawn(function task() {
-      try {
-        yield OS.File.copy(this.path, this.backupPath + ext);
-      } catch (ex if this._isNoSuchFile(ex)) {
-        // Ignore exceptions about non-existent files.
-      } catch (ex) {
-        Cu.reportError("Could not backup session state file to " +
-                       dest + ": " + ex);
-        throw ex;
+        Cu.reportError("Could not write session state file " + this.path
+                       + ": " + ex);
       }
     }.bind(this));
   },
 
-  removeUpgradeBackup: function(ext) {
-    return TaskUtils.spawn(function task() {
-      try {
-        yield OS.File.remove(this.backupPath + ext);
-      } catch (ex if this._isNoSuchFile(ex)) {
-        // Ignore exceptions about non-existent files.
-      }
-    }.bind(this));
+  writeLoadStateOnceAfterStartup: function (aLoadState) {
+    return SessionWorker.post("writeLoadStateOnceAfterStartup", [aLoadState]);
   },
 
-  wipe: function ssfi_wipe() {
-    let self = this;
-    return TaskUtils.spawn(function task() {
-      let exn;
-      // Erase session state file
-      try {
-        yield OS.File.remove(self.path);
-      } catch (ex if self._isNoSuchFile(ex)) {
-        // Ignore exceptions about non-existent files.
-      } catch (ex) {
-        // Report error, don't stop immediately
-        Cu.reportError("Could not remove session state file: " + ex);
-        exn = ex;
-      }
-
-      // Erase any backup, any file named "sessionstore.bak[-buildID]".
-      let iterator = new OS.File.DirectoryIterator(OS.Constants.Path.profileDir);
-      for (let promise of iterator) {
-        let entry = yield promise;
-        if (!entry.isDir && entry.path.startsWith(self.backupPath)) {
-          try {
-            yield OS.File.remove(entry.path);
-          } catch (ex) {
-            // Report error, don't stop immediately
-            Cu.reportError("Could not remove backup file " + entry.path + " : " + ex);
-            exn = exn || ex;
-          }
-        }
-      }
-
-      if (exn) {
-        throw exn;
-      }
-    });
-
+  moveToBackupPath: function () {
+    return SessionWorker.post("moveToBackupPath");
   },
 
-  _isNoSuchFile: function ssfi_isNoSuchFile(aReason) {
-    return aReason instanceof OS.File.Error && aReason.becauseNoSuchFile;
+  createBackupCopy: function (ext) {
+    return SessionWorker.post("createBackupCopy", [ext]);
+  },
+
+  removeBackupCopy: function (ext) {
+    return SessionWorker.post("removeBackupCopy", [ext]);
+  },
+
+  wipe: function () {
+    return SessionWorker.post("wipe");
   }
 };
+
+// Interface to a dedicated thread handling I/O
+let SessionWorker = (function () {
+  let worker = new PromiseWorker("resource:///modules/sessionstore/SessionWorker.js",
+    OS.Shared.LOG.bind("SessionWorker"));
+  return {
+    post: function post(...args) {
+      let promise = worker.post.apply(worker, args);
+      return promise.then(
+        null,
+        function onError(error) {
+          // Decode any serialized error
+          if (error instanceof PromiseWorker.WorkerError) {
+            throw OS.File.Error.fromMsg(error.data);
+          } else {
+            throw error;
+          }
+        }
+      );
+    }
+  };
+})();
diff --git a/browser/components/sessionstore/src/moz.build b/browser/components/sessionstore/src/moz.build
index bdd2ee4c104..41e67e1b765 100644
--- a/browser/components/sessionstore/src/moz.build
+++ b/browser/components/sessionstore/src/moz.build
@@ -16,6 +16,7 @@ EXTRA_JS_MODULES = [
     'DocumentUtils.jsm',
     'SessionMigration.jsm',
     'SessionStorage.jsm',
+    'SessionWorker.js',
     'XPathGenerator.jsm',
     '_SessionFile.jsm',
 ]
diff --git a/browser/components/sessionstore/test/browser_354894_perwindowpb.js b/browser/components/sessionstore/test/browser_354894_perwindowpb.js
index 78e861ba08f..d2814dfe721 100644
--- a/browser/components/sessionstore/test/browser_354894_perwindowpb.js
+++ b/browser/components/sessionstore/test/browser_354894_perwindowpb.js
@@ -196,18 +196,13 @@ function test() {
       options = {private: true};
     }
 
