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).