If the Firefox user interface (menu bar) is in the wrong language or if you want to change the current language then you can get Firefox in the language of your choice here:

• Firefox 9.0.x: http://www.mozilla.com/en-US/firefox/all.html

wie kann man firefox 10 von der englisch version auf deutsch umstellen? danke emilpichlmeier

Thank you for your help. Your link to install Firefox new have solved the problem.

Thank you very much hagemann

welcher link hilft um von Englisch auf Deutsch umzustellen?

Download the German Firefox version via the above mentioned page:

• Firefox 10.0.x: http://www.mozilla.org/en-US/firefox/all.html

Uninstall the current (English) Firefox version via the Control Panel to remove the English language pack and install the German Firefox version.

Wie kann ich von Englisch auf Deutsch umstellen?

Just do as I wrote above and download the German (de) Firefox version.
Uninstall the currently installed English version to remove the en-US or en-GB language pack.

You can check the value of the general.useragent.locale pref on the about:config page and set that value to language code of the version that you have downloaded (e.g. "de") if there is still a wrong locale set.