The user has the choice to not use these features. It's not like Firefox was sending data to AI companies by default. But if you want to completely make them disappear, so you can live in your fantasy world where LLMs were never invented, then yes, that's a niche personal preference and an advanced customization. That's why it goes under about:config.
PS: I do actually find Google's ai thing in the search useful now and again, so no fantasy world.
The master AI switch doesn't actually change whether the browser uses AI features - it never does unless you specifically run them. What it does is hides them from the user, pretending they don't exist.
Browsers that don't respect their users' choices about using AI do things like automatically download large models in the background, integrate cloud-based speech recognition and synthesis as an API available to any website and make the default search engine which they also own show LLM slop above actual results.