CantoFish is a popup Cantonese-English dictionary for Firefox. It is based on the code from Chinese Perapera-kun, which in turn was based on Rikaichan for Japanese.
A few different strategies were used to generate Cantonese readings for the Mandarin based CC-CEDICT (http://usa.mdbg.net/chindict/chindict.php?page=cedict) and adso (http://adsotrans.com) data sets. Along with the generated readings, a large number of human checked compound word readings were provided by Adam Sheik's CantoDict project (http://www.cantodict.org). Additionally, CantoFish supports a number of colloquial Cantonese characters and terms which don't appear in standard Chinese.
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.