When you first open a new Nightly install there's literally a banner across the entire browser telling you that it's collecting metrics, along with a button that displays more information and allows you to customize what data gets sent. The whole point of offering a Nightly build is so that they can have feedback on performance regressions and crashes. Characterizing it as "spying" is simply FUD.

And the event I'm linking to is a scenario where Mozilla updated nightly with no warning to share more data without telling me, against the policy stated about the setting in the browser. And a response by a Mozilla engineer saying that they thought this was perfectly acceptable behavior.