S3.Translator adds the power of Google Translate’s automatic translations to your browser!
It supports translation of selected text, entered phrase or whole webpage between any of 100+ languages.
New Feature: Language learning!
Now you can not only translate, but also learn the selected language!
Automatic translation of subtitles on YouTube!
For enable/disable: Open settings - tab Translate - YouTube Subtitles
Options: Actions: "Selected text" and "Single word" More Info »
The extension uses the Google Translate so it provides more than 80 languages which you can use for quick translation.
Also, the extension able to auto-detect the language of the original text.
There are a lot of simple and complex extensions for Firefox browser but they all have same problem: when translating, the format of selected text is lost, while S3.Google Translator retains the HTML formatting of web pages.