>"It's no longer the Firefox that I knew in 2004"
I would hope not. If it were, it would lack a hell of a lot of useful improvements, including new standards, performance, etc.
>''it's just AI this and sponsored tab that. It's proof that open source can be enshittified."
Um, it is an optional browsing mode. You don't have to use it. You can probably even disable it so you don't see it. I am not a fan of all this AI stuff, and it doesn't bother me. Mozilla/Firefox isn't "going AI", it is just a mode that hooks into whatever cloud AI site you want it to use. No different than when they added the search stuff. You have full control what to use (or not use it at all).
Like it or not, sometimes things need to be added that many of us find useless. But for Firefox to stay relevant, they do need to keep up with some of the "trendy" stuff. Thankfully, something like this is not difficult to add and doesn't really use any resources.
You know that "sponsored" home page or tab stuff? A click on two settings and it they off, permanently. Settings> Home> New Windows and Tabs> Homepage> Blank Page and New Tabs> Blank Page. Or you can keep them on and customize what remains on them, including turning off "sponsored" anything.