The Studies mechanism is clear, public and fully transparent. You can see what they've done. You can see what they plan to do. It's all out in the open.
At the end of the day the question is whether you can trust Mozilla. I trust them more than most entities, including many that push changes through apt-get.
Yes, Studies, that mechanism so open that end users didn't know about it until they found they were participating in some random Augmented Reality marketing collaboration with a TV show.
I would love to see a link to the code related to various Studies. So far I've not been able to find any. All you seem to get told is the name of any studies you're in, and only if you go over to about:studies and go looking.
This is not to say I don't trust Mozilla. I do. I trust them far more than I do Google / Chrome. It's hard not to see Studies as a privacy nightmare, though, and the level of power it has is disturbing.
At the end of the day the question is whether you can trust Mozilla. I trust them more than most entities, including many that push changes through apt-get.