...and that is the interface.
Having read the instructions on the author's website I failed to open an EPUB document which I had in a folder. After reading some of the reviews and making several attempts I managed to read the book.
I would have liked to be able to reopen the book and carry on reading without having to set a bookmark.

It is evident that Firefox users are unhappy with the Add-on updates needed for the upcoming new requirements.
It would be helpful if there was a mozillaZine Forum for this Add-on so that users and the author could interact publicly.