S3.Google Translator is a handy little add-on for Firefox that can translate blocks of text, entire webpages or specific words quickly.
Download and install it from here.
After it is installed, it will show up in Firefox address bar.
When browsing a webpage, if it needs to be translated, simply select the destination language and hit the “Translate the page” globe icon. The entire translation is handled by Google Translation and is fairly accurate.
Similarly, to translate a specific word, enter it in the second row text input box and hit the “Translate entered words”. The translated version will pop up at the bottom of Firefox window.
Pretty easy and useful to use add-on.