This was one of my favorite add-ons, but sadly it doesn't seem to be supported any longer. Most people like myself, had used this on blogs or forums. And now the button doesn't show up on most modern designs (it's not the browser, it's the websites).
For example right now I see the spelling iron, but on several other tab windows such as WordPress, vBulletin, XenForo, ect... Nothing.
The add-on it's self is also using older code.
I tried to contact the developer. Was willing to even pay them to develope it further.... no reply.