How to manually configure Firefox / Waterfox language

  • Go to
  • Choose the Firefox version, ex: 46.0
  • Choos the architecture, ex: linux-x86_64
  • Go into the xpi folder
  • Choose a language, ex: fr.xpi
  • Click on the choosen language xpi file and enable the patch
  • Restart Firefox
  • In the URL navigation bar, wirte about:config
  • In the Filter field, write general.useragent.locale
  • Double-click on the key to edit it and eplace the value en-US with fr-FR for example
  • Restart Firefox

Firefox / Waterfox is now in the newly configured language.