Wouldn't it be super handy if you could stick whatever you wanted on Firefox's New Tab page? That's exactly what Mark-It by Matt allows.
Mark-It turns your New Tab page into a text editor that supports markdown. I use it to store commonly used bookmarks, notes and bits of text that i often paste in forums and comments.
Mark-It is not super polished and it doesn't support all of the markdown syntax, but it is simple and it gets the job done.
Prior to Marl-It i was using Clippings until i realized that the developer is injecting quite a bit of CSS into every page you visit for no good reason. While Clippings has more features than Mark-It, i believe it could make the browser much easier to fingerprint and so it's a poor choice for anyone that cares about privacy. Besides, the fewer add-ons, the better.