This add-on is under active development and it's not complete as a full kiosk. This add-on implements Kiosk functionality to be used by the public, to prevent access to the running browser. Steps to run:

Install the extension and verify you see the rounded-square icon on the browser and/or the context menu
You can open the preferences via the add-ons area for ModernKiosk, right-clicking on the context menu or right clicking on the icon in the tab bar
You can set the kiosk URL that you want in the Preferences, or
You can browse to the URL that you want as the default kiosk URL and set it in the context menu or right-clicking in the tab bar icon, right-click on the icon and select "Set as the kiosk URL"
To start the kiosk, right click on the tab icon or the context menu and select on "Enter kiosk mode" to start in the Kiosk URL in full screen

To exit, quit the browser. We will add more functionality later to further prevent access to the OS. To do:

Whitelist domain so user cannot browse to other domains
Prevent user access to the device's operating system
Close all other windows except the one in full screen
Set up a special keystroke ("exit key combination" or EKC) to exit the kiosk mode
Set up an optional password/pin such that, when set, the user must enter it after entering the EKC

Want any more features? --> https://github.com/racketlogger/ModernKiosk/issues