In our app the public directory specified in firebase.json contains several HTML files. The main is called "index.html", the others are "index-full.html" and "facebook-channel.html". After "firebase deploy" the "index-full.html" becomes the default index file and following URLs all load contents of the same file "index-full.html":
If "index-full.html" was renamed to "index-full.html-off", then the correct "index.html" is picked up as the default index file.