Rated 4 out of 5 stars
I love this extension, which enables me to read all my epub files inside my browser seamlessly. However, one thing which I did not like so much about this add-on is that, by default it saves all the epub files inside your firefox profile directory and depending on how big your epub library is, it might consume a lot of space. I used an workaround of creating a link from my profile directory to the target where I actually want to save my epubs. In Linux you can use ln -s command for this and in Windows (Vista+), you can use mklink command for this.
cd <Firefox Profile Directory>
mklink -J epub <Target Epub Diretory>