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.
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".