The default UI for Firefox and Thunderbird does not provide an easy way
to switch between multiple installed language packs for the application,
because ordinary users aren't expected to install multiple language packs
However, some developers or users who speak multiple languages may wish
to install language packs and easily switch between multiple languages.
The locale-switcher extension provides simple menu to switch between
languages. The extension adds a "Languages" item on the "Tools" menu
under the existing Extensions/Themes options, allowing the user can
select a language.

WWW: http://benjamin.smedbergs.us/switch-locales/