I'm making an android project that will use both Firebase (cloud) & Sqlite to store my data.
In this scenario, is it a good idea to create a contentprovider?
If your application won't communicate with another apps, you don't need any content provider in this case, because you are using Firebase and Sqlite also together. You can use Firebase for your online operations and you can use Sqlite to manage your offline operations. Firebase offline capabilities may be a solution for you also or you can make a custom solution, Firebase and Sqlite together. Please check documentation: https://firebase.google.com/docs/database/android/offline-capabilities
You need Content provider, when you want to share your data with other apps. If your scenario doesn't have any data that is being provided to or being shared with other apps, you don't need content provider.
Additionally you can share your Firebase database with other applications via Firebase Console settings. You can create one Firebase Realtime Database and share it with many other applications. You truly have many options, you have to decide what you need in your scenario and decide on a particular way from many alternatives.
If you need to share your data with another application, please check this documentation: https://developer.android.com/training/basics/intents/index.html