This is about adding a language pack that has translations for the user interface. Is that what you are after or do you want to select the preferred language(s) used by webpages ? *https://support.mozilla.org/en-US/kb/use-firefox-another-language *https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages ---- You can try to install a language pack via the Add-ons website. Note that for spell checking you need to install a dictionary. *https://addons.mozilla.org/firefox/language-tools/