Install the language pack for the desired languages.
       https://addons.mozilla.org/firefox/language-tools/ 
   Type about:config into the address bar and press Enter.
   To bypass the warning, press the big button labeled "I'll be careful, I promise!".
   In the search box, paste intl.locale.matchOS
   In the search results, if intl.locale.matchOS is set to true, double-click it to set its value to false.
   In the search box, paste general.useragent.locale
   In the search results, double-click general.useragent.locale and set its value to the desired language.
   Restart Firefox for the change to take effect.

Kedves bajas! Ez a kérdés már meg lett válaszolva egy másik fórumon: https://support.mozilla.org/hu/questions/1018686 Gingerbread Man válasza: If you need to switch between different interface languages, Install the language pack for the desired languages. https://addons.mozilla.org/firefox/language-tools/ Type about:config into the address bar and press Enter. To bypass the warning, press the big button labeled "I'll be careful, I promise!". In the search box, paste intl.locale.matchOS In the search results, if intl.locale.matchOS is set to true, double-click it to set its value to false. In the search box, paste general.useragent.locale In the search results, double-click general.useragent.locale and set its value to the desired language. Restart Firefox for the change to take effect.