I am thinking of making a game, but it involves staying in fullscreen mode always. You can only leave full screen mode on entering a password. Pressing Esc or toggling the fullscreen key shouldn’t exit full screen mode.
I can’t divulge more details, but it involves leaving a person to interact with content of the page until another person who knows the password stops the interaction.
EDIT: I know it is not user friendly, but it is for a special purpose. It is there to leave kids to interact with, and be left alone with. If they escape out of the browser they would be able to tamper with other things on the computer which i dont want them too.
Since you don’t need any more answers like “this is not user friendly” i would recommend you to have a look at some add-ons for firefox or chrome. You should have a look at some “Kiosk-Mode” add-ons.
Kiosk-mode is made for use cases where you have a pc in a public place and want to provide internet access. Maybe you can use one of those add-ons and configure it to get what you want.
If not, you should think about writing your own plugin for firefox or chrome in this add-on context you should have control over such things.