Firefox trick: use the hidden window to access DOM APIs without a window reference

foo.js

// You may sometimes be doing something in non-UI add-on code,
// but want access to nice DOM APIs, like navigator.sendBeacon().
// Use the hidden window, which is accessible via the AppShell service:

// If you're in an SDK addon, do this to get a Cu reference:
// const { Cu } = require('chrome');
Cu.import('resource://gre/modules/Services.jsm');

// this is your window:
const win = Services.appShell.hiddenDOMWindow;

// you can access all the APIs directly:
Services.appShell.hiddenDOMWindow.navigator.sendBeacon(url, data);