gecko-dev/dom/u2f/tests/README.md

Note:

While conceptually similar to the tests for Web Authentication (dom/webauthn), the tests for U2F require an iframe while window.u2f remains hidden behind a preference, though WebAuthn does not. The reason is that the window object doesn't mutate upon a call by SpecialPowers.setPrefEnv() the way that the navigator objects do, rather you have to load a different page with a different window object for the preference change to be honored.