I haven't migrated off of Firebase but this is something I've thought a lot about as I've selected Firebase for many of my client projects.
As far as I can tell there is no open-source alternative with feature parity that makes it easy to migrate away from Firebase. I'd be curious to hear about how people have done this.
This is actually what I'm afraid of - specifically vendor lock-in and being stuck with a stack that has high operating cost. From some of the comments I've seen, Firebase has high operating costs. But I'm still planning on testing this out myself with a small project soon.
As far as I can tell there is no open-source alternative with feature parity that makes it easy to migrate away from Firebase. I'd be curious to hear about how people have done this.