Explain Like I'm Five

Problem is, when you don't oppose stuff like that, stuff like that gets added more and more and it's all opt-out and some day you'll have an update and something's turned on by default and you don't realize that for a year or so and then you're like "shit, was this really on all the time". Even worse when they hide settings well in the UI, or use dark patterns to annoy or trick you to enable a setting that's actually bad for you.

Opt-out stuff is just bad, even in small doses. It's always kind of a scam. I wish Mozilla wouldn't need that kind of stuff. I mean they could be the knight-in-shining-privacy-armor browser, compared to Chrome/Edge/Opera/.... But they are all similar unfortunately (by default). Yes, Firefox is still less worse than Chrome/Edge/Opera are by default. But "less worse" doesn't equal "good". Yes, you can configure Firefox to behave well, and by using a good preconfigured user.js these settings also will stick after updates. But you shouldn't have to do that in the first place. The common user doesn't do that and shouldn't have to. The Firefox forks like LibreWolf or Mullvad Browser for example do not have anything bad enabled by default. And it's likely they won't ever have anything bad enabled after updates. So it is possible. The only reason the common browser makers aren't doing it is because that gives them (or their business partners) less data/money.