Thing is, there's a large (or at least certainly vocal) contingent of users (and mostly techies, to boot) that view "AI" as the Devil, and transformer models as the original sin, and they want to refuse to partake, wholesale.
This feature seems to be a nod to people with this worldview.
EDIT: See e.g. https://news.ycombinator.com/item?id=47133786 liking AI features to defecating on your food. It's not a technical objection, it's a principled one.
I figure, hey, at least Mozilla listened and provided the opt-out. It could be worse. I also happen to be in the "food defecating analogy" camp, and I can give the developer an unenthusiastic thumbs up for at least listening to the peanut gallery this time.
Ideally they wouldn't make the product bad, with a badness opt-out, in the first place, but everyone in Silicon Valley's got to feed the AI monkey. So I guess this is the best we can expect.
More like excuse to add more of the features. "See, we gave you kill switch"
I use AI for coding, I don't want it on my browser. Actually I don't want it on Firefox. Let Brave be the AI browser. They could have used AI themselves to actually make their pathetic new split tab view usable.
And that's in turn because product managers keep calling everything "AI" and shoving the bad kind in every feature they possibly can.