Sounds like it has nothing to do with microservices and more that they just blew everything up instead of fixing their firebase schema. But hey, sometimes that's way simpler.
Microservices didn't particularly cause technical issues. Just unnecessary operational overhead. While I'd probably go with a monolith if I was building for scratch, in this case we've stuck with ~5 microservices (we did have over 10) as it works well enough.
Firebase is fundamentally limited though. You simply cannot do things like joins and even filtering and updating is very limited compared to a SQL database. Of course you can implement these things in code, and precompute them and store them denormalised. But at that point you're implementing your own database and it's much less work to move the data to a different database platform. We did a gradual migration which made it relatively painless, although it was still a lot of work. And we're able to move much faster on new features now we have the flexibility of a fully-featured database.
