Google Sign-in Popup Blocker
zkbro explains in a post how to configure Librewolf.
One of the mentioned add-ons catched my eye: Google Sign-in Popup Blocker
This should block the annoying sign-in with Google dialogs that popup on some websites.
As this sounds promising, I installed the add-on in my Firefox.
I briefly tested it on a site that uses the annoying sign-in with Google dialog, and this time the dialog did not appear.
To be sure I confirmed it in the developer tools where it mentioned that loading the dialog was blocked by the add-on 🎉