Migration Wizard Reference

The migration wizard is the piece of UI that allows users to migrate from other browsers to Firefox. Firefox has had a legacy migration wizard for many years, and this has historically been a top-level XUL dialog window. This documentation is not for the legacy migration wizard, but is instead for an in-progress replacement migration wizard that can be embedded in the following contexts:

  1. In a top level stand-alone dialog window

  2. In a tab dialog modal

  3. Within privileged about: pages, like about:welcome, and about:preferences

To accommodate these contexts, the new migration wizard was developed as a reusable component using pure HTML, with an architecture that decouples the control of the wizard from how the wizard is presented to the user. This architecture not only helps to ensure that the wizard can function similarly in these different contexts, but also makes the component viewable in tools like Storybook for easier development.

High-level Overview

The following diagram tries to illustrate how the pieces of the migration wizard fit together:


MigrationWizard reusable component

The MigrationWizard reusable component (<migration-wizard>) is a custom element that can be imported from migration-wizard.mjs. The module is expected to load in a DOM window context, whereupon the custom element is automatically registered for that document.

After binding to the document, the MigrationWizard dispatches a MigrationWizard:Init custom event, which causes a MigrationWizardChild to instantiate and be associated with it. After receiving the migrator state from the MigrationWizardParent, the MigrationWizardChild will dispatch a MigrationWizard:Ready event on the MigrationWizard, mainly to aid in testing.

Notably, the MigrationWizard does not contain any internal logic or privileged code to perform any migrations or to directly interact with the migration mechanisms. Its sole function is to accept input from the user and emit that input as events. The associated MigrationWizardChild will listen for those events, and take care of calling into the MigrationWizard to update the state of the reusable component. This means that the reusable component can be embedded in unprivileged contexts and have its states presented in a tool like Storybook.

If the MigrationWizard is embedded in a dialog, it should have the dialog-mode attribute set on it so that dialog-appropriate buttons and styles are applied.


The MigrationWizardConstants module exports a single object of the same name. The properties of that object are constants that can be used to set the state of a MigrationWizard instance using MigrationWizard.setState.


The MigrationWizardChild is a JSWindowActorChild (see JSActors) that is responsible for listening for events from a MigrationWizard, and then either updating the state of that MigrationWizard immediately, or to message its paired MigrationWizardParent to perform tasks with MigrationUtils.


While a MigrationWizardChild can run in a content process (for out-of-process pages like about:welcome), it can also run in parent-process contexts - for example, within a tab dialog or standalone window dialog. The same flow of events and messaging applies in all contexts.

The MigrationWizardChild also waives Xrays so that it can directly call the setState method to update the appearance of the MigrationWizard. See XrayVision for much more information on Xrays.

class MigrationWizardChild()

This class is responsible for updating the state of a <migration-wizard> component, and for listening for events from that component to perform various migration functions.


General event handler function for events dispatched from the <migration-wizard> component.

  • event (Event) – The DOM event being handled.




Calls the setState method on the <migration-wizard> component. The state is cloned into the execution scope of this.#wizardEl.

  • state (object) – The state object that a <migration-wizard> component expects. See the documentation for the element’s setState method for more details.


The MigrationWizardParent is a JSWindowActorParent (see JSActors) that is responsible for listening for messages from the paired MigrationWizardChild to perform operations with MigrationUtils. Effectively, all of the heavy lifting of actually performing the migrations will be kicked off by the MigrationWizardParent by calling into MigrationUtils. State updates for things like migration progress will be sent back down to the MigrationWizardChild to then be reflected in the appearance of the MigrationWizard.

Since the MigrationWizard might be embedded in unprivileged documents, additional checks are placed in the message handler for MigrationWizardParent to ensure that the document is either running in the parent process or the privileged about content process. The JSActors registration for MigrationWizardParent and MigrationWizardChild also ensures that the actors only load for built-in documents.

class MigrationWizardParent()

This class is responsible for communicating with MigrationUtils to do the actual heavy-lifting of any kinds of migration work, based on messages from the associated MigrationWizardChild.


General message handler function for messages received from the associated MigrationWizardChild JSWindowActor.

  • message (ReceiveMessageArgument) – The message received from the MigrationWizardChild.




This document is meant for being loaded in a tab modal or window modal dialog, and embeds the MigrationWizard reusable component, setting dialog-mode on it. It listens for dialog-specific events from the MigrationWizard, such as MigrationWizard:Close, which indicates that a “Cancel” button that should close the dialog was clicked.

Pages like about:preferences or about:welcome can embed the MigrationWizard component directly, rather than use migration-dialog.html.