Download German version of Installer from here:

• www.mozilla.org/en-US/firefox/all.html

run installer

Thank you very much. no my problem is solved. regards!

Indeed you get by extra downloading a Firefox-version in German (see above). But installing this you lose a lot, e.g. the bookmarks. The problem is, that the language changes from German zu English due to the automatic Firefox update. Before updating Firefox does not inform the users nor does Firefox ask the users whether they want the update. Users do not have any protection against this arbitraryness. The only solution is to use another browser.