The start-up guides, tutorials and documentation in general for Firebase are pretty outstanding.
There is 1GB database storage for the free tier as compared to Supabase's 500MB. Not that I think there is anything wrong with Supabase, I intend to try it out someday.
Also if you are doing any sort of personal front-end project, even using a free cluster from MongoDB can be a lot of work and setup, with Firebase (specifically Fire store and Google Authenticator) the implementation of BaaS is quite easy to get up and running.
It's pretty easy to understand the Fires store security rules as well, and if you ever have a hard time trying to figure something out, there is good community support and YouTube tutorials for most topics.