TL;DR: yes you can, but why would you want to do this?
You can just upload GeoJSON using JSON file using Firebase console and it technically works. The data can then be loaded using REST API, and you will get same GeoJSON as result of .json query. However, this is not very effective as you can query same dataset back, or refer specific object by index (e.g. get first, second object etc). Then there is question why you would use Firebase for this, better then use some Geo-specific data storage, e.g. CartoDB, Github etc which has map visualisations etc.
Firebase Database is meant for real-time data, so good use case would be e.g. tracking device GPS locations (points with timestamps, based on user ID-s). This means that you need specific data structure to enable queries by user, time or location, stored GeoJSON does not give such possibilities, even after you have loaded it to the Firebase. That's why GeoFire uses a bit different structure: adds GeoHash keys to each object, so you use this as quick spatial filter and you can do location-based queries with this. Also you can add timestamp as key and query based on this range and probably you want to use user id as higher level key. With GeoFire data structure you can even subscribe to real-time location-based notifications (area entry/leave).