-    let newWin = OpenBrowserWindow(options);
-    newWin.addEventListener("load", function(aEvent) {
-      newWin.removeEventListener("load", arguments.callee, false);
-      newWin.gBrowser.addEventListener("load", function(aEvent) {
-        newWin.gBrowser.removeEventListener("load", arguments.callee, true);
-        TEST_URLS.forEach(function (url) {
-          newWin.gBrowser.addTab(url);
-        });
+    whenNewWindowLoaded(options, function (newWin) {
+      TEST_URLS.forEach(function (url) {
+        newWin.gBrowser.addTab(url);
+      });
 
-        executeSoon(function() testFn(newWin));
-      }, true);
-    }, false);
+      executeSoon(() => testFn(newWin));
+    });
   }
 
   /**
@@ -230,20 +225,16 @@ function test() {
 
       // Open a new window
       // The previously closed window should be restored
-      newWin = OpenBrowserWindow({});
-      newWin.addEventListener("load", function() {
-        this.removeEventListener("load", arguments.callee, true);
-        executeSoon(function() {
-          is(newWin.gBrowser.browsers.length, TEST_URLS.length + 1,
-             "Restored window in-session with otherpopup windows around");
+      whenNewWindowLoaded({}, function (newWin) {
+        is(newWin.gBrowser.browsers.length, TEST_URLS.length + 1,
+           "Restored window in-session with otherpopup windows around");
 
-          // Cleanup
-          newWin.close();
+        // Cleanup
+        newWin.close();
 
-          // Next please
-          executeSoon(nextFn);
-        });
-      }, true);
+        // Next please
+        executeSoon(nextFn);
+      });
     });
   }
 
@@ -259,32 +250,24 @@ function test() {
       // Enter private browsing mode
       // Open a new window.
       // The previously closed window should NOT be restored
-      newWin = OpenBrowserWindow({private: true});
-      newWin.addEventListener("load", function() {
-        this.removeEventListener("load", arguments.callee, true);
-        executeSoon(function() {
-          is(newWin.gBrowser.browsers.length, 1,
-             "Did not restore in private browing mode");
+      whenNewWindowLoaded({private: true}, function (newWin) {
+        is(newWin.gBrowser.browsers.length, 1,
+           "Did not restore in private browing mode");
 
-          // Cleanup
-          newWin.BrowserTryToCloseWindow();
+        // Cleanup
+        newWin.BrowserTryToCloseWindow();
 
-          // Exit private browsing mode again
-          newWin = OpenBrowserWindow({});
-          newWin.addEventListener("load", function() {
-            this.removeEventListener("load", arguments.callee, true);
-            executeSoon(function() {
-              is(newWin.gBrowser.browsers.length, TEST_URLS.length + 1,
-                 "Restored after leaving private browsing again");
+        // Exit private browsing mode again
+        whenNewWindowLoaded({}, function (newWin) {
+          is(newWin.gBrowser.browsers.length, TEST_URLS.length + 1,
+             "Restored after leaving private browsing again");
 
-              newWin.close();
+          newWin.close();
 
-              // Next please
-              executeSoon(nextFn);
-            });
-          }, true);
+          // Next please
+          executeSoon(nextFn);
         });
-      }, true);
+      });
     });
   }
 
@@ -312,21 +295,17 @@ function test() {
           popup2.close();
 
           // open a new window the previously closed window should be restored to
-          newWin = OpenBrowserWindow({});
-          newWin.addEventListener("load", function() {
-            this.removeEventListener("load", arguments.callee, true);
-            executeSoon(function() {
-              is(newWin.gBrowser.browsers.length, TEST_URLS.length + 1,
-                 "Restored window and associated tabs in session");
+          whenNewWindowLoaded({}, function (newWin) {
+            is(newWin.gBrowser.browsers.length, TEST_URLS.length + 1,
+               "Restored window and associated tabs in session");
 
-              // Cleanup
-              newWin.close();
-              popup.close();
+            // Cleanup
+            newWin.close();
+            popup.close();
 
-              // Next please
-              executeSoon(nextFn);
-            });
-          }, true);
+            // Next please
+            executeSoon(nextFn);
+          });
         }, true);
       }, false);
     });
@@ -366,22 +345,18 @@ function test() {
           // but instead a new window is opened without restoring anything
           popup.close();
 
-          let newWin = OpenBrowserWindow({});
-          newWin.addEventListener("load", function() {
-            newWin.removeEventListener("load", arguments.callee, true);
-            executeSoon(function() {
-              isnot(newWin.gBrowser.browsers.length, 2,
-                    "Did not restore the popup window");
-              is(TEST_URLS.indexOf(newWin.gBrowser.browsers[0].currentURI.spec), -1,
-                 "Did not restore the popup window (2)");
+          whenNewWindowLoaded({}, function (newWin) {
+            isnot(newWin.gBrowser.browsers.length, 2,
+                  "Did not restore the popup window");
+            is(TEST_URLS.indexOf(newWin.gBrowser.browsers[0].currentURI.spec), -1,
+               "Did not restore the popup window (2)");
 
-              // Cleanup
-              newWin.close();
+            // Cleanup
+            newWin.close();
 
-              // Next please
-              executeSoon(nextFn);
-            });
-          }, true);
+            // Next please
+            executeSoon(nextFn);
+          });
         }, true);
       }, false);
     }, true);
@@ -402,27 +377,23 @@ function test() {
 
         newWin = undoCloseWindow(0);
 
-        newWin2 = OpenBrowserWindow({});
-        newWin2.addEventListener("load", function() {
-          newWin2.removeEventListener("load", arguments.callee, true);
-          executeSoon(function() {
-            is(newWin2.gBrowser.browsers.length, 1,
-               "Did not restore, as undoCloseWindow() was last called");
-            is(TEST_URLS.indexOf(newWin2.gBrowser.browsers[0].currentURI.spec), -1,
-               "Did not restore, as undoCloseWindow() was last called (2)");
+        whenNewWindowLoaded({}, function (newWin2) {
+          is(newWin2.gBrowser.browsers.length, 1,
+             "Did not restore, as undoCloseWindow() was last called");
+          is(TEST_URLS.indexOf(newWin2.gBrowser.browsers[0].currentURI.spec), -1,
+             "Did not restore, as undoCloseWindow() was last called (2)");
 
-            browserWindowsCount([2, 3], "browser windows while running testOpenCloseRestoreFromPopup");
+          browserWindowsCount([2, 3], "browser windows while running testOpenCloseRestoreFromPopup");
 
-            // Cleanup
-            newWin.close();
-            newWin2.close();
+          // Cleanup
+          newWin.close();
+          newWin2.close();
 
-            browserWindowsCount([0, 1], "browser windows while running testOpenCloseRestoreFromPopup");
+          browserWindowsCount([0, 1], "browser windows while running testOpenCloseRestoreFromPopup");
 
