It was only a matter of time until someone implemented a translator add-on for Mozilla Firefox. The add-on currently supports the following languages: English, French, German, Portuguese, Spanish, Italian, Simplified Chinese, Korean, Japanese and Arabic. You may not translate between all languages though, for example you may only translate from Simplified Chinese to English but not to other languages.
The add-on is configured to use Google Translation as the default Translation service. You may change this behavior if you prefer Yahoo Babelfish, PROMPT, Altavista, FreeTranslation or IM Translator. The add-on places an icon in the status bar. Right-Clicking that icon opens a menu with all the available translation options and a link to the preferences.
The add-on can be configured to translate the website in the current tab, a new window, a new tab or the sidebar. You may also select to add a quick translation option when the icon in the status bar is left clicked. You have to select the languages for quick translation by yourself.
The translator add-on is an excellent time saver if you have been using translation services before. Sure, you do not necessarily need the add-on to translate the websites. You could also do it manually by visiting the translation service and pasting the url to translate into the form field.
Related posts:
Microsoft Translator For Microsoft OfficeFirefox Extension gTranslate
Google Translator Toolkit
Translators wanted for Ghacks
Translate Bots added to Google Talk
Skype Translator
Desktop Client For Google Translate
Google Translate Software Client


gTranslate has been out since July ‘05
https://addons.mozilla.org/firefox/918/history/
gTranslate is very useful. Been using this addon nearly every day, to translate English/Spanish words (I’m German). I give this extension a try tomorrow.
Another good add-on is the Quick Translation (qtl)
Hello!
How about testing “Quick TransLation” extension?
https://addons.mozilla.org/ru/firefox/addon/5641
It works rather like vocabulary, but for me it is even better.