This add-on is a little bit wonky. It allows you to adjust font qualities, mostly; you can set up a dark mode, mostly; it remembers where you were in a EPUB, only if you set a bookmark. Nor does it have a library setting, convenient if you keep your ebooks in one place. Also, if you switch full-screen mode on or off, you will see that the reader does not adjust accordingly.

But it does read EPUBs at least as well as another wonky reader (Nook), so I am happy to cut it some slack and grant it five stars.