Any language:
wiktionary + google translate with mouse over / other options:
http://www.toptip.ca/2010/10/firefox-extension-google-dictionary-and.html
Just Chinese-->English:
mouse over translation:
https://addons.mozilla.org/fr/firefox/addon/perapera-kun-chinese-popup-tra/
wiktionary + google translate with mouse over / other options:
http://www.toptip.ca/2010/10/firefox-extension-google-dictionary-and.html
Just Chinese-->English:
mouse over translation:
https://addons.mozilla.org/fr/firefox/addon/perapera-kun-chinese-popup-tra/