Do you mean that you currently have an English Firefox version and want to change this to the German version? 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