first of all let me thank you for this excellent addon. However there are a few issues I would like to draw your attention to.
1. this addon creates a copy of the file in profile directory. moreover if that file (the copy) is accessed, one more copy of it created. for someone who has many EPUBs, this will result in multiple copies of the same file leading to unneccessary wastage of space. instead of copying the file it is desirable to create a shortcut (or soft link).
2. this addon has changed the default behaviour of EPUB files to directly open ... this is a problem if one wants to download the file to his PC in a preferred location. you may say that "right click -> save link" would rectify the problem. however some sites (manybooks.net) do not send a permalink to the file but instead dynamically generate data from a script. so it becomes impossible to download the file. sure it is downloaded in the profile directory (but the download is not logged in the download manager!!) but it is a pain in the ass to go to the profile directory in case one wants to copy in in a USB-stick or open with another reader.
I hope the above is counted as genuine concern. i think it would be a nice idea to have a 'options' configurator where one can set how to handle files got directly from internet and files opened from PC.