Some of us are looking forward to an upcoming browser called Ladybird that promises to be cleaner. They're also developing it from the ground up, to fight the monoculture problem I mentioned, so it's not another Chromium, Webkit, or Gecko (Firefox's engine is Gecko) based browser.
I only found out about it earlier this year, and their monthly newsletter does indicate things like how many PRs were done that month, so there is work going into it. I can't tell you how closely it cleaves to its projected schedule, only that work is being done, and Cloudflare sees enough worthwhile to have given them a $100,000 grant.
You can disable firefox's ai more explicitly in the advanced "about:config" options. You can disable anything with "chat" or ".ml"
There's a few, and one is just whether it's showing the sidebar, but one option outright disables it. I'll check on my PC when I get to it (on phone now).
Edit: alright, at my computer:
In Firefox, using about:config in the URL bar gets you to the advanced options.
Then, you have an empty page with a options search bar: you can search for .ml. to bring up the "machine learning" options.
I think the big ones to look for are: browser.ml.chat.enabled and browser.ml.enable
I also turned off a few others: extensions.ml.enabled browser.ml.chat.sidebar browser.ml.chat.page