The Add-on sounds great but it seems that it does not work with an actual Version of Firefox. Spellchecking is activated in general, just like the add-on itself. But nothing has changed at all and I don't see a way to actually use it. :-(
I was looking for a good spell checker to proof sites I have developed before going live and this tool is brilliant. Its nice that it uses the built in spell checker so i can select which dictionary to use. It would be nice if it could highlight the wrongly spelled word rather than just underline it as fonts are hard to see if they are wrong.