Adding for anybody that wants to disable it all:
user_pref("browser.ml.enable", false); // general switch for machine learning features in Firefox, though it might only apply to some of them, https://bugzilla.mozilla.org/show_bug.cgi?id=1971973#c11
user_pref("browser.ml.chat.enabled", false); // AI Chatbot (https://docs.openwebui.com/tutorials/integrations/firefox-sidebar/#additional-about-settings)
user_pref("browser.ml.chat.sidebar", false);
user_pref("browser.ml.chat.menu", false); // remove "Ask a chatbot" from tab context menu
user_pref("browser.ml.chat.page", false); // remove option in page context menu (https://github.com/mozilla/policy-templates/issues/1230)
user_pref("extensions.ml.enabled", false); // might only be relevant for app developers
user_pref("browser.ml.linkPreview.enabled", false);
user_pref("browser.tabs.groups.smart.enabled", false); // "Use AI to suggest tabs and a name for tab groups" in settings
user_pref("browser.tabs.groups.smart.userEnabled", false);
user_pref("pdfjs.enableAltTextModelDownload", false); // "This prevents downloading the AI model unless the user opts in (by enabling the toggle to "Create alt text automatically" from "Image alt text settings" when viewing a PDF)"
user_pref("pdfjs.enableGuessAltText", false); // (disabling this might be redundant when AltTextModelDownload is disabled)