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').
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.