The built-in AI staff,you referred to, is nothing but an accelerator to integrate with 3rd-party or self-hosted LLMs. It's quite similar to choosing a search engine in settings.
This feature itself is lightweight and can be disabled in settings if not required.
The built-in AI staff [sic]... is... an accelerator to integrate with 3rd-party or self-hosted LLMs.
Users are only shown Big Tech "3rd-party" options. Mozilla made this choice intentionally.
Since Mozilla is clearly capable of developing an add-on that is not forcefully installed on user's devices, they should remove the built-in thing that endorses the highly unethical chatbots run by Google, OpenAI, etc.