Furigana Toggle (Firefox Addon)

This addon will let you toggle furigana on pages that have it embedded. That is of only limited use as a lot of sites in Japanese won’t have furigana embedded. Generally speaking, IPA furigana (Firefox, Chrome), which adds furigana to pages that don’t already have it supported, would be the better alternative but that only allows you to toggle the furigana on and off for the entire page. If you prefer to only see furigana when you hover over a word, then you can use this addon with the IPA furigana addon to achieve that.