Neowin has a brief warning that Mozilla plans to collect anonymized user data. The given reason is to better understand how people use Firefox. Perhaps the most alarming aspect of this plan is that it is opt-out rather than opt-in. This is very far from the early days of Firefox when it had previously touted privacy as one of its main advantages.

As stated in the Google Groups announcement thread, they intend to use RAPPOR:

[Update @ 20170824_152224 UTC: fixed bad link to Google Groups thread.]