Shame on you for making studies like this opt-out. Look, I get that making it opt-in would reduce your sample size but this kind of thing isn't acceptable for a browser that's supposed to respect the user -- you're literally using dark patterns. Expressed consent should be the standard.
You're targeting Nightly users, the very people who know enough to make an informed decision as to whether they want to participate in the study, please just let us make an informed decision. Have a big modal pop-over when the browser starts, explain the experiment and give the user an unbiased choice -- don't select participate by default, don't make the decline button gray and sad, don't shame the user with the decline button text. I would have been excited to participate.