That Firefox would fully intend to insert full page unskippable adverts of it's own into unrelated websites is a major accusation and there is evidence this was an accident.
Looks more to me like bleepingcomputer purposefully sensationalized the issue as clickbait.
https://news.ycombinator.com/item?id=36077360
seem to indicate that Mozilla intended for the popup to be shown if the user is AFK for 20 minutes but that timer malfunctioned