Actually I think I'm just old. To me "Firebase" still means the "Firebase Realtime Database" but after they got acquired by Google, they took the Firebase brand and started building all kinds of features that would let non-backend engineers ship apps without needing a backend, including an improved version of the realtime database (Firestore).

Somehow I've always read "Firebase alternative" as "alternative to Firebase Realtime Database" but now I see that what Supabase & co mean is "stuff you can put on a server once and then you won't need to do any backend stuff anymore", ie the current positioning of the Firebase brand.

I stand corrected.

(sidenote, Google's Firebase acquisition has to be one of the best-executed devtool acquisitions right? Nobody would've been surprised if they had just gobbled Firebase up, spread the team across GCP, put the product on life support and after a year told customers to just move to GCP proper, somehow. Instead they realized that "GCP Easy Mode" was worth having a separate brand for and they executed excellently on that)