If the Firefox user interface (toolbars) is in the wrong language or if you want to change the current language, download Firefox in the language of your choice. *https://www.mozilla.org/en-US/firefox/all/#product-desktop-release You can try "Clear the startup cache..." via the "Help -> More Troubleshooting Information" (about:support) page. *https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox