--- a/browser/components/migration/content/migration.js
+++ b/browser/components/migration/content/migration.js
@@ -38,17 +38,17 @@ var MigrationWizard = { /* exported Migr
     this.isInitialMigration = entryPointId == MigrationUtils.MIGRATION_ENTRYPOINT_FIRSTRUN;
 
     if (args.length > 1) {
       this._source = args[1];
       this._migrator = args[2] instanceof kIMig ? args[2] : null;
       this._autoMigrate = args[3].QueryInterface(kIPStartup);
       this._skipImportSourcePage = args[4];
       if (this._migrator && args[5]) {
-        let sourceProfiles = this._migrator.sourceProfiles;
+        let sourceProfiles = this.spinResolve(this._migrator.getSourceProfiles());
         this._selectedProfile = sourceProfiles.find(profile => profile.id == args[5]);
       }
 
       if (this._autoMigrate) {
         // Show the "nothing" option in the automigrate case to provide an
         // easily identifiable way to avoid migration and create a new profile.
         document.getElementById("nothing").hidden = false;
       }
@@ -62,38 +62,56 @@ var MigrationWizard = { /* exported Migr
     os.removeObserver(this, "Migration:Started");
     os.removeObserver(this, "Migration:ItemBeforeMigrate");
     os.removeObserver(this, "Migration:ItemAfterMigrate");
     os.removeObserver(this, "Migration:ItemError");
     os.removeObserver(this, "Migration:Ended");
     MigrationUtils.finishMigration();
   },
 
+  spinResolve(promise) {
+    let canAdvance = this._wiz.canAdvance;
+    let canRewind = this._wiz.canRewind;
+    let canCancel = this._canCancel;
+    this._wiz.canAdvance = false;
+    this._wiz.canRewind = false;
+    this._canCancel = false;
+    let result = MigrationUtils.spinResolve(promise);
+    this._wiz.canAdvance = canAdvance;
+    this._wiz.canRewind = canRewind;
+    this._canCancel = canCancel;
+    return result;
+  },
+
+  onWizardCancel() {
+    return this._canCancel;
+  },
+
   // 1 - Import Source
   onImportSourcePageShow() {
     // Show warning message to close the selected browser when needed
-    function toggleCloseBrowserWarning() {
+    let toggleCloseBrowserWarning = () => {
       let visibility = "hidden";
       if (group.selectedItem.id != "nothing") {
-        let migrator = MigrationUtils.getMigrator(group.selectedItem.id);
+        let migrator = this.spinResolve(MigrationUtils.getMigrator(group.selectedItem.id));
         visibility = migrator.sourceLocked ? "visible" : "hidden";
       }
       document.getElementById("closeSourceBrowser").style.visibility = visibility;
-    }
+    };
     this._wiz.canRewind = false;
 
     var selectedMigrator = null;
     this._availableMigrators = [];
 
     // Figure out what source apps are are available to import from:
     var group = document.getElementById("importSourceGroup");
     for (var i = 0; i < group.childNodes.length; ++i) {
       var migratorKey = group.childNodes[i].id;
       if (migratorKey != "nothing") {
-        var migrator = MigrationUtils.getMigrator(migratorKey);
+        var migrator = this.spinResolve(MigrationUtils.getMigrator(migratorKey));
         if (migrator) {
           // Save this as the first selectable item, if we don't already have
           // one, or if it is the migrator that was passed to us.
           if (!selectedMigrator || this._source == migratorKey)
             selectedMigrator = group.childNodes[i];
           this._availableMigrators.push([migratorKey, migrator]);
         } else {
           // Hide this option
@@ -143,25 +161,25 @@ var MigrationWizard = { /* exported Migr
       Services.telemetry.getHistogramById("FX_MIGRATION_SOURCE_BROWSER")
                         .add(MigrationUtils.getSourceIdForTelemetry("nothing"));
       document.documentElement.cancel();
       return false;
     }
 
     if (!this._migrator || (newSource != this._source)) {
       // Create the migrator for the selected source.
-      this._migrator = MigrationUtils.getMigrator(newSource);
+      this._migrator = this.spinResolve(MigrationUtils.getMigrator(newSource));
 
       this._itemsFlags = kIMig.ALL;
       this._selectedProfile = null;
     }
     this._source = newSource;
 
     // check for more than one source profile
-    var sourceProfiles = this._migrator.sourceProfiles;
+    var sourceProfiles = this.spinResolve(this._migrator.getSourceProfiles());
     if (this._skipImportSourcePage) {
       this._wiz.currentPage.next = "homePageImport";
     } else if (sourceProfiles && sourceProfiles.length > 1) {
       this._wiz.currentPage.next = "selectProfile";
     } else {
       if (this._autoMigrate)
         this._wiz.currentPage.next = "homePageImport";
       else
@@ -184,54 +202,57 @@ var MigrationWizard = { /* exported Migr
 
     var profiles = document.getElementById("profiles");
     while (profiles.hasChildNodes())
       profiles.firstChild.remove();
 
