I would love to see the bookmark toolbar in the new tab page (a la Chrome) to save on vertical space. If you disable the bookmark toolbar on chrome, the bookmark toolbar is instead displayed on the new tab page. Considering that vertical real estate is at a premium, It would be a useful change. Since the activity stream is meant to be the new tab page in FF 56, it would be great to include this feature on the roadmap.
Here is a filed bugzilla report from 2012: https://bugzilla.mozilla.org/show_bug.cgi?id=727668