Spellcheck not working on YouTube and Reddit (and possibly other) comment feilds.
And, apparently not even on this site (see "feilds" above)!
I have tried troubleshooting mode and a clean profile with no addons.
At least on YouTube, I've noticed this issue for several months.
This is a better link for downloading dictionaries:
Download and install the en-US version of Firefox to get a built-in US dictionary. Other locales may lack dictionaries for licensing reasons.