https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
にある
We are working on a general fix that doesn’t use the Studies system and will keep this blog post updated accordingly. We will share a more substantial update in the coming days.
は、66.0.4をリリースしたからこの話はおしまいって理解でよろしくて？