As a startup, managing my own database, backups and even the schemas/migrations are all overhead. Next to that, I needed both Backend and Frontend ways to write to the database. With firebase this is possible, this saved us some time: Some API calls were not needed because I could directly fetch data in the FE.
Offline support & realtime data updates is also supported out of the box. No need to write your own websockets.
Once the startup grows, moving to a different relational database might make sense. But in a pre-product-market-fit startup, Firebase is a good, and cheaper fit!
The pricing model of firebase firestore is a bit risky. But it saves a lot of time to get quickly to market.