The extension is unsigned.

[olliebean]

Firefox v48 no longer has any option for allowing unsigned extensions. Hopefully the extension can become signed so it will continue to be usable?

[joscha]

Firefox changed the way add-ons are built - they don't allow add-ons any longer that are built with cfx and I currently don't have the time to update the add-on code and the whole build system. I'd be happy to accept a pull request however.