There are lots of spellcheck add-ons. Which one do you mean?
That said, you won't actually need a spellcheck add-on in most cases because spellchecking is included in TenFourFox. It should be active. If it's not, go to about:config and set layout.spellcheckDefault to 1. A US-English disctionary is included in the default TFF download. If you install one of our langpacks you'll get an additional dictionary for this language in most cases.