I think that Rikai-chan has been supplanted by Perapera-kun these days. They both use the same dictionary file, though. Incidentally, that dictionary can be safely hacked to work in modern versions of FireFox (use Nightly Tester tools or hack the extension directly... I think you edit maxVersion in the RDF file, but I'm working from memory here). The dictionary files haven't been updated in forever and so Firefox thinks they're not compatible any more.

Perapera-kun link: https://addons.mozilla.org/en-US/firefox/addon/3343/