Total shot in the dark,

I had this problem for a while. It turns out the whole time my firefox was set to a different dialect of english (British) lol. I think it may have been set that way by default because of Ubuntu being headquartered in the UK lol

My spellcheck seems to be working fine these days though, could you list some examples of common words that you’ve had to add to the dictionary? I can check how mine responds when I get home