Das Add-On ansich ist ganz gut. Das UI ist sehr altbacken, funktioniert aber.

Allein die Möglichkeit, die wohl einem Firefox Update zum fiel, ePubs direkt aus Windows oder einem Download zu öffnen fehlt inzwischen. Dadurch ist der Nutzen des Add-Ons leider kaum noch gegeben, da ist es sinnvoller extra Software zu installieren.

эх блин все ссылки в книге сплошным цветом, не годится

This is not a bad addon. It reads EPUB files like it says it does. But compared to the old addon, it has some issues caused by the new Firefox security. There is no longer a catalog of books that one has read in the past, so one can go back and continue reading that book. The Epubreader icon is only in the toolbar, and is no longer on the Tools menu of Firefox. I can't find any link to the Github project, I'm not sure it even has a Github project. There is no link in the toolbar icon to take one to the Mozilla addon page, the support page at http://www.epubread.com, or the Github page to enter issues.

I've read several EPUB books so far with no issues. It just needs a bit more polish as the author had to start over and rewrite this for Firefox 54.

When I make the Firefox window short and thin, then I can't see the bottom lines of the chapter. But this only happens when the FF window is too small. I'm going to have to use a standalone reader software because I can't find an addon that works with my ebooks.

EDIT: There are some EPUBs that are made by Pandoc which EPUBreader has a problem with. The problem is EPUBReader does not show the last few lines of the EPUB chapter if the Firefox window is not wide enough. This problem seems to stem from any EPUBs made by Pandoc.

The new version... hmm... catalogue has gone, or has been hidden well. So instead of the ease with epub I now - well until I downgraded - have to browse around hoping to find what I am looking for. Why did that go?
Besides the new layout.. hmm... maybe fine if you read a book from one end to another, but not great for anything but that.

...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.