There's a nice extension called Scribe that lets you save your form contents into a file. But it didn't do the trick for me, for two reasons:
- It only works for Firefox 1.0, and refuses to install if you have Firefox 1.5.
- To save your form, you have to go File > Save Entry..., then a file dialog pops up and you have to navigate to where you want to save your file, type in a filename... blah blah blah. There's no way i'd be conscientious enough to do that every few minutes.
Be warned: this is a quick and dirty hack, and it does not delete old files! You may end up with a lot of little files piling up in that directory after a while. But that's better than losing work, i think.
(And yes, i know there are clients for LiveJournal that can autosave. I don't want one.)