there's a tool for firefox users called rikai that enables you to look up words from a toolbar and lets you mouse over japanese words and translates them. it also breaks the kanji down giving you the readings and meanings.
click here for rikaiif you decide to download the dictionary/toolbar, make sure you download at least one expansion or it won't work. but it downloads so fast, that you shouldn't have a problem downloading all of it in one sitting. once all of this is done, go to tools and click addons. enable rikai if it isn't already and add both the searchbar and the button. rikai looks like a little red smiley face and he has a star when he's enabled. i hope this helps.