Hi Filipe,
Ah! Now I understand your question.
From my experience, Firebase push notifications show when you have the app on foreground on iOS but the same is not true for Android (which I assume is the operative system of the device you are using with this behavior).
You're going to have the change to determine this behavior in a future version which is currently under development:

Either you can go ahead and download it or you can wait for this version to be marked as stable and then download it.
Kind Regards,
João