Oracle Cloud (called OCI) has an always-free offering, you don't need credits. You can just sign up and use some small quantity of resources for nothing indefinitely. That includes a managed Oracle database:
https://docs.oracle.com/en/cloud/paas/autonomous-database/se...
There's also some sort of startup credits program with a brochure here, apparently you can just fill out a form and get some credits with an option to apply for more. But I don't know much about that. I've used the always-free programme for some personal stuff and it worked fine so I never needed to think about credits.
https://www.oracle.com/a/ocom/docs/free-cloud.pdf
I have to admit I'm not really familiar with Firebase, I thought that was some managed service for mobile apps, but Oracle DB comes with some stuff that sounds similar. And Oracle Cloud is an AWS-style cloud, it has a ton of high level services for things.
ORDS is a REST binding layer that lets you export tables, views, stored procedures and NoSQL JSON document stores over HTTP without writing a middleman server yourself. You can drive it directly from the browser. ORDS supports OAuth2 or can be integrated with custom auth schemes from what I understand. I've not used ORDS myself yet but probably will in the near future.
https://www.oracle.com/database/technologies/appdev/rest.htm...
Firebase IIRC when it first launched was known for push streaming of changes. Oracle DB lets you subscribe to the results of SQL queries and get push notifications when they change, either directly via driver callbacks or into a message queue for async processing later. It's pretty easy to hook such notifications up to web sockets or SSE or similar, in fact I've done that in my current project.
There's also a thing called APEX which is a bundled visual low-code app builder. I've never used it but I've used apps built with it, and it must be quite flexible as they all had a lot of features and looked very different. You can tell you're using an APEX app because they have a lot of colons in the URLs for some reason. Here's a random example of one from outside of Oracle that exports a database of dubious scientific research papers:
https://dbrech.irit.fr/pls/apex/f?p=9999:1::::::
I'm not holding it up as a great example, there are probably better examples out there, it's just a one that came to mind that's public and I used before.