It’s used to check for website breaches. From How to stop Firefox from making automatic connections:

Firefox Monitor warns you if your online accounts were involved in a known data breach. For more information, see Firefox Password Manager - Alerts for breached websites.

To get the latest login breach information and more, Firefox connects to firefox.settings.services.mozilla.com

To disable, see here.

You can also run VirtualBox with KVM as a backend.

It’s the Schmidtstedter Knoten in Erfurt.

Also add acoup.blog##.darkmode-layer to your filters.