The Google Translate Firefox Extension lets you translate between any of the languages supported by Google Translate from the Firefox address bar.
The extension can be installed from the Firefox add-on repository: https://addons.mozilla.org/en-US/firefox/addon/google-translate-all-languages/.
gt FROM_LANGUAGE TO_LANGUAGE STRING_TO_TRANSLATE
FROM_LANGUAGE and TO_LANGUAGE are the two-letter code representing the language.
Example:
gt en es hello friend
This will open Google Translate using the URL https://translate.google.com/#en/es/hello friend, which will translate "hello friend" to "Hola amigo".
Please visit https://www.vegard.net/google-translate-firefox-extension/ for more information.