HTML Ruby allows Firefox to render common ruby annotation (furigana) used in many CJK language websites and other extensions such as Furigana Inserter. It is a simple, fast, and light-weight alternative to XHTML Ruby Support.
Below in the Development Channel, there is a beta of the next version of HTML Ruby geared towards newer versions of Firefox.
Firefox 38 has built-in support for ruby annotations. Please uninstall HTML Ruby if you have updated to Firefox 38 or later.