"CantoFish is a popup Cantonese dictionary for Firefox. It contains over 200,000 entries and works with both traditional and simplified Chinese characters. Both the Yale and Jyutping romanization systems are supported, as well as Mandarin pinyin."
rikaichan is a popup Japanese-English/German/French/Russian dictionary extension for Firefox. Features: * Simple to use, just hover the mouse on top of a Japanese word. * Automatically de-inflects verbs and adjectives. * Has an optional toolbar that allows you to manually type the word to lookup. * Detailed kanji view shows meaning/keyword in English, on/kun readings, and other information. * Hiragana, katakana and half-width katakana are treated the same making it possible to lookup stylized/emphasized words.