Earlier quoted context omitted.
Because people like chat bot sidebars. My code editor has a built-in chat bot sidebar that I use every day. It's not a huge stretch that people who use chatbot sidebars in other applications would also want one in their browser. ChatGPT is the #6 most popular website in the world, why wouldn't a browser want tighter integration with such a popular kind of service?
Should Firefox build in a separate side bar for every popular website? Would you want a Facebook side bar and Facebook account integration? I wouldn't.
Facebook is mostly scrolling the timeline and passive consumption. It doesn't benefit from being on the side because the content you interact with on Facebook is completely separate from the content on your other tabs.
In contrast, LLMs have ongoing conversations that the user can come back to, and each conversation might relate to multiple tabs that the user is working on. On top of that, it's a very common occurrence that the user has questions about, or a task to be done using the content of the current page. This makes LLM and chatbot integration much more useful than a Facebook integration.
Also if you have the Facebook Messenger installed, Firefox already gives you an integration to share things with your Facebook contacts.