I came here to mention exactly this. I don't mind the certificate issue (as long as there is a post mortem and they learn something from it).
I was wondering how my addons came back automatically (without me having to upgrade to 66.0.4) and I found out about this studies thing, which I never consented to. I feel violated. And the problem is, what browser am I supposed to use from now on? Lynx? Sigh
ETA: I am (was) a proud Firefox user since it was called Firebird, and changing browsers never crossed my mind before (even if Chrome felt faster some times). At this exact moment, I have zero trust on Mozilla, just like I have zero trust on Google (Chrome). Extremely frustrated and disappointed.