     // Note that this block is still reached even if the user chose 'From File'
     // and we canceled the dialog.  When that happens, _migrator will be null.
     if (this._migrator) {
-      var sourceProfiles = this._migrator.sourceProfiles;
+      var sourceProfiles = this.spinResolve(this._migrator.getSourceProfiles());
 
       for (let profile of sourceProfiles) {
         var item = document.createElement("radio");
         item.id = profile.id;
         item.setAttribute("label", profile.name);
         profiles.appendChild(item);
       }
     }
 
     profiles.selectedItem = this._selectedProfile ? document.getElementById(this._selectedProfile.id) : profiles.firstChild;
   },
 
   onSelectProfilePageRewound() {
     var profiles = document.getElementById("profiles");
-    this._selectedProfile = this._migrator.sourceProfiles.find(
+    let sourceProfiles = this.spinResolve(this._migrator.getSourceProfiles());
+    this._selectedProfile = sourceProfiles.find(
       profile => profile.id == profiles.selectedItem.id
     ) || null;
   },
 
   onSelectProfilePageAdvanced() {
     var profiles = document.getElementById("profiles");
-    this._selectedProfile = this._migrator.sourceProfiles.find(
+    let sourceProfiles = this.spinResolve(this._migrator.getSourceProfiles());
+    this._selectedProfile = sourceProfiles.find(
       profile => profile.id == profiles.selectedItem.id
     ) || null;
 
     // If we're automigrating or just doing bookmarks don't show the item selection page
     if (this._autoMigrate)
       this._wiz.currentPage.next = "homePageImport";
   },
 
   // 3 - ImportItems
   onImportItemsPageShow() {
     var dataSources = document.getElementById("dataSources");
     while (dataSources.hasChildNodes())
       dataSources.firstChild.remove();
 
-    var items = this._migrator.getMigrateData(this._selectedProfile, this._autoMigrate);
+    var items = this.spinResolve(this._migrator.getMigrateData(this._selectedProfile,
+                                                               this._autoMigrate));
     for (var i = 0; i < 16; ++i) {
       var itemID = (items >> i) & 0x1 ? Math.pow(2, i) : 0;
       if (itemID > 0) {
         var checkbox = document.createElement("checkbox");
         checkbox.id = itemID;
         checkbox.setAttribute("label",
           MigrationUtils.getLocalizedString(itemID + "_" + this._source));
         dataSources.appendChild(checkbox);
@@ -299,19 +320,19 @@ var MigrationWizard = { /* exported Migr
 
     var singleStart = document.getElementById("homePageSingleStart");
     singleStart.setAttribute("label", mainStr);
     singleStart.setAttribute("value", "DEFAULT");
 
     var appName = MigrationUtils.getBrowserName(this._source);
 
     // semi-wallpaper for crash when multiple profiles exist, since we haven't initialized mSourceProfile in places
-    this._migrator.getMigrateData(this._selectedProfile, this._autoMigrate);
+    this.spinResolve(this._migrator.getMigrateData(this._selectedProfile, this._autoMigrate));
 
-    var oldHomePageURL = this._migrator.sourceHomePageURL;
+    var oldHomePageURL = this.spinResolve(this._migrator.getSourceHomePageURL());
 
     if (oldHomePageURL && appName) {
       var oldHomePageLabel =
         brandBundle.getFormattedString("homePageImport", [appName]);
       var oldHomePage = document.getElementById("oldHomePage");
       oldHomePage.setAttribute("label", oldHomePageLabel);
       oldHomePage.setAttribute("value", oldHomePageURL);
       oldHomePage.removeAttribute("hidden");
@@ -332,25 +353,28 @@ var MigrationWizard = { /* exported Migr
 
   // 5 - Migrating
   onMigratingPageShow() {
     this._wiz.getButton("cancel").disabled = true;
     this._wiz.canRewind = false;
     this._wiz.canAdvance = false;
 
     // When automigrating, show all of the data that can be received from this source.
-    if (this._autoMigrate)
-      this._itemsFlags = this._migrator.getMigrateData(this._selectedProfile, this._autoMigrate);
+    if (this._autoMigrate) {
+      this._itemsFlags =
+        this.spinResolve(this._migrator.getMigrateData(this._selectedProfile,
+                                                       this._autoMigrate));
+    }
 
     this._listItems("migratingItems");
     setTimeout(() => this.onMigratingMigrate(), 0);
   },
 
-  onMigratingMigrate() {
-    this._migrator.migrate(this._itemsFlags, this._autoMigrate, this._selectedProfile);
+  async onMigratingMigrate() {
+    await this._migrator.migrate(this._itemsFlags, this._autoMigrate, this._selectedProfile);
 
     Services.telemetry.getHistogramById("FX_MIGRATION_SOURCE_BROWSER")
                       .add(MigrationUtils.getSourceIdForTelemetry(this._source));
     if (!this._autoMigrate) {
       let hist = Services.telemetry.getKeyedHistogramById("FX_MIGRATION_USAGE");
       let exp = 0;
       let items = this._itemsFlags;
       while (items) {