I have the pug experience study active and I don't recall the browser asking about it.
From the studies about page linked from about:studies...
"When a study is available, you will automatically be enrolled if you meet the criteria. There will be occasions where we might prompt you for participation first."
Just saw also that if you opt-in for the "Allow Firefox Developer Edition to send technical and interaction data to Mozilla" then it automatically checks the studies checkbox for you. I would wonder if I checked the allow sharing at some point in the past, or during installation, with no mention of the studies option. So it was presumed to opt me into the studies automatically.