I think some of the answer is expressed in the article:

To service and retain this loyal userbase then, you might imagine that Mozilla would address their needs and concerns with what made Phoenix a great first version back in 2002. A lightweight and versatile standards-compliant and open-source web browser with acceptable privacy standards, and without any other non-browser features attached to it. Just a browser, only a browser, and above all, a fast browser.

Instead, Mozilla appear to be following a course calculated to alarm rather than retain these users. Making themselves an AI-focused organisation, neglecting their once-unbeatable developer network, and trying to sneak data gathering into their products.