Maybe this may work or it gives you info why spell check does not work : https://support.mozilla.org/en-US/questions/987216#answer-536562 I am not coming up with anything from after Nov 14 2017 release date of Quantum. So all answers may not work but they do continue to be a theme as per above URL. It would be a Firefox vs Microsoft issue. Might find more of the same issue in below. https://www.google.com/search?q=firefox+speel+check+and++Outlook+365&ie=utf-8&oe=utf-8&client=firefox-b Please let us know if this solved your issue or if need further assistance.