1) search for firebase at the top of the forum page. what you need to do as regards b4a is pretty clear. (plus it works). what you need to do as regards setting up a firebase app with google is another matter. unclear how much you understand about that aspect. (not difficult, but you have to follow some steps). the abovementioned forum search does get into that, but you still have to go to google to set up your firebase app before you can do anything on the b4a end.
2) there is nothing stopping multi-user access to databases using b4a. and not necessarily related to firebase. the multi-user part is managed by your database. if you can access a webpage using b4a, you'll have no problem getting to the front door of a database that allows access via a webpage. are you running a database currently? have you tried building a little test with b4a to access a webpage? again, search the forum for http. that will lead you to httputil2. works perfectly fine without a whole lot of work. if you're already running a database that allows users to log in via http(s), you should see a familiar page when you try the same thing through b4a.