Please note that Firefox v57+ will no longer support XUL-based addons like Rikaichan.
XUL-based addons are still supported in Firefox ESR (for now), and Firefox-based browsers like Waterfox and Pale Moon.
Thanks to Kalamandea for working on a WebExtension-based version that is compatible with Firefox v57+. It's not yet complete, but you can checkout his work at GitHub
There is also an alternate version called Rikaichamp at AMO
that is compatible with Firefox 57+.
The files listed below are updated more frequently than those listed above, but are not "signed." These files
will only work on Firefox versions that do not use signature verification or on versions that will allow
you to disable it. Examples:
Firefox Developer Edition
Thunderbird can use both signed and unsigned addons.