I've found this the other day, and find it to be quite useful to learn kanji on webpages.
https://addons.mozilla.org/en-US/firefox/addon/6178
It's a plugin for Firefox that will add hiragana over the kanji. You can turn it off and on at your leisure, and also set it to where it only displays the furigana on the first occurrence of kanji.
This works great with my Mac, and it does support Windows, and Linux. I've found though you need to increase the font size to read some words.