[RossM]

Re: ML - I think it might be possible to do some sort of generic popup-detection: a container that's initially invisible, containing a clickable that the mouse accelerates towards shortly after becoming visible. With opt-in, they could use those detections that aren't reported to identify good use-cases (though sending passive data like that might not be very Mozilla).

The tricky bit will be getting rid of the darkened full-screen overlay as well as the modal, but I guess you could track element visibility changes/removals around the same instant as the mouse click.