I think you all will appreciate this, I found it today. Unlike the 金山词典 that most Chinese people use to get pop-up Chinese and English translations this one works through Firefox so is compatible with Mac.
Go to this website (https://addons.mozilla.org/en-US/firefox/addon/3349) to download it. Don’t forget after restarting Firefox to right-click (ctrl-click for Mac users) on your browser’s toolbar and drag the little green book icon into your toolbar. That’s how you toggle the feature on and of.
Enjoy!
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For Google Chrome users – Zhongwen.com has a popup translator extension for you to check out.
Started using a different one by Nciku which can be found here Nciku Toolbar.