Session Manager does this while expanding greatly on the session save/restore capabilities built into Firefox.
Another alternative is to do your writing in a real text editor. This is what
It's All Text allows you to do in your $EDITOR. Start with the existing text from the textarea, edit, and when you save and quit the contents are put back into the textarea. Quite handy for a number of reasons, but reliability is a good one because you can always save regularly to a real file while you are at it.
Personally, I do this with
Vimperator which uses vim, of course. This extension does much, much more though.
