I have changed my review to 5 stars. It seems the problem of files being stored in the Firefox Profile directory has been fixed. The update obviously failed to delete the many unused files in the %mozillaprofile% directory and so I was unaware. I thank the developer for responding so quickly and clearing this matter up. BTW, deleting the \epub directory reduced my profile backup size by about 90%.

OLD REVIEW BELOW, WHICH NO LONGER APPLIES
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I have been using this add-on for years. I normally backup my Firefox Profile directory and I have been wondering why it keeps growing in size at an alarming rate. I discovered today that EPubReader stores it's data, including copies of old ePub files I've read and which I no longer need INSIDE the profile folder. This is a huge problem and for this reason I REMOVED this add-on today. Yes, I know the trick of creating a link to another folder, but it's a hassle and not worth it to keep track of so many symbolic or hard links. Any add-on that uses so much space should have a setting to choose the location of its data files.