https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

By default, Firefox is configured to automatically check spelling, If it is not working then it is either disabled or you do not have a Dictionary installed. Only a few language locales comes with a dictionary due to licensing reasons.

See https://addons.mozilla.org/firefox/language-tools/ for dictionaries if you need one. Ignore the language packs as they change language of the user interface like the menus and such.

Websites can disable the Firefox spell checker via a spellcheck="false" attribute of an input field or textarea or editable element and possibly provide its own spelling checker. You can right-click in the text area or input field and use "Inspect" to check in the Inspector if this text area has a spellcheck="false" attribute. You can remove this spellcheck="false" attribute or change it to spellcheck="true" via a double-click or via "Edit As HTML" to make the "Check Spelling" item in the right-click context menu reappear. Firefox will disable "Check Spelling" in the right-click context menu of a text area if the web page disables the spelling checker via a spellcheck="false" attribute of this element.

• https://firefox-source-docs.mozilla.org/devtools-user/page_inspector/how_to/examine_and_edit_html/