For me, it started back when "sponsored tiles" were first announced in 2014. On the surface it was obviously advertisements, but many defenders tried to argue that it was a "good thing"
Then there was the proprietary Pocket extension baked into the browser with no easy removal. Again, many defenders tried to argue it was a "good thing"
Then the "studies" channel was used to push a Mr Robot ad. It's unclear how it was aligned with the values, but defenders tried to argue it was a "good thing".
They partnered with Cliqz to collect data and make recommendations. Again, defenders tried to argue it was a "good thing"
They partnered with Booking.com to push advertisements, going so far as to argue that they didn't receive any monetary compensation and that it was just a "social experiment". Again, defenders tried to argue it was a "good thing"
This is just a sampling of the events in the last 8 years (sponsored tiles was 2014). Every single time, they may have received some sort of benefit, but a number of users who bought into firefox for the security and privacy aspects ... felt betrayed and left. Because if it isn't about the privacy, what is the USP of firefox? "Not google" is only a small part of the user base.