Rated 4 out of 5 stars

The developer, Polarcloud, posted this information on their website (https://www.polarcloud.com/getrcx/)

Alternatives to rikaichan that work with Firefox 57+ (Quantum):

https://addons.mozilla.org/en-US/firefox/addon/rikaichanext/
https://addons.mozilla.org/en-US/firefox/addon/rikaichamp/