JavaScript to add "Fullscreen" context menu item.

fullscreen.js

function addFullScreenMenu () {
    var menu = document.createElement('menu');
    var item = document.createElement('menuitem');
    menu.setAttribute('id', 'fsmenu');
    menu.setAttribute('type', 'context');
    item.setAttribute('label', 'Fullscreen');
    item.addEventListener('click', function (e) {
        if (window.fullScreen) {
            document.body.mozCancelFullScreen();
        } else {
            document.body.mozRequestFullScreen();
        }
    });
    menu.appendChild(item);
    document.body.appendChild(menu);
    document.body.setAttribute('contextmenu', 'fsmenu');    
}