For more information about JavaScript settings in Mozilla Firefox, refer to the following article from Mozilla Support: 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.
To enable JavaScript and / or cookies, refer to the information in the relevant section below according to your web browser: 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