Write your posts in an external editor
I just discovered this little nifty Firefox extension:
I know that some people here write their posts in an external editor before posting them here, because they think it is more comfortable.
With this plugin it becomes a bit easier to do so. Just install it, restart Firefox.
Go to Tools -> Extensions, and select "External Editor" in the list, click "Options" and then "Browse" to search your Editor (Notepad, OpenOffice, MS Word, iWrite or whatever ..)
When you rightlick into a textfield now (you even may have started typing) choose "edit" to open your external editor, write your post there and e.g. use spellchecking, thesaurus etc. When you're done, close the editor, save the document, and the text appears in the browser's textfield.