I think Facebook did a study that making options opt-in means only a tiny tiny percentage of users will ever activate them. People never look around in settings.

I suppose if - after you click away the popup that says "Thank you for loving Firefox"(1) - a popup shows that says "Hey, hey, look at me, look we have this new feature, it'll blow you away. Do you want to enable it?" would be obnoxious but satisfies the idea of "opt-in".

(1) https://bugzilla.mozilla.org/show_bug.cgi?id=1791524 - I still remember how icked I was seeing this popup.