If you're looking to build something that runs locally like a desktop app, I'd second @ugnvs's (and Reddit suggestion) to make a lightweight Python script which uses either WebKitGTK or QWebEngine (based on Chromium). There's also Electron (Chromium). That way, you have full control over the window, and everyone using it will have a consistent experience with your web app/game.
Fun fact! Ubuntu MATE Welcome was a WebKitGTK program.
I gave up a long time ago with Firefox having site-specific windows (especially for PWAs). It was once possible, but they removed the feature. The only embedded Gecko I can recall is Wine's Internet Explorer! 