I think you need to install a new or different dictionary. It’s dumb, but that’s how it works - spellcheck dictionaries are provided on the Mozilla extension store. You could try a new one.
There is something seriously wrong with Firefox if the default spellcheck dictionary is so bad that I might need to install a different one, especially for the English language. Hell they should just be able to merge theirs with Chromium's. Regardless, as I said I don't normally use spellcheck and I trust my own judgement over the computer's so it isn't a big deal for me.