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
I don't know exactly what anyone else experienced. What I experienced is: I was away from my computer and noticed that my Syncthing[0] folders were out of date, so I used KDE Connect[1] to make sure Syncthing was running on my PC, which opens a tab connecting to Syncthing on the local machine in the default browser - Firefox in my case. Some time later, I unlocked my PC and found the advertisement on top of my Syncthing admin page.
So yes, it popped something up while I was AFK, though I can't guess whether it would have done so if I hadn't remotely triggered a new tab. Needless to say, I was very surprised to see that behavior from Firefox, and even more surprised that despite posts to reddit and HN complaining, I didn't find a report in Bugzilla.