I recently learned how to develop a backend with Nodejs, Express, and MongoDB. For me, it's super simple and easy to learn if you have some free time (a guide that I found). You can then easily deploy the API to Heroku within a few lines of code if you want to use the backend on the web (more info).
This will be helpful if you want to have more control over your API since Firebase could be limited sometimes. Moreover, integrating firebase into your application might be something unpleasant (it took my team 2 weeks to get firebase going...). Of course, this approach might be more time-consuming due to its learning curve, but I believe it'd be a more helpful and future-proof experience.
In my opinion, if you don't want to customize your backend that much, firebase would be sufficient and more robust since it has some nice built-in features (including Google authentication and the real-time database)
