Firefox Spell Check ?

When typing in a post on GC , my fingers work faster than my brain and I get a few typos. Now my old browser Chrome ( and MS Edge ) highlight the error and I can correct it.
Due to Chrome getting a bit cranky I have transferred to Firefox Quantum. Very pleased with it , but the spell check does not operate in GC . In the options I have ticked "Check your spelling as you type " , but it does not highlight them . How can I get this to operate ?

TIA

 

Not sure if it is what highlights misspellings in red for me, but I have an Add on Dictionary.

 

@Palustris , thanks ! Just download a Firefox add on dictionary , and all is OK now . I can mistype to my hearts content now.

 

Good oh!