-            // Next please
-            executeSoon(nextFn);
-          });
-        }, true);
+          // Next please
+          executeSoon(nextFn);
+        });
       });
     });
   }
diff --git a/browser/components/sessionstore/test/browser_524745.js b/browser/components/sessionstore/test/browser_524745.js
index b531f4c8d2f..d984ec38b8c 100644
--- a/browser/components/sessionstore/test/browser_524745.js
+++ b/browser/components/sessionstore/test/browser_524745.js
@@ -10,37 +10,34 @@ function test() {
 
   waitForExplicitFinish();
 
-  let window_B = openDialog(location, "_blank", "chrome,all,dialog=no");
-  window_B.addEventListener("load", function(aEvent) {
-    window_B.removeEventListener("load", arguments.callee, false);
+  whenNewWindowLoaded({ private: false }, function (window_B) {
+    waitForFocus(function() {
+      // Add identifying information to window_B
+      ss.setWindowValue(window_B, uniqKey, uniqVal);
+      let state = JSON.parse(ss.getBrowserState());
+      let selectedWindow = state.windows[state.selectedWindow - 1];
+      is(selectedWindow.extData && selectedWindow.extData[uniqKey], uniqVal,
+         "selectedWindow is window_B");
 
+      // Now minimize window_B. The selected window shouldn't have the secret data
+      window_B.minimize();
       waitForFocus(function() {
-        // Add identifying information to window_B
-        ss.setWindowValue(window_B, uniqKey, uniqVal);
-        let state = JSON.parse(ss.getBrowserState());
-        let selectedWindow = state.windows[state.selectedWindow - 1];
-        is(selectedWindow.extData && selectedWindow.extData[uniqKey], uniqVal,
-           "selectedWindow is window_B");
+        state = JSON.parse(ss.getBrowserState());
+        selectedWindow = state.windows[state.selectedWindow - 1];
+        ok(!selectedWindow.extData || !selectedWindow.extData[uniqKey],
+           "selectedWindow is not window_B after minimizing it");
 
-        // Now minimize window_B. The selected window shouldn't have the secret data
-        window_B.minimize();
-        waitForFocus(function() {
-          state = JSON.parse(ss.getBrowserState());
-          selectedWindow = state.windows[state.selectedWindow - 1];
-          ok(!selectedWindow.extData || !selectedWindow.extData[uniqKey],
-             "selectedWindow is not window_B after minimizing it");
+        // Now minimize the last open window (assumes no other tests left windows open)
+        window.minimize();
+        state = JSON.parse(ss.getBrowserState());
+        is(state.selectedWindow, 0,
+           "selectedWindow should be 0 when all windows are minimized");
 
-          // Now minimize the last open window (assumes no other tests left windows open)
-          window.minimize();
-          state = JSON.parse(ss.getBrowserState());
-          is(state.selectedWindow, 0,
-             "selectedWindow should be 0 when all windows are minimized");
-
-          // Cleanup
-          window.restore();
-          window_B.close();
-          finish();
-        });
-      }, window_B);
-  }, false);
+        // Cleanup
+        window.restore();
+        window_B.close();
+        finish();
+      });
+    }, window_B);
+  });
 }
diff --git a/browser/components/sessionstore/test/browser_819510_perwindowpb.js b/browser/components/sessionstore/test/browser_819510_perwindowpb.js
index 26937134c68..087ea4f4a29 100644
--- a/browser/components/sessionstore/test/browser_819510_perwindowpb.js
+++ b/browser/components/sessionstore/test/browser_819510_perwindowpb.js
@@ -174,23 +174,7 @@ function forceWriteState(aCallback) {
 }
 
 function testOnWindow(aIsPrivate, aCallback) {
-  let win = OpenBrowserWindow({private: aIsPrivate});
-  let gotLoad = false;
-  let gotActivate = false;
-  win.addEventListener("activate", function onActivate() {
-    win.removeEventListener("activate", onActivate, false);
-    gotActivate = true;
-    if (gotLoad) {
-      executeSoon(function() { aCallback(win) });
-    }
-  }, false);
-  win.addEventListener("load", function onLoad() {
-    win.removeEventListener("load", onLoad, false);
-    gotLoad = true;
-    if (gotActivate) {
-      executeSoon(function() { aCallback(win) });
-    }
-  }, false);
+  whenNewWindowLoaded({private: aIsPrivate}, aCallback);
 }
 
