All of these add-ons should have working versions for Firefox 4+. If I happen to know of any good extensions for Chrome, I'd be happy to add them.
Firefox Add-ons
rikaichan – a popup translator for Foxfox. Simply point to any word or Kanji while the add-on is activated to see it's meaning and pronunciation. You must also install a dictionary. Indispensable.
Furigana Inserter – adds furigana (small hiragana) to Kanji in webpages. Requires that you also install HTML Ruby and a dictionary, which needs to be unzipped using 7-zip. Once you have the unzipped file, drag it into a Firefox window. Finally, disable the option "process inserted content".