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 6.0.x: http://www.mozilla.com/en-US/firefox/all.html
• Firefox 3.6.x: http://www.mozilla.com/en-US/firefox/all-older.html

• Uninstall the current Firefox version, but make sure that you do not remove your personal data.
• Install the new Firefox version of the wanted language.

Instead if you want to change the default language for content to display a page in a language instead of another, you have to:

1. open the settings
2. go to Content -> Languages
3. click Choose...
4. add your language of choice and set the desidered order of importance if you have more than one listed.