Your question is too general, there are huge differences between firebase and sql. Do you need it for some project? Then one is likely more suitable than the other, but we can't tell without knowing what it's for. Or do you want to become a database specialist in general? At a research level?

My shot at an answer (given above caveat): you could start by getting better at SQL. Try to learn general SQL and skip the platform-specific extensions. It'll be a good choice for many backends (websites, apps, etc).

In my opinion if you want to start learning firebase then create some simple project like url shortening and if you want to learn mongoDB then create some RESTful api with CRUD operation . Learn the concepts of Schema , Model, auth for firebase , Migration etc . You can find some awesome tutorials for these thinks in the web and youtube