Firefox has been literally funded by Google's Ads, that Search deal having kept Mozilla alive. With all due respect, Firefox not being connected to AdTech is a hallucination.
Actually, all 3 major browser engines are directly funded by Google's Ads. And while you may have noticed that Mozilla and Apple have been singing the privacy tune, you should've also noticed that they never did anything to upset their cash cow.
Mozilla diversifying their revenue would be an improvement IMO. But whatever they did in the past, people got mad, because many imagine that such a complex piece of software could be developed for free or from the donations of individuals that ad-block YouTube instead of paying for Premium.