Chrome also switches to fullscreen, but it shows their fullscreen warning at the bottom (very easy to miss) after the OS Auth UI goes away. This is an OS prompt, not a Firefox one, but we do know that the promise is pending if that's useful. Or at least GeckoView does! It may not be available to the Fenix front end.
Bug 1865413 Comment 4 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Chrome also switches to fullscreen, but it shows their fullscreen warning at the bottom (very easy to miss) after the OS Auth UI goes away. The WebAuthn prompt is an OS prompt, not a Firefox one, but we do know that the promise is pending if that's useful. Or at least GeckoView does! It may not be available to the Fenix front end.