I've GPLed the javascript that runs the new Rikai
If anyone wants to help, either to make it better, get Opera/Safari support working, etc. please dig in.
Also, the popup code could easily be modified to work as a firefox plugin. I already have code to do the Japanese lookup in my rikaixul plugin, so it should just be a matter of connecting everything together. Still, it's not a good first firefox project, as developing plugins can get pretty hairy.