This is one of the dumbest things I’ve ever read.
Mozilla is a non-profit whose mission is to keep the internet freely available and privacy focused. See https://foundation.mozilla.org/en/who-we-are/
Firefox is built by open source developers who overwhelmingly have those same values. They have also been at this for many years now and have given us no real reason to doubt this commitment.
They sell those at a loss to bring in customers to buy other higher margin items. What else is Spotify selling?
Just use 7zip. It’s so much better.