 function waitForTabLoad(aWin, aURL, aCallback) {
diff --git a/browser/components/sessionstore/test/browser_833286_atomic_backup.js b/browser/components/sessionstore/test/browser_833286_atomic_backup.js
index bc21ae39983..a04a6ed8ce5 100644
--- a/browser/components/sessionstore/test/browser_833286_atomic_backup.js
+++ b/browser/components/sessionstore/test/browser_833286_atomic_backup.js
@@ -78,9 +78,9 @@ function testWriteNoBackup() {
   let array = yield OS.File.read(path);
   gSSData = gDecoder.decode(array);
 
-  // Manually trigger _SessionFile.createBackupCopy since the backup once
+  // Manually trigger _SessionFile.moveToBackupPath since the backup once
   // promise is already resolved and backup would not be triggered again.
-  yield _SessionFile.createBackupCopy();
+  yield _SessionFile.moveToBackupPath();
 
   nextTest(testWriteBackup);
 }
@@ -140,4 +140,4 @@ function testNoWriteBackup() {
   is(ssBakData, gSSBakData, "sessionstore.bak is unchanged.");
 
   executeSoon(finish);
-}
\ No newline at end of file
+}
diff --git a/browser/components/sessionstore/test/browser_dying_cache.js b/browser/components/sessionstore/test/browser_dying_cache.js
index 41e22d864d3..169a07539ab 100644
--- a/browser/components/sessionstore/test/browser_dying_cache.js
+++ b/browser/components/sessionstore/test/browser_dying_cache.js
@@ -14,10 +14,11 @@ function test() {
 function runTests() {
   // Open a new window.
   let win = OpenBrowserWindow();
-  yield whenWindowLoaded(win);
+  yield whenDelayedStartupFinished(win, next);
 
   // Load some URL in the current tab.
-  win.gBrowser.selectedBrowser.loadURI("about:robots");
+  let flags = Ci.nsIWebNavigation.LOAD_FLAGS_REPLACE_HISTORY;
+  win.gBrowser.selectedBrowser.loadURIWithFlags("about:robots", flags);
   yield whenBrowserLoaded(win.gBrowser.selectedBrowser);
 
   // Open a second tab and close the first one.
diff --git a/browser/components/sessionstore/test/browser_input.js b/browser/components/sessionstore/test/browser_input.js
index 56681ca4fe9..3c135eecec0 100644
--- a/browser/components/sessionstore/test/browser_input.js
+++ b/browser/components/sessionstore/test/browser_input.js
@@ -19,7 +19,7 @@ function runTests() {
   // because we always collect data for tabs of active windows no matter if
   // the window is dirty or not.
   let win = OpenBrowserWindow();
-  yield waitForLoad(win);
+  yield whenDelayedStartupFinished(win, next);
 
   // Create a tab with some form fields.
   let tab = gBrowser.selectedTab = gBrowser.addTab(URL);
diff --git a/browser/components/sessionstore/test/browser_pageshow.js b/browser/components/sessionstore/test/browser_pageshow.js
index 0130288e368..3560b61af59 100644
--- a/browser/components/sessionstore/test/browser_pageshow.js
+++ b/browser/components/sessionstore/test/browser_pageshow.js
@@ -23,7 +23,7 @@ function runTests() {
   // because we always collect data for tabs of active windows no matter if
   // the window is dirty or not.
   let win = OpenBrowserWindow();
-  yield waitForLoad(win);
+  yield whenDelayedStartupFinished(win, next);
 
