CantoFish is a popup Cantonese-English dictionary for Firefox. It contains over 200,000 entries and works with both traditional and simplified Chinese characters. The Yale and Jyutping romanization systems are supported, as well as Mandarin pinyin.
Here is more about CantoFish from the developers:
To enable CantoFish, right click inside a web page and select CantoFish. Then simply hover over a Chinese word to see the definition and pronunciation. To switch between romanization settings (Yale, Jyutping, and Mandarin Pinyin), go into the Add-ons menu, then find CantoFish and click the Options button. Change the Romanization dropdown, and click OK. It is possible to specify both a primary and a secondary romanization. If a secondary romanization is specified, it will be displayed alongside the primary in the popup. This makes it possible to display both Cantonese and Mandarin readings at the same time.
Learn more about CantoFish from the Firefox Add-ons website.