Mozilla have used various telemetry tools over time to assist with product development. It's possible they're trying to covertly track users but unlikely. It should certainly be known and public but does not necessarily equate to spying.
Intent makes absolutely no difference. If an application is sending any information, anywhere that is not controlled by the user, and only the user, even if the user can clearly see all of it, it is, by definition, not private. No matter what Mozilla says, anonymity cannot be guaranteed. If they want feedback, they should ask for permission after a crash is detected locally. It should offer the options "No"; "No, never ask again"; and "Yes, ask again for future errors". It should explicitly not offer "Yes, always send" and "Always send" sure as hell should not be the default in a "private browser". The only way you can truly trust someone is when you don't have to.
