Firebase Analytics incorrect active user count

As looptheloop88 correctly answered uninstalling and reinstalling the app is considered new active user. If your app provides IDFA you will also log the IDFA identifier. Linking your app to BigQuery will export the raw data to BigQuery table. You can then count the number of distinct IDFA values. That will give you the number of distinct devices your app was installed. Please note that IDFA can be reset by the user so the distinct IDFA is not exactly the number of distinct devices but will be reasonable close.

Active users are the unique users who initiated sessions for a period of time. When you install, uninstall and install the app again, then that will be counted as two active users.

If you'd like to learn more about this Firebase dashboard metric, check out this Firebase help documentation.