Firefox dsoe nto spel chcek fomr feilds. This comment didn't get any spell checking whatsoever. Last time I investigated this the suggestion was to install a plugin, a hoop I think is unreasonable to make people jump through. I'm using a vanilla Firefox install on a Mac. I've not fiddled with it in any way.
update It seems there is a preference:
https://dl.sendcat.com/8BEOaD0qwKgHhIizbRLif8G572x4RgKGGLvcp...
It was selected already but I still don't get spell checking.