WITH runs in your browser, but you can install it and it'll open full-screen like a native app — no browser bar, no address bar.
Open Settings inside the app (tap the gear, top-right) and tap Install App. Confirm the prompt.
Tap the Share button (the box with the arrow) → "Add to Home Screen" on iPhone, or look for the install icon in Safari's address bar / Share → "Add to Dock" on Mac.
No install support on Firefox, on any device — that's a Firefox limitation, not a WITH one. Just keep the tab open, everything still works.
Once installed, open it like any other app on your device.