I found another Chinese dictionary plugins for Firefox, it work with the latest Firefox 3.x, is DictCN.
When you move the mouse cursor to the english word it will display the explanation as floating tips display, it support simplified chinese and traditional chinese. It support english audio as well. It can be configured by the right click on the 'D' icon at the status bar as well.
I think is a good replacement if you don't like Oak dictionary.