This add-on is excellent, and it doesn't ask for any permissions. You can do a lot with MD, and it also lets you use a bit of HTML. Handy for, let's say, adding some custom search engine links to your NTP. Unfortunately, the FF version doesn't seem to sync the notes between Firefox profiles.

Ingenious concept. Works flawlessly. A thing that can be improved is that the tab does not occupy the whole width of the screen.