https://www.bleepingcomputer.com/news/software/firefox-to-use-windows-bits-service-for-background-updates/

In the near future Mozilla Firefox will utilize Windows Background Intelligent Transfer Service, or BITS, to download software updates in the background. This is the first step in the eventual release of a standalone "Update Agent" that will perform updates even when the browser is closed.

Currently Mozilla Firefox will check for new updates when you open the browser and either display a notification that an update is available or automatically install it.