ImTranslator

If you regularly visit websites written in languages you don't understand, or if you often contact people from other countries, ImTranslator will greatly help you. This add-on gives Firefox a multilingual boost by simultaneously translating everything you type into any of the 20+ languages ImTranslator supports. With this tool, you can translate the text you type into different languages, check it for mistakes, fix unreadable messages, look up words, print results, and send emails. All these translations are based on the PROMT and Google translation servers. It includes the following modules: translation editor, virtual keyboard, spell check, dictionary, translation decoder, and mail client.