Scribefire is an excellent add-on for many reasons beyond submitting to blogs.
I use it as a simple Markdown and HTML editor on the fly, for image links, code snippets, such as Youtube embeds and the like.
I even keep a tab just for keeping notes and backing up my clipboard. It is an all-in-one, go-to kind of accessory.
I also use it for Facebook notes. It works for the basic markups. It is a nice place for working on a note offline, and then just copying the code once you are finished. You'll notice that FB just excludes the unused code for the features FB does not support.