When it comes to Firefox Nightly and development versions like that, I completely agree, telemetry makes perfect sense! It’s helpful to the developers to know when stuff goes wrong and under which conditions, and also which features are used the most and deserve the most focus.
But still, the two conditions from my post (which I clarified a bit better by editing my post) should apply, in my opinion. For example, there could be a popup dialogue when the program is run for the first time that says something like: “Thank you for using our development version. We would like to collect such and such data from you, do you consent? For more info, please visit this webpage that tells you exactly what data we collect.”
The only way you deserve the trust of your users is by being 100% transparent and honest. One violation of that is enough to destroy all trust.