Frankly I don't really even want an opt-in. If Mozilla wants to go build an AI browser, they can do that, but it should be a separate project; don't transition Firefox into being an AI browser. I don't want to use an "AI browser with AI features disabled", whether through an opt-in option or an opt-out option.
This is the grudging half-measure.
Many would have preferred the updates to come with a form asking for on or off. It didn't, so they complained, and this was the answer.