rikaichan is an extension that adds a Japanese to English/German/French/Russian dictionary.

Just hover the mouse on top of a word, and a popup appears. Automatically de-inflects verbs and adjectives. Detailed kanji view.

Here are some key features of "rikaichan":
· 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.

Requirements:
· Firefox 1.0 - 2.0.0.* ALL

What's New in This Release:
· The enter key can now be used to switch between the dictionaries. Thanks to Charles for the hint on how to make it finally work on a Mac. :)
· The kanji display is now configurable.
· Fixed a problem in preferences that prevented the display of some options.
· Fixed version information to make it work on Firefox 2.0b2+.