It's more that a lot of people really don't care one way or another, and will neither go out of their way to opt-in or opt-out.

Additionally, it's not that Mozilla just disregards user privacy here: differential privacy being used would mean that no user has to reveal their private information, but looking at all the data in aggregate would still allow Mozilla to gain useful information on how to make Firefox better.

So, let me see if I can follow your argument.

Because most people don't care, it was decided to implement a feature that is flat out contrary to people caring.

Management decisions like this don't exactly inspire confidence about the future of the browser.

The people that don't care one way or another are mostly using Chrome.