I've uploaded to the Sandbox the version 0.6 of Write Area. This version (besides the bump to support FF3b4) does add the ability to use the spellchecker from inside the editor.
Thanks to the chrome privileges it's possible to access directly to the internal data about the spellcheck and that way I can add it to the context menu, just as easy as in any other textarea, there's no need to choose between the FCKeditor context menus to manipulate the content or the Firefox default menus to use the spellcheck.
I'm gonna leave it a few days and if everything is fine then I'll review a few details and then I'll ask to make the extension public, so I would ask that anyone that it's reading this, please, go to the Firefox extensions site, if you don't have an account, then apply for one, it takes no time at all, when you are registered then login and install the Write Area extension.
Now go to a site where you can use HTML in a textarea, (for example this Blogger site, Slashdot or any other of the thousands of sites available), open the context menu and select "Edit with Write Area", and you'll see the editor. Play with it, give it a go and when you are done, then go back to the Write Area page and write a review (if it's nice I'll be happier, but any review is good as long as it's done with good will)