01 / The real difference
One asks you to decide the shape now. The other does not.
Postgres is relational: tables, columns, relationships, rules enforced by the database. Firebase is document-based: you write objects and work out the structure as you go.
That difference decides almost everything else. Relational means more thinking up front and fewer surprises later. Document means you move immediately and meet the surprises further down the road.
02 / Take Firebase if
Shipping this month is the priority
You need auth, storage and sync on day one. Sign-in with the major providers, files, and realtime updates without building any of it.
It is a mobile app. The client libraries are genuinely excellent, and offline sync (changes made without signal reconciling later) is most of what a mobile app needs and is unpleasant to write yourself.
You do not have an ops person. Nothing to patch, back up or scale.
The shape is genuinely unknown. When the product will change weekly, a schema you rewrite weekly is a tax.
03 / Take Postgres if
You will be asking questions of the data
Reporting matters. “Revenue by region by month, excluding refunds” is one query in SQL and an afternoon of application code in a document store. This is the single most common regret.
Things relate to other things. Orders to customers to products to invoices. Relational databases exist for exactly this.
Correctness matters more than speed of change. Constraints the application cannot forget, enforced where the data lives.
You want predictable cost. Postgres is priced by the machine. Firebase is priced by reads, writes and storage, which is cheap while small and can surprise you when a screen refreshes more than you expected.
04 / The middle
Supabase, if the split is what is bothering you
If you want Firebase’s conveniences without giving up relational data, Supabase is Postgres with auth, storage, realtime and row-level security on top. You get the head start and a database you could take elsewhere.
It is not a free lunch: it is one stack per project, which matters if you are running several small products, but for a single product it removes most of the trade.
05 / What moving costs
Be honest about the exit before you enter
Leaving Postgres is a database migration: unpleasant, well understood, done every day. Leaving Firebase means rebuilding the data layer and often the auth, because the structure of your data was shaped by the tool.
That is not an argument against it. It is an argument for choosing it deliberately rather than by default.
06 / Honestly
Pick by the question you will ask in year two
Not by what is fastest this month. If in eighteen months someone will ask for a report that joins four things together, choose relational now and save yourself the rebuild. If the honest answer is that you do not know whether the product will exist in eighteen months, choose speed and accept the bill if it works.
Tell us what the product has to do and we will tell you which one we would choose, and what it will cost you to change your mind later.