For people using NixOS, you can wrap these configs in extraPrefs in wrapFirefox firefox-unwrapped:
Example: Put this expression(using lockPref to hardcode the config values) in environment.systemPackages(assuming "with pkgs"):
(wrapFirefox firefox-unwrapped {
extraPrefs =
(
''
lockPref("browser.ml.enable", false);
lockPref("browser.ml.chat.enabled", false);
lockPref("browser.ml.chat.hideFromLabs", true);
lockPref("browser.ml.chat.hideLabsShortcuts", true);
lockPref("browser.ml.chat.page", false);
lockPref("browser.ml.chat.page.footerBadge", false);
lockPref("browser.ml.chat.page.menuBadge", false);
lockPref("browser.ml.chat.menu", false);
lockPref("browser.ml.linkPreview.enabled", false);
lockPref("browser.ml.pageAssist.enabled", false);
lockPref("browser.tabs.groups.smart.enabled", false);
lockPref("browser.tabs.groups.smart.userEnable", false);
lockPref("extensions.ml.enabled", false);
''
);
})