(No affiliation, just what I've picked up.)
HN doesn't depend on Firebase, the HN API does but HN isn't built on top of it (and presumably the API is down). HN is a monolith which uses a disk as it's database, it goes down sometimes when eg the disk fails, and it can't scale so under heavy load it starts failing requests, but it's about as robust to cloud outages as it's possible to be.
