browser.ml.chat.enabled set to false
    browser.ml.chat.menu set to false
    browser.ml.chat.page set to false
    browser.ml.chat.page.footerBadge set to false
    browser.ml.chat.page.menuBadge set to false
    browser.ml.chat.shortcuts set to false
    browser.ml.chat.sidebar set to false
    browser.ml.enable set to false
    browser.ml.linkPreview.enabled set to false
    browser.ml.pageAssist.enabled set to false
    browser.tabs.groups.smart.enabled set to false
    browser.tabs.groups.smart.userEnable set to false
    extensions.ml.enabled set to false

That should do it.

Can also use the user config override if you want to do it without having to do that every time you install FF somewhere new (put user.js in the root folder of your firefox profile).

    user_pref("browser.ml.chat.enabled", false);
    user_pref("browser.ml.chat.menu", false);
    user_pref("browser.ml.chat.page", false);
    user_pref("browser.ml.chat.page.footerBadge", false);
    user_pref("browser.ml.chat.page.menuBadge", false);
    user_pref("browser.ml.chat.shortcuts", false);
    user_pref("browser.ml.chat.sidebar", false);
    user_pref("browser.ml.enable", false);
    user_pref("browser.ml.linkPreview.enabled", false);
    user_pref("browser.ml.pageAssist.enabled", false);
    user_pref("browser.tabs.groups.smart.enabled", false);
    user_pref("browser.tabs.groups.smart.userEnable", false);
    user_pref("extensions.ml.enabled", false);

It's a garbage feature that no one appears to have asked for.