Reusable widgets/components


Different Firefox surfaces make use of similar UI elements such as cards, menus, toggles, and message bars. A group of designers and developers have started working together to create standardized versions of these elements in the form of new web components. The intention is for these components to encapsulate our design system, ensure accessibility and usability across the application, and reduce the maintenance burden associated with supporting multiple different implementations of the same UI patterns.

Many of these components are being built using the Lit library to take advantage of its templating syntax and re-rendering logic. All new components are being documented in Storybook in an effort to create a catalog that engineers and designers can use to see which components can be easily lifted off the shelf for use throughout Firefox.

Adding new design system components

We have a ./mach addwidget scaffold command to make it easier to create new reusable components and hook them up to Storybook. Currently this command can only be used to add a new Lit based web component to toolkit/content/widgets. In the future we may expand it to support options for creating components without using Lit and for adding components to different directories.

To create a new component, you run:

# Component names should be in kebab-case and contain at least 1 -.
./mach addwidget component-name

The scaffold command will generate the following files:

└── toolkit
    └── content
        ├── tests
        │   └── widgets
        │       └── test_component_name.html # chrome test
        └── widgets
            └── component-name # new folder for component code
                ├── component-name.css # component specific CSS
                ├── component-name.mjs # Lit based component
                └── component-name.stories.mjs # component stories

It will also make modifications to toolkit/content/ to add chrome:// URLs for the new files, and to toolkit/content/tests/widgets/chrome.ini to enable running the newly added test.

After running the scaffold command you can start Storybook and you will see placeholder content that has been generated for your component. You can then start altering the generated files and see your changes reflected in Storybook.

Unfortunately for now the browser_all_files_referenced.js test will fail unless your new component is immediately used somewhere outside of Storybook. We have plans to fix this issue, but for now you can get around it by updating this array to include your new chrome filepath.

Using new components

Once you’ve added a new component to toolkit/content/widgets and created chrome:// URLs via toolkit/content/ you should be able to start using it throughout Firefox. You can import the component into html/xhtml files via a script tag with type="module":

<script type="module" src="chrome://global/content/elements/your-component-name.mjs"/>

Common pitfalls

If you’re trying to use a reusable component but nothing is appearing on the page it may be due to one of the following issues:

  • Omitting the type="module" in your script tag.

  • Not specifying the html: namespace when using a custom HTML element in an xhtml file. For example the tag should look something like this:

  • Adding a script tag to an inc.xhtml file. For example when using a new component in the privacy section of about:preferences the script tag needs to be added to preferences.xhtml rather than to

  • Trying to extend a built-in HTML element in Lit. Because Webkit never implemented support for customized built-ins Lit doesn’t support it either. That means if you want to do something like:

    customElements.define("cool-button", CoolButton, { extends: "button" });

    you will need to make a vanilla custom element, you cannot use Lit. For an example of how this works in practice, see moz-support-link.