Windows 10's Action Centre is a great way of having all your notifications in one place on your computer, but unfortunately, not everything makes use of it. This means that you'll often find yourself actioning some notifications from the Action Centre, while still relying on other notification methods in other apps and software - such as e-mails, in-software notifications, and more.
While Windows 10 has been around for some time now, developers are taking their time to implement Windows 10's Action Centre. Google's Chrome recently added support for the Action Centre. Now, Firefox is working on implementing support, with the feature set to arrive in Firefox 64, as confirmed by a comment on Mozilla's Bugzilla (via Windows Latest).
As a result of this feature enhancement, Firefox will stop delivering notifications from within the browser itself, instead feeding all notifications to the Action Centre, providing a consistent notification experience.