Since Google Firebase is still a relatively new platform, some features are still in development. One of the most important is analytics.
Firebase does have analytics on how many messages were sent but currently this information can be difficult to read and may be mixed in with data from your native app.
However, we would currently recommend tracking usage from within your Google Analytics account. From here, you can go to Acquisition -> Campaigns -> All Campaigns.
The source will be "BLOX Broadcast" and the medium will be "push notification."