   // Create a tab with two history entries.
   let tab = gBrowser.selectedTab = gBrowser.addTab("about:blank");
diff --git a/browser/components/sessionstore/test/browser_windowRestore_perwindowpb.js b/browser/components/sessionstore/test/browser_windowRestore_perwindowpb.js
index 3ee34513816..ebe774b5b16 100644
--- a/browser/components/sessionstore/test/browser_windowRestore_perwindowpb.js
+++ b/browser/components/sessionstore/test/browser_windowRestore_perwindowpb.js
@@ -13,9 +13,7 @@ function test() {
 
   // Load a private window, then close it 
   // and verify it doesn't get remembered for restoring
-  var win = OpenBrowserWindow({private: true});
-
-  whenWindowLoaded(win, function onload() {
+  whenNewWindowLoaded({private: true}, function (win) {
     info("The private window got loaded");
     win.addEventListener("SSWindowClosing", function onclosing() {
       win.removeEventListener("SSWindowClosing", onclosing, false);
diff --git a/browser/components/sessionstore/test/head.js b/browser/components/sessionstore/test/head.js
index 85c62a9a172..ac2849b581b 100644
--- a/browser/components/sessionstore/test/head.js
+++ b/browser/components/sessionstore/test/head.js
@@ -289,39 +289,34 @@ function closeAllButPrimaryWindow() {
   }
 }
 
+/**
+ * When opening a new window it is not sufficient to wait for its load event.
+ * We need to use whenDelayedStartupFinshed() here as the browser window's
+ * delayedStartup() routine is executed one tick after the window's load event
+ * has been dispatched. browser-delayed-startup-finished might be deferred even
+ * further if parts of the window's initialization process take more time than
+ * expected (e.g. reading a big session state from disk).
+ */
 function whenNewWindowLoaded(aOptions, aCallback) {
   let win = OpenBrowserWindow(aOptions);
-  let gotLoad = false;
-  let gotActivate = (Cc["@mozilla.org/focus-manager;1"].getService(Ci.nsIFocusManager).activeWindow == win);
-
-  function maybeRunCallback() {
-    if (gotLoad && gotActivate) {
-      win.BrowserChromeTest.runWhenReady(function() {
-        executeSoon(function() { aCallback(win); });
-      });
-    }
-  }
-
-  if (!gotActivate) {
-    win.addEventListener("activate", function onActivate() {
-      info("Got activate.");
-      win.removeEventListener("activate", onActivate, false);
-      gotActivate = true;
-      maybeRunCallback();
-    }, false);
-  } else {
-    info("Was activated.");
-  }
-
-  win.addEventListener("load", function onLoad() {
-    info("Got load");
-    win.removeEventListener("load", onLoad, false);
-    gotLoad = true;
-    maybeRunCallback();
-  }, false);
+  whenDelayedStartupFinished(win, () => aCallback(win));
   return win;
 }
 
+/**
+ * This waits for the browser-delayed-startup-finished notification of a given
+ * window. It indicates that the windows has loaded completely and is ready to
+ * be used for testing.
+ */
+function whenDelayedStartupFinished(aWindow, aCallback) {
+  Services.obs.addObserver(function observer(aSubject, aTopic) {
+    if (aWindow == aSubject) {
+      Services.obs.removeObserver(observer, aTopic);
+      executeSoon(aCallback);
+    }
+  }, "browser-delayed-startup-finished", false);
+}
+
 /**
  * The test runner that controls the execution flow of our tests.
  */
diff --git a/browser/components/sessionstore/test/unit/test_backup.js b/browser/components/sessionstore/test/unit/test_backup.js
index 5efa4358c72..9e25b966cf9 100644
--- a/browser/components/sessionstore/test/unit/test_backup.js
+++ b/browser/components/sessionstore/test/unit/test_backup.js
@@ -19,7 +19,7 @@ function pathBackup(ext) {
 
 // Ensure that things proceed smoothly if there is no file to back up
 add_task(function test_nothing_to_backup() {
-  yield _SessionFile.createUpgradeBackupCopy("");
+  yield _SessionFile.createBackupCopy("");
 });
 
