EventSourceTest.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using BestHTTP;
using BestHTTP.ServerSentEvents;
using System;

public class GetRequest : MonoBehaviour
{
    // Start is called before the first frame update
    IEnumerator Start()
    {
         string firebaseHost = "https://fir-testdatabase-<<REMOVED>>.firebaseio.com/foo.json";

        HTTPRequest request = new HTTPRequest(new Uri(firebaseHost), OnRequestFinished);
        request.Send();

            var eventSource = new EventSource(new Uri(firebaseHost));
            // Start to connect to the server
            eventSource.OnMessage += OnMessage;
            eventSource.Open();

         var unireq =  UnityEngine.Networking.UnityWebRequest.Get(firebaseHost);
         unireq.SetRequestHeader("Accept", "text/event-stream"); // Removing this clearly works on webgl, but not with streaming. 
         var response = unireq.SendWebRequest();

        int bytesSoFar = 0;

         while (!unireq.downloadHandler.isDone)
         {             
             if (unireq.downloadHandler.text.Length != bytesSoFar)
             {
                string allText = $"Bytes: {unireq.downloadHandler.text.Length} : " + unireq.downloadHandler.text;

                Debug.Log(allText); 

                bytesSoFar = unireq.downloadHandler.text.Length;
             }

             yield return null;
         }
         Debug.Log("IsDone()" + unireq.responseCode);

    }

    // The OnRequestFinished() function's implementation might be this:

    void OnRequestFinished(HTTPRequest request, HTTPResponse response)
    {
        Debug.Log("Request Finished! Text received: " + response.DataAsText);
    }      

    private void OnMessage(EventSource eventSource, Message message)
    {
        Debug.Log(string.Format("Message: <color=yellow>{0}</color>", message));
    }
}