Is this about the language used in the Firefox user interface or about the language used by webpages ? If the Firefox user interface (toolbars) is in the wrong language or if you want to change the current language then download Firefox in the language of your choice. *https://www.mozilla.org/en-US/firefox/all/#product-desktop-release *https://support.mozilla.org/en-US/kb/use-firefox-another-language *https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages