Nice! I was working on something similar, except that it was a chrome extension that put a follow/unfollow button by the username, and required authentication via google credentials. But, since I didn't keep a limit on the number of people you could follow, as the number of people I was following increased, waiting for Firebase responses to return (for ordering comments by id) started taking too long and killed the user experience.
I wish there was a way to do a bulk query (ala Elasticsearch), so I could get all comment ids in one request. The response time would still increase with increasing number of users, but it would be marginal compared to the current situation.