I know I am holding it wrong etc! But I really tried in earnest, as a fanboy of firebase, for quite a long time. The problems I had were with basic things. You have companies, a company can have many users, users might belong to more than one company (hello Slack...) and then there can be relationship between users.
Putting aside the problem between chair & keyboard.
Another difference is more if you make a mistake in your relational schema, you can SQL your way out of it - add an extra join or group by. And you can also fairly easily migrate you way out of it to a new schema that is the right structure.
This requires actual code with firebase, and a lot of patience, and probably a lot more downtime. So you need more of a waterfall approach, I would suggest, to design a schema ahead of time, and know all of your requirements. NoSQL document-oriented schemas just aren't flexible (unless the DB supports something like materialized views to help you get out of it)