e.g. if I have hugosite.firebaseapp.com and cname/A records set up so it loads at domain.com, will google ever be confused and rank the firebase app site?
I try to avoid using “Google” as a stand-in for “search engine”, but in this case you should Google your question. It is in their help docs about rank and DNS records. This forum is about Hugo, but Hugo doesn’t affect DNS in any way you are describing.