I use this:

    $ cat $(which whatsapp)
    #!/bin/dash
    # A kiosk is full-screen by design. You need to go to about:config and toggle
    # full-screen-api.ignore-widgets to true to prevent our window manager from
    # creating an actual full-screen window.
    # Additionally, you need to set the name to "kiosk" which will set the app id
    # under sway to "kiosk" so that in sway you can disable full-screen on
    # startup.
    firefox --name kiosk -P whatsapp --kiosk https://web.whatsapp.com/