There isn't an automatic way of doing this as far as I know, but it shouldn't be too difficult to implement yourself.
- Create a list of UUIDs of users who have installed your app
- Add a UUID for each user upon install in your Firebase database for whatever app you're concerned with tracking the users of.
- In that same block of code that you add the UUID of the current user, update another node in your Firebase database, perhaps called numUsers, with the size of the list of users.
Note that this will really show you number of installs on unique devices rather than users, unless you come up with a way to use the user's Google account or something similar and can guarantee that they don't have different accounts on different devices, etc, but it will get you a very good idea of how many total installs you have accumulated over time.
If you want to decrement the count when a user uninstalls your app, there is a very, VERY hacky way of doing this on Android where you listen to a certain file being removed, but I'm honestly not sure if that still works. However, hypothetically if you get that working, you would maybe be able to reach Firebase in time to decrement the count, I'm really not sure. You'd have to test this yourself. This is a different method of listening for uninstalls that has been accepted as an answer, but I have not tested it myself.