You're forgetting that people who want to use the features older versions had but not anymore are likely the same people who have been using Firefox for a long

You're forgetting that people who want to use the features older versions had but not anymore are likely the same people who have been using Firefox for a long time. These are the same people who drive Firefox adoption by being experienced long-time users giving advice to the young'uns. If you lose that core user base, you'll lose the rest of 80 million to Google/Microsoft ads in no time.