Firebase emulator doesn't display ReactsJS components

I'm trying to host a ReactJS App on Firebase. The site is rendered correctly via normal ReactJS npm start but the ReactJS components don't show when started via the Firebase emulator with firebase emulators:start

ReactJS (localhost)

Firebase emulator (localhost)

I've tried changing public to build as seen in the link below, but then I get a blank screen with /cannot get: firebase deploy gives welcome screen

What do I do wrong?