Right click in the text input area, and 2 or 3 from the bottom there should be a "check spelling" or similar option.
this is the one i installed before, but for some reason it didn't worksHave you installed dictionaries for your languages?
Just getting the language pack might not be enough, but you need the dictionary as well
(or does it work on other sites and not just here?)
https://addons.mozilla.org/en-US/firefox/language-tools/