It is opt in, this article is click bait.
Directly from the horses mouth:
"In Firefox, you’ll never be locked into one ecosystem or have AI forced into your browsing experience. You decide when, how or whether to use it at all. You’ve already seen this approach in action through some of our latest features like the AI chatbot in the sidebar for desktop or Shake to Summarize on iOS.
Now, we’re excited to invite you to help shape the work on our next innovation: an AI Window. It’s a new, intelligent and user-controlled space we’re building in Firefox that lets you chat with an AI assistant and get help while you browse, all on your terms. Completely opt-in, you have full control, and if you try it and find it’s not for you, you can choose to switch it off."
https://blog.mozilla.org/en/firefox/ai-window/