新着情報の無効化

about:configで編集するなら
browser.newtabpage.activity-stream.asrouter.providers.whats-new-panel
これを検索して
{"id":"whats-new-panel","enabled":true,"type":"remote-settings","bucket":"whats-new-panel","updateCycleInMs":3600000}
を
{"id":"whats-new-panel","enabled":false,"type":"remote-settings","bucket":"whats-new-panel","updateCycleInMs":3600000}
に変更

user.jsに書くなら
user_pref("browser.newtabpage.activity-stream.asrouter.providers.whats-new-panel",
"{\"id\":\"whats-new-panel\",\"enabled\":false,\"type\":\"remote-settings\",\"bucket\":\"whats-new-panel\",\"updateCycleInMs\":3600000}");

これでOK