Only a few language locales of Firefox like en-US and I think Français comes with a dictionary for the built-in inline spellchecker to use due to licensing reasons. https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker The spell checker does not make use of the Language setting in Settings. It uses Dictionaries whether the one that language of Firefox comes with or ones you installed. To uninstall a Dictionary you installed, you go to Hamburger menu > Extensions and themes (Ctrl+Shift+A) or about:addons and click on Dictionaries on left. You can click on the three horizontal dots and Remove. There are English (Australian), English (British), English (Canadian), English (New Zealand), English (US) Dictionaries in the right column to choose from. Ignore the left one as that is Language Packs and are not used by the spell checker. https://addons.mozilla.org/firefox/language-tools/