This is how the feature works: for example, you go into the Mozilla website then you want to download it as an application into your local computer. When downloaded, the application will automatically go into a folder that Firefox will create which is called Firefox Apps or Mozilla Firefox Apps; the directory where the app is gonna be installed can also be picked by the user.
How to delete an app: If the user wants to delete an application, there will be 3 dots or the "menu" at the top of the window and there, there will be an option called Uninstall <AppName> which is, by the name itself, will uninstall the application. When clicked, there will be a popup which is a checkbox if the user wants to clear the data of the app.
Additional features (each additional feature will be found on the menu):
This feature is also available on Google Chrome and Microsoft Edge.