 // Create a file, back it up, remove it
@@ -29,14 +29,14 @@ add_task(function test_do_backup() {
   yield OS.File.writeAtomic(pathStore, content, {tmpPath: pathStore + ".tmp"});
 
   do_print("Ensuring that the backup is created");
-  yield _SessionFile.createUpgradeBackupCopy(ext);
+  yield _SessionFile.createBackupCopy(ext);
   do_check_true((yield OS.File.exists(pathBackup(ext))));
 
   let data = yield OS.File.read(pathBackup(ext));
   do_check_eq((new TextDecoder()).decode(data), content);
 
   do_print("Ensuring that we can remove the backup");
-  yield _SessionFile.removeUpgradeBackup(ext);
+  yield _SessionFile.removeBackupCopy(ext);
   do_check_false((yield OS.File.exists(pathBackup(ext))));
 });
 
diff --git a/browser/devtools/profiler/commands.js b/browser/devtools/profiler/commands.js
index cbbb9282ffe..e94414bb8c9 100644
--- a/browser/devtools/profiler/commands.js
+++ b/browser/devtools/profiler/commands.js
@@ -69,7 +69,7 @@ gcli.addCommand({
         throw gcli.lookup("profilerAlreadyStarted2");
 
       panel.toggleRecording();
-      return gcli.lookup("profilerStarted");
+      return gcli.lookup("profilerStarted2");
     }
 
     return gDevTools.showToolbox(context.environment.target, "jsprofiler")
diff --git a/browser/devtools/profiler/test/browser_profiler_cmd.js b/browser/devtools/profiler/test/browser_profiler_cmd.js
index 42e00ae3092..db0af1dc1bb 100644
--- a/browser/devtools/profiler/test/browser_profiler_cmd.js
+++ b/browser/devtools/profiler/test/browser_profiler_cmd.js
@@ -48,7 +48,7 @@ function testProfilerStart() {
     deferred.resolve();
   });
 
-  cmd("profiler start", gcli.lookup("profilerStarted"));
+  cmd("profiler start", gcli.lookup("profilerStarted2"));
   return deferred.promise;
 }
 
diff --git a/browser/devtools/shared/Parser.jsm b/browser/devtools/shared/Parser.jsm
index 42b32c07cb2..ef302a83d58 100644
--- a/browser/devtools/shared/Parser.jsm
+++ b/browser/devtools/shared/Parser.jsm
@@ -1474,7 +1474,9 @@ let SyntaxTreeVisitor = {
       aCallbacks.onArrayExpression(aNode);
     }
     for (let element of aNode.elements) {
-      if (element) {
+      // TODO: remove the typeof check when support for SpreadExpression is
+      // added (bug 890913).
+      if (element && typeof this[element.type] == "function") {
         this[element.type](element, aNode, aCallbacks);
       }
     }
diff --git a/browser/locales/en-US/chrome/browser/devtools/gclicommands.properties b/browser/locales/en-US/chrome/browser/devtools/gclicommands.properties
index 5b1a9ce47e0..ceda6a70e6b 100644
--- a/browser/locales/en-US/chrome/browser/devtools/gclicommands.properties
+++ b/browser/locales/en-US/chrome/browser/devtools/gclicommands.properties
@@ -1251,9 +1251,9 @@ profilerNotFound=Profile not found
 # start the profiler.
 profilerNotStarted3=Profiler has not been started yet. Use 'profile start' to start profiling
 
-# LOCALIZATION NOTE (profilerStarted) A very short string that indicates that
+# LOCALIZATION NOTE (profilerStarted2) A very short string that indicates that
 # we have started recording.
-profilerStarted=Recording...
+profilerStarted2=Recording…
 
