We assume Mozilla is now just another ad-pandering jackwagon because that is the only safe assumption when presented with the facts: a major change in management, followed by removal of "we won't sell your data" across all of their marketing.
Their userbase is arguably mostly people who put up with browser disparity for the sake of peace of mind about their data. That type of user will abandon Firefox the same way we did Chrome.
That being said, I also don't think the browser "duopoly" is as entrenched as market share suggests. As Chromium is open source, the cost to the consumer of switching browsers is minimal; Chrome, Edge, Brave, Vivaldi, Opera and un-googled Chromium all offer very similar browsing experiences and the main difference is small UI things. If Google does something anti-consumer, people can jump ship very easily. Other corporations can also fork Chromium very easily (as some have already done).