There's an opt-out, but not an opt-in for Google Analytics. Unless you're referring to simply blocking it via a content blocker script.
The users above are referring to opting in/out of the tracking rather than the blocking of the tracking - so GA on Firefox by that standard is opt-in, even though in a Firefox specific context the setting is opt-out.
Obviously the EU wouldn't be cool with "but a competing browser with a small market share blocks us by default anyway".
What I meant to say: third-party tracking is still opt-out, since you need to actively enable the content blocking to avoid being tracked. The blocking of said content trackers is opt-in of course, but that's not what I was referring to.
It's not really an importance distinction in the end though.