If you have disabled JavaScript using a browser add-on that allows you to customize your JavaScript settings, you need to re-enable JavaScript using this add-on. JavaScript is enabled for all websites by default in Mozilla Firefox.
This is because the Avast Store is unable to load and function correctly without these settings enabled. When you make a purchase via the Avast Store, you may be notified that you need to enable JavaScript and / or cookies in your web browser. Enabling JavaScript and cookies in your web browser