Check out this question here: Firebase Storage Video Streaming
To achieve exactly what you're looking for you would have to upload your video in many different chunks and download them one by one.
And according to this answer on that post you can start streaming before the video is fully loaded, but it will still attempt to download the entire file.
While it is possible to achieve what you're trying to do with Firebase, this isn't really what cloud storage is meant for. If you want to achieve YouTube style loading as you go you should look into a dedicated video streaming service such as ziggeo
Another option, depending on your needs, would be to upload your videos to YouTube and stream them directly from your app.