Via @mwl@io.mwl.io on Mastodon:

using \#firefox or a derivative? Setting browser.ml.chat.enabled=false turns off the brand-new AI features.

You're welcome.

(RIP my mentions, muting this)

in the address bar type:
"about:config" then press enter.
You will see an "accept the risks" warning. Continue.
in the search bar, type:
"browser.ml.chat.enabled"
All the way to the right, you'll see two buttons, click the left one to toggle the true to false if it is not false already.