Not bad, but there are issues caused by new Firefox security Rated 3 out of 5 stars
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.