What is the further plan to make it more privacy protecting? In whatever way it may end up in an add-on or in Firefox itself in the future, you likely (hopefully) don’t wwant to send the browser history to the two parties mentioned. At least not everything.
So may you integrate some way of locally filtering the browser history, like the current about:newtab page of Firefox does?
The results from this experiment will help us understand if it’s worth further investing into development of these experiences and evaluate privacy-aware solutions.
Remember this is an opt-in experiment that is helping understand how users value these experiences. The problems inherent in the data collection required to make recommendation engines valuable is clear, the goal is to do better for users and the web.