 # LOCALIZATION NOTE (profilerNotReady) A message that is displayed whenever
 # an operation cannot be completed because the profiler has not been opened yet.
diff --git a/toolkit/devtools/server/actors/script.js b/toolkit/devtools/server/actors/script.js
index e2460515e9f..fa64ab587f0 100644
--- a/toolkit/devtools/server/actors/script.js
+++ b/toolkit/devtools/server/actors/script.js
@@ -2849,7 +2849,7 @@ function fetch(aURL, aOptions={ loadFromCache: true }) {
       try {
         NetUtil.asyncFetch(url, function onFetch(aStream, aStatus) {
           if (!Components.isSuccessCode(aStatus)) {
-            deferred.reject("Request failed: " + url);
+            deferred.reject(new Error("Request failed: " + url));
             return;
           }
 
@@ -2858,7 +2858,7 @@ function fetch(aURL, aOptions={ loadFromCache: true }) {
           aStream.close();
         });
       } catch (ex) {
-        deferred.reject("Request failed: " + url);
+        deferred.reject(new Error("Request failed: " + url));
       }
       break;
 
@@ -2876,7 +2876,7 @@ function fetch(aURL, aOptions={ loadFromCache: true }) {
       let streamListener = {
         onStartRequest: function(aRequest, aContext, aStatusCode) {
           if (!Components.isSuccessCode(aStatusCode)) {
-            deferred.reject("Request failed: " + url);
+            deferred.reject(new Error("Request failed: " + url));
           }
         },
         onDataAvailable: function(aRequest, aContext, aStream, aOffset, aCount) {
@@ -2884,7 +2884,7 @@ function fetch(aURL, aOptions={ loadFromCache: true }) {
         },
         onStopRequest: function(aRequest, aContext, aStatusCode) {
           if (!Components.isSuccessCode(aStatusCode)) {
-            deferred.reject("Request failed: " + url);
+            deferred.reject(new Error("Request failed: " + url));
             return;
           }
 
@@ -2934,6 +2934,7 @@ function convertToUnicode(aString, aCharset=null) {
  *        An optional prefix for the reported error message.
  */
 function reportError(aError, aPrefix="") {
+  dbg_assert(aError instanceof Error, "Must pass Error objects to reportError");
   let msg = aPrefix + aError.message + ":\n" + aError.stack;
   Cu.reportError(msg);
   dumpn(msg);
diff --git a/toolkit/devtools/server/main.js b/toolkit/devtools/server/main.js
index 9544bc858b9..66ec873da06 100644
--- a/toolkit/devtools/server/main.js
+++ b/toolkit/devtools/server/main.js
@@ -24,8 +24,7 @@ let wantLogging = Services.prefs.getBoolPref("devtools.debugger.log");
 Cu.import("resource://gre/modules/jsdebugger.jsm");
 addDebuggerToGlobal(this);
 
-let promise = Cu.import("resource://gre/modules/commonjs/sdk/core/promise.js").Promise;
-const { defer, resolve, reject, all } = promise;
+loadSubScript.call(this, "resource://gre/modules/commonjs/sdk/core/promise.js");
 
 Cu.import("resource://gre/modules/devtools/SourceMap.jsm");
 
@@ -198,11 +197,11 @@ var DebuggerServer = {
   get initialized() this._initialized,
 
   /**
-   * Performs cleanup tasks before shutting down the debugger server, if no
-   * connections are currently open. Such tasks include clearing any actor
-   * constructors added at runtime. This method should be called whenever a
-   * debugger server is no longer useful, to avoid memory leaks. After this
-   * method returns, the debugger server must be initialized again before use.
+   * Performs cleanup tasks before shutting down the debugger server. Such tasks
+   * include clearing any actor constructors added at runtime. This method
+   * should be called whenever a debugger server is no longer useful, to avoid
+   * memory leaks. After this method returns, the debugger server must be
+   * initialized again before use.
    */
   destroy: function DS_destroy() {
     if (!this._initialized) {