EDIT: 09/06/2020
Issue has been fixed. 5 star extension, should be standard.

EDIT: 23/11/2017
Issue persists; check
https://sites.google.com/a/duneland.k12.in.us/sites-tutorial/add-a-calendar-to-your-site#TOC-EXAMPLE-CALENDARS

The full width and half width calendars fail to show up with the extension enabled, but work fine with it disabled. Blacklisting the websites works, but it's strange as to why this happens at all. Doesn't happen with the Chrome version of this extension.

Odd - prevents embedded Google Calendar from displaying properly. Otherwise, works on video websites.

Extension works as intended, however some keyboard keys are taken over by Firefox and thus can't be mapped to controls in this extension.

Props to active developer for reading comments.