DictCn is an online English to Chinese (and vise 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.
You may click on the 'D' icon on the status-bar to activate and deactivate DictCN (color of the icon changes correspondingly). You may also press ctrl-alt-i (since 3.1.4) (ctrl-alt-d 3.1.3 and previous) short-key to do the job.
By Moving the mouse over the translation, you can hear the speech, and a click on it will bring you to the detailed explanation page.