Firebase is Google Cloud platform developed by Google for creating mobile and web applications. It was originally an independent company founded in 2011. In 2014, Google acquired the platform.
It can be used as the backend for your project as it has a lot of great services like :
Cloud Firestore is a flexible, scalable database for mobile, web, and server development.
Real-time Database is a noSQL database and It's better used in smaller projects.
Hosting you can host your website like that www.example.web.com
Authentication provides ready Authentication backend for login and sign up, You can have a free database with millions of users.
Cloud Storage for storing images and files
There are a lot of more services for Firebase like Machine learning but we'll stick to what benefits a Frontend Developer.
So, let's go back to our question
As a Frontend developer always wants to learn about consuming APIs and read, write, update and delete data so you search for free APIs to train on but you don't have Full Control on the API so Firebase will let you code whatever your idea to make a Full-Stack Application without learning backend.
Firebase has free and paid plans (except Authentication is free )but you can use the free plans for learning and making projects for your portfolio. Here's a link for Firebase Pricing Plans.
Here The Net Ninja you can learn Firebase Stuff from Shaun the owner of the channel.