Dictcn

DictCn is an online English to Chinese (and vice versa) dictionary extension for Firefox. It is a handy real-time language tool with audio support.

To use DictCN, simply hover the mouse (coming with 3.0) upon the word you want to translate, or select it (drag or double-click to highlight the word), the translation (pronunciation and explanation) will show up soon in a layer on the same web page.