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.
Also note the weasel language of their statement: "We’re continuously working to understand the best ways to communicate with people who use Firefox. ...".
"Communicate" my *ss. It really makes my blood boil how the Mozilla management hijacked Firefox for their unethical bullshit (because it happens again and again, as soon as the dust has settled over the last 'accident').
They did this a couple years ago as well to similar backlash, that time with a plugin that they force-installed for users.
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
But Firefox has all kind of promo things (the latest I saw was adverts on their overview/links page - which you can also disable), so the presence of a config item for this doesn't mean they intended for it to show up where it did.
Wow, that would have been a whole lot worse than what actually happened!
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.