https://medium.com/firefox-context-graph/make-firefox-better-with-pioneer-10c82d0f9301
Of course, Pioneers will be able to opt out of individual studies and Mozilla will be up front about what information is being collected at any given time so that Pioneers are able to make an informed choice. Weve also put in place a number of other limits to make sure our Pioneers are protected:
Data about individuals will not be shared publicly or sold. Raw data will only be available to a small number of researchers, and those researchers will only have access to studies relevant to their work. Aggregate information will only be disclosed in ways that minimize risk to user privacy.
Proposed studies are reviewed by a team dedicated to ensuring user privacy and will not proceed without their approval.
Users are given a random ID unique to Pioneer. It will not be correlated with any other user information and will be the only identifier that links your data on our servers.
Pioneer will not attempt to collect cookies, account names, passwords, email addresses, or POST form values. No probe will look at what happens in Private Browsing windows.
eta: also this page
https://addons.mozilla.org/en-US/firefox/addon/firefox-pioneer/
About this Add-on
Pioneers are willing to contribute sensitive data to help make Firefox better. Once you have installed the add-on, you may be included in studies about how you use the browser. Some of these studies will be passive and simply listen. Others may alter the behavior of your browser a little bit to see if it improves how you browse the web. Others may occasionally ask a question to help us understand how youre interacting with a page.
Participation is voluntary, and you can remove yourself by disabling the add-on after you install it.