I cannot add support for WebXR to Firefox if they do not support it. caniuse.com says FF doesn't support it: https://caniuse.com/?search=webxr
This is what Google AI results say about it (take it with a grain of salt):
Desktop Firefox does not natively support the WebXR Device API. While you can manually force the experimental dom.vr.webxr.enabled flag to true via about:config, to actually run immersive VR/AR experiences, you must either integrate a WebXR Polyfill or install the WebXR API Emulator add-on.