This actually seems to mean that you are opted in. The setting is extremely poorly named. "app.shield.optoutstudies.enabled" must mean "Are Studies, which are "opt-out", enabled"?
Here's a brand new profile on firefox stable, downloaded a few moments ago:
> An finally proof you are opted out by default on the public release of 57: https://www.dropbox.com/s/3zpwunknngy71pw/Screen%20Shot%2020....
This actually seems to mean that you are opted in. The setting is extremely poorly named. "app.shield.optoutstudies.enabled" must mean "Are Studies, which are "opt-out", enabled"?
Here's a brand new profile on firefox stable, downloaded a few moments ago:
https://i.imgur.com/kZiGAjG.png
Or see this privacy guide which recommends false for this setting: https://aaronhorler.com/articles/firefox-privacy.html