WhatsApp is not displayed in Firefox for Android

This is probably because the window.orientation value is not available in Firefox for Android. All phones should support it, computers not. Supporting all phones with user agents is difficult so there is probably no way to fix this.

Oh, okay. Interesting implementation with window.orientation. Firefox has support for window.screen.orientation which is on a standards track [1] – but this also works on desktop so it’s not useful here.

What about using the following code?

if(window.orientation || navigator.userAgent.match(/android/i)) {
    console.log('mobile!');
}

[1] https://www.w3.org/TR/screen-orientation/