hello, you can download and install the language version of your choice (vietnamese too!) from https://www.mozilla.org/en-US/firefox/all/

If you want to be able to switch the language of the user interface then you can install additionally language packs. You can find language packs here: * https://addons.mozilla.org/firefox/language-tools/ * http://download-origin.cdn.mozilla.net/pub/firefox/releases/51.0.1/mac/xpi/ The chosen language is set via the general.useragent.locale pref on the <b>about:config</b> page. You can open the <b>about:config</b> page via the location/address bar. You can accept the warning and click "I'll be careful" to continue. *http://kb.mozillazine.org/about:config See also: * http://kb.mozillazine.